Topic 8 Chemistry of Carbon Compounds 31
Transcript of Topic 8 Chemistry of Carbon Compounds 31
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Topic 8 Chemistry of Carbon Compounds
3131UnitUnit
31.1 Introduction
Synthesis of carbon compounds is a major activity in industrial research laboratories because it allows chemists to make new molecules, which may have useful properties, from readily available molecules.
Suppose we need to plan a synthetic route to prepare propanone from 2-bromopropane.
Typical reactions of selected functional groups
HH
2-bromopropane propanone
C C H
H
H
O H
C
H
H
C
Br
H
C
H
H
C
H
H
?
The synthesis may take two steps as follows:
H C C C
H
H
H H H
Br H
2-bromopropane propan-2-ol
H C C C
H
H
H H
OH H
propanone
H C C C
H
H
H H
O H
H
To be able to design such a synthesis, we require a good knowledge of typical reactions of the functional groups concerned (i.e. –Br,
–OH and C
O
) and the conversion of one functional group into another.
In this unit, we are going to discuss important reactions of members of some homologous series. This will enhance your knowledge of typical chemical reactions of selected functional groups.
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Unit 31 Typical reactions of selected functional groups
31.2 Important reactions of alkanes
We have discussed two important reactions of alkanes in Topic 7 Fossil Fuels and Carbon Compounds. These are combustion and substitution reactions.
Combustion of alkanes
In a good supply of oxygen, alkanes undergo complete combustion to produce carbon dioxide and water, and release much heat. The following equation represents the complete combustion of methane, CH4.
CH4(g) + 2O2(g) CO2(g) + 2H2O(l)
Reaction with halogens — substitution reactions
The reaction of an alkane with chlorine occurs when a mixture of the two is irradiated with ultraviolet light or heated. For example, methane reacts with chlorine to yield a mixture of carbon compounds. The reaction of an alkane with a halogen is a substitution reaction, called halogenation.
CCl4(l) + HCl(g)
CHCl3(l) + HCl(g)
CH2Cl2(l) + HCl(g)Cl2
Cl2
Cl2
CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)UV light
or heat
UV lightor heat
UV lightor heat
UV lightor heat
combustion 燃燒 substitution reaction 取代反應 halogenation 鹵化作用
Alkanes react with bromine in a similar manner.
The products of the reactions between alkanes and halogens are known as haloalkanes.
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Topic 8 Chemistry of Carbon Compounds
Chlorination of methylbenzene takes place in the gas phase in the presence of ultraviolet light or when heated.
methylbenzene (chloromethyl)benzene (dichloromethyl)benzene (trichloromethyl)benzene
CH3 CH2Cl CHCl2 CCl3
(g)UV lightor heat
Cl2(g)
UV lightor heat
Cl2(g)
UV lightor heat
Cl2(g)
31.3 Addition reactions of alkenes
In Topic 7, we learnt that the most characteristic reaction of compounds containing carbon-carbon double bonds is an addition reaction. When a symmetrical reagent X — X adds to a carbon-carbon double bond, each X atom becomes attached to one carbon atom of the double bond. The double bond changes to a single bond.
When an unsymmetrical reagent X — Y adds to a carbon-carbon double bond, the X atom becomes attached to one carbon atom of the double bond and the Y atom becomes attached to the other.
C C
X X
addition reactionXX+CC
C C
X Y
YXaddition reaction
+CC
Addition of hydrogen and halogens are examples of addition of a symmetrical reagent. Addition of hydrogen halides are examples of addition of an unsymmetrical reagent.
addition reaction 加成反應
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Unit 31 Typical reactions of selected functional groups
31.5 Addition of halogens to alkenes
In Topic 7, we have already seen that when we mix bromine (dissolved in an organic solvent) with ethene, they react immediately to form 1,2-dibromoethane. Chlorine also undergoes addition reaction with ethene.
C C
Cl Cl
Cl Cl(g)
H H
H H
H H(l)
H H
(g) +
1,2-dichloroethane
C C
31.4 Addition of hydrogen to alkenes
In the presence of a catalyst, hydrogen adds across a carbon-carbon double bond of ethene to form ethane which is a saturated compound.
If a platinum or palladium catalyst is used, the process takes place under normal laboratory conditions. Nickel is a cheaper but less efficient catalyst. It needs to be finely powdered and the gases need to be at 150 °C and 5 atmospheric pressures for hydrogenation to occur.
hydrogenation 加氫作用
(g) + C C
H H
Pt catalystH H(g)
H H
H(g)H
ethaneethene
H H
H H
C C
Ethene reacts explosively with fluorine, but the reaction with iodine is rather slow.
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Topic 8 Chemistry of Carbon Compounds
Test for unsaturation
Often the test for an alkene involves shaking the substance under test with aqueous bromine rather than pure bromine. Decolorization of the aqueous bromine occurs if an alkene is present. In the case of ethene, a mixture of 2-bromoethanol, 1,2-dibromoethane and hydrogen bromide is obtained.
BrBr
C C
Br Br
C C
Br OH
H H +
H H
2-bromoethanol
H H
H H
H H
H H
1,2-dibromoethane
in water
in organic solvent
C C
Br Br
H H + HBr
H H
1,2-dibromoethane
+C C
If the aqueous bromine is dilute, 2-bromoethanol will be the main product of the reaction. This does not affect what you see — the aqueous bromine still changes from yellow-brown to colourless (Fig. 31.1).
Fig. 31.1 Decolorization of aqueous bromine by ethene
Remember that we can also use the addition reaction of an alkene with cold acidified dilute potassium permanganate solution as a test for alkenes. The purple permanganate solution changes from purple to colourless quickly when shaken with an alkene.
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Unit 31 Typical reactions of selected functional groups
31.1 Studying the addition reactions of alkenes
Write an equation for each of the following reactions and name the product in each case.
a) Methylpropene with bromine (in organic solvent)
b) Cyclopentene with hydrogen
c) Cyclohexene with bromine (in organic solvent)
Solution
a)
b)
c)
C CH H (in organic solvent)
H
Br Br
C
H
H1,2-dibromo-2-methylpropane
C CH H(g) + Br2 (in organic solvent)
H
C
H
H
CH3 CH3
Pt catalyst
cyclopentane
(l) + H2(g) (l)
Br
1,2-dibromocyclohexane
(l) + Br2 (in organic solvent)
Br
(in organic solvent)
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Topic 8 Chemistry of Carbon Compounds
31.6 Addition of hydrogen halides to alkenes
Ethene reacts with gaseous hydrogen halides at room temperature.
1 Identify A, B and C in the following reactions.
a)
b)
c)
2 Write an equation for each of the following reactions:
a) hexa-1,4-diene with excess hydrogen; and
b) cyclohexa-1,3-diene with bromine (in organic solvent).
Br2 (in organic solvent)CH3CH2CH AC
CH2CH3
CH2CH2CH3
H2 / PtB
CH2CH3
C OH
CH2BrCH2
Ethene also reacts with concentrated aqueous solutions of hydrogen halides in the cold.
Br(g) C C
bromoethane
H H
H H
(g) + H H H(l)
H H
H Br
C C
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Unit 31 Typical reactions of selected functional groups
When a molecule HA adds to an asymmetric alkene, the major product is the one in which the hydrogen atom attaches itself to the carbon atom already carrying the larger number of hydrogen atoms.
Some asymmetrical alkenes are shown below.
When a hydrogen halide adds to an asymmetrical alkene, two products are possible.
For example, the addition of hydrogen bromide to propene could give either 1-bromopropane or 2-bromopropane.
In fact, the product is almost entirely 2-bromopropane. The Russian chemist, Markovnikov, formulated a rule for predicting the major addition product formed. Markovnikov’s rule says that:
Markovnikov’s rule 馬科尼科夫規則
C2H5CH3
CH3
C
H
C
H
H
propene
CH3
C C
H
2-methylpent-2-ene 1-methylcyclohexene
CH3
CH3
H H
H
(g) + H Br(g)
C C
1-bromopropane
H H(l)
H H
H Br
C
H
H
C C
2-bromopropane
H
H H
Br H
C
H
H
H(l)
C C
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Topic 8 Chemistry of Carbon Compounds
Thus, for the reaction between propene and hydrogen bromide, 2-bromopropane is the major product.
For the reaction between 2-methylpent-2-ene and hydrogen bromide, 2-bromo-2-methylpentane is the major product.
For the reaction between 1-methylcyclohexene and hydrogen bromide, 1-bromo-1-methylcyclohexane is the major product.
CH3
Br(g)
H
H H
(g) + H C C
major product
H H(l)
H H
Br H
C
H
H
C C
minor product
H
H H
H Br
C
H
H
this carbon atom carries more hydrogen atoms
+C C H(l)
CC
HCH3
C2H5CH3
(l) + Br(g)H C C
HCH3
Br H
CH3 C2H5(l)
this carbon atom carries morehydrogen atom
major product
+ C C
HCH3
H Br
CH3 C2H5(l)
minor product
(l) + Br(g)H
this carbon atom carries morehydrogen atoms
major product minor product
CH3
(l) +
CH3
Br
CH3
H
Br
(l)
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Unit 31 Typical reactions of selected functional groups
31.2 Predicting the major products of the addition of hydrogen halides to alkenes
Predict the major product of each of the following reactions:
a) b) c)?
CH2 HBr?
HCl
(D is deuterium, an isotope of hydrogen)
CC
C2H5H
CH3H
?DBr
Fig. 31.2 summarizes the addition reactions of alkenes.
RCH CH2
alkene
RCHBrCH2Br
RCHCH3
RCHCH2
Br
OH OH
diol
R H
C C
H H
polyalkene
bromoalkane
dibromoalkane
RCH2CH3
alkane
RCHCH2Br
OH
bromoalcohol
Br2
(in organic solvent)
H2(g)Pt catalyst
polymerization in the plastics industry
cold alkalinedilute potassiumpermanganatesolution
HBr(g) Br2(aq)
n
Fig. 31.2 A summary of the addition reactions of alkenes
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Topic 8 Chemistry of Carbon Compounds
Solution
a) b)
c)
31.3 Preparing a haloalkane from an alkene
What alkene would you start with to prepare the following haloalkane? There may be more than one possibility.
CH2HBr
CH3
Br
this carbon atom carries more hydrogen atoms
HClCl
CC
C2H5H
CH3H
this carbon atom carries more hydrogen atoms
DBrC
H
H
D Br
C
C2H5
CH3
? CH3CH2CCH2CH2CH3
CH3
Cl
CH3
CH3CH CCH2CH2CH3 or CH3CH2C CHCH2CH3 or CH3CH2CCH2CH2CH3
CH3
CH3
CH3CH2CCH2CH2CH3
Cl
HCl
CH2
Solution
The haloalkane can be prepared by the reaction of an alkene with HCl. The carbon atom bearing the Cl atom in the haloalkane must be one of the carbon atoms in the carbon-carbon double bond of the reactant.
There are three possibilities, any one of them could give the desired product.
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Unit 31 Typical reactions of selected functional groups
1 Predict the major product of each of the following reactions.
a) b) CH3CH2CH2CH=CH2 + HCl
2 Consider the reactions of 1-ethylcyclopentene shown below:
a) Give the reagent(s) and condition(s) for the conversion of 1-ethylcyclopentene to ethylcyclopentane.
b) Draw the structural formula of the product X.
3 Draw the structural formula of an alkene you would start with to prepare each of the following haloalkanes.
a) b)
C CH2
CH3
CH3
+ Hl
CH2CH3ethylcyclopentane
major product XHBr1-ethylcyclopentene
?
CH3CH2CHCH2CH2CH3
Br CH2CH3
I
hydrolyzed 被水解
31.7 Substitution reactions of haloalkanes
Substitution reactions are typical reactions of haloalkanes. For example, a substitution reaction takes place between a haloalkane and hydroxide ions, in which the haloalkane is hydrolyzed to form an alcohol. Take 1-bromobutane, for example:
CH3CH2CH2CH2Br(l) + OH–(aq) CH3CH2CH2CH2OH(aq) + Br–(aq)
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Topic 8 Chemistry of Carbon Compounds
Haloalkanes are usually hydrolyzed by heating under reflux with sodium hydroxide solution (Fig. 31.3). Often ethanol is added to act as a solvent. These substitution reactions can be used to prepare alcohols.
In the reflux condenser, all the vapour is condensed back into the flask. This prevents any loss of the reaction mixture*.
For a given alkyl group, the iodo compound reacts most readily, the bromo compound less so and the chloro compound reacts less readily. This is because the carbon-halogen bond becomes progressively stronger from I to Cl.
Order of reactivity: RI > RBr > RCl
The C–F bond is so strong that the fluoroalkanes are extremely unreactive.
Fig. 31.3 Apparatus used for heating an haloalkane with sodium hydroxide solution under reflux
R X
haloalkane
R OHNaOH(aq)
alcoholreflux
The boiling points of haloalkanes are not high. We have discussed this in Unit 29.
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Unit 31 Typical reactions of selected functional groups
31.4 Comparing the rate of hydrolysis of haloalkanes
A student carried out an experiment to compare the rate of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane.
A general equation for the hydrolysis is:
OH + H+ + X–X + H2OR R
The experiment consisted of three steps.
Step 1 Three solutions (each containing 2 cm3 of ethanol and 1 cm3 of dilute silver nitrate solution) were put into different test tubes.
Step 2 The test tubes were placed in a water bath at 60 °C.
Step 3 5 drops of 1-chlorobutane, 1-bromobutane and 1-iodobutane were added separately to the test tubes.
The student found that a silver halide precipitate appeared in each test tube.
silver chloride produced by
1-chlorobutane
silver bromide produced by
1-bromobutane
silver iodide produced by 1-iodobutane
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Topic 8 Chemistry of Carbon Compounds
31.8 Reactions of alcohols
There are three main categories of alcohols: primary (1°), secondary (2°) and tertiary (3°). This classification is based on the number of alkyl groups attached to the carbon bearing the hydroxyl group (–OH). A primary alcohol has one alkyl group attached, a secondary alcohol has two alkyl groups attached and a tertiary has three alkyl groups attached.
For convenience sake, methanol (CH3OH) is classified as a primary alcohol, even though it has no alkyl group attached to the carbon bearing the –OH group.
R1
R2
R1
OH OH
C OH
H
R
H
C
H
R
secondary (2°) alcohol
CR
tertiary (3°) alcohol
C OH
H
H
H
primary (1°) alcohol*
a) What was the purpose of adding silver nitrate solution to each reaction mixture?
b) The student found that the precipitate appeared first in the test tube containing 1-iodobutane, followed by 1-bromobutane and then 1-chlorobutane. Explain this order.
Solution
a) The silver nitrate solution was added to follow the rate of hydrolysis of the haloalkanes. Halide ions produced from the hydrolysis would form a precipitate with the silver ions.
Ag+(aq) + X–(aq) AgX(s)
b) The rate of hydrolysis of haloalkanes is related to how easily the C–X bond breaks. As the C–I bond is weaker than the C–Br and C–Cl bonds, the C–I bond breaks most readily. Hence the rate of hydrolysis of 1-iodobutane is the highest. The precipitate appeared first in the test tube containing it.
As the C–Cl bond is the strongest, the bond is the most difficult to break. Hence the rate of hydrolysis of 1-chlorobutane is the lowest.
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Unit 31 Typical reactions of selected functional groups
Substitution reactions of alcohols with halides
Alcohols react with hydrogen halides (HI, HBr and HCl) to form haloalkanes. This reaction involves the cleavage of the carbon-oxygen bond of the alcohol.
Draw the structural formula for each of the following alcohols and state whether it is primary, secondary or tertiary.
a) Propan-1-ol b) 2-methylbutan-2-ol
c) Cyclohexanol
OH + HXR X + H2OR
Examples of different categories of alcohols are shown below:
Although all alcohols behave similarly in some reactions, the primary, secondary and tertiary alcohols react differently in certain reactions.
C
H
H
H
H
H
C OH
ethanol(1° alcohol)
HC C
HH
H OH
H C
H
H
propan-2-ol(2° alcohol)
HC C
CH3H
H OH
H C
H
H
methylpropan-2-ol(3° alcohol)
2-methylcyclopentanol(2° alcohol)
CH3
OH
The order of reactivity of alcohols in substitution reactions with halides is 3° > 2° > 1°.
The order of reactivity of the hydrogen halides is HI > HBr > HCl (HF does not react).
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Topic 8 Chemistry of Carbon Compounds
Reaction with hydrogen chloride
Bubble dry hydrogen chloride through the anhydrous alcohol in the presence of anhydrous zinc chloride as a catalyst.
We can also prepare chloroalkanes by refluxing concentrated hydrochloric acid using zinc chloride as a catalyst.
ZnCl2 catalystCH3CH2CH2OH(l) + HCl(g) CH3CH2CH2Cl(l) + H2O(l)
1-chloropropane
Fig. 31.4a An experimental set-up for the reaction between hydrogen bromide and alcohol
refluxCH3CH2CH2OH(l) + HCl(aq) CH3CH2CH2Cl(l) + H2O(l)
ZnCl2 catalyst1-chloropropane
Tertiary alcohols react reasonably rapidly with concentrated hydrochloric acid without a catalyst. For example, methylpropan-2-ol reacts with concentrated hydrochloric acid at room temperature. 2-chloro-2-methylpropane is formed in the reaction.
Reaction with hydrogen bromide
Normally, the reaction is carried out by refluxing together a mixture of alcohol, sodium bromide and concentrated sulphuric acid (Fig. 31.4a).
C CH3(l) + HCl(aq)
CH3
OH
CH3 C CH3(l) + H2O(l)
CH3
Cl
CH3
room temperature
2-chloro-2-methylpropane
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Unit 31 Typical reactions of selected functional groups
Concentrated sulphuric acid reacts with sodium bromide to produce hydrogen bromide* which reacts with the alcohol.
The mixture is then warmed to distil off the bromoalkane (Fig. 31.4b).
CH3CH2OH(l) + HBr(g) CH3CH2Br(l) + H2O(l)
bromoethane(boiling point 38 °C)
reflux
Fig. 31.4b A laboratory set-up for separating bromoalkane from the reaction mixture
Reaction with hydrogen iodide
In this case, the alcohol is reacted with a mixture of sodium iodide and concentrated phosphoric acid*. Concentrated phosphoric acid reacts with sodium iodide to produce hydrogen iodide* which reacts with the alcohol. The iodoalkane is distilled off.
refluxCH3CH2OH(l) + HI(g) CH3CH2I(l) + H2O(l)
iodoethane(boiling point 72 °C)
Phosphoric acid is used instead of concentrated sulphuric acid because sulphuric acid oxidizes iodide ions to iodine and produces very little hydrogen iodide.
Concentrated sulphuric acid and sodium bromide react according to the following equation:
H2SO4(l) + NaBr(s) NaHSO4(s) + HBr(g)
Concentrated phosphoric acid and sodium iodide react according to the following equation:
H3PO4(l) + NaI(s) NaH2PO4(s) + HI(g)
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Topic 8 Chemistry of Carbon Compounds
Reaction with phosphorus halides
We can also convert alcohols to haloalkanes by reaction with phosphorus halides.
To prepare a bromoalkane or an iodoalkane, the alcohol is usually heated under reflux in a water bath with a mixture of red phosphorus and bromine or iodine (Fig. 31.5). Phosphorus first reacts with bromine or iodine to give phosphorus trihalide. The halide then reacts with the alcohol to give the corresponding haloalkane which can be distilled off.
3CH3CH2OH(l) + PBr3(l) 3CH3CH2Br(l) + H3PO3(l)
bromoethane phosphorousacid
reflux
Phosphorus pentachloride reacts with alcohols in the cold*.
roomtemperature
CH3CH2CH2OH(l) + PCl5(s) CH3CH2CH2Cl(l) + POCl3(l) + HCl(g)
1-chloropropane phosphorustrichloride oxide
The formation of hydrogen chloride fumes upon reaction with phosphorus pentachloride is a test for the presence of a hydroxyl group in a compound.
Fig. 31.5 A laboratory set-up for the reaction of ethanol with PBr3
(the reaction should be carried out inside a fume cupboard)
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Unit 31 Typical reactions of selected functional groups
Reaction with sulphur dichloride oxide (thionyl chloride)
Sulphur dichloride oxide (thionyl chloride) has the molecular formula SOCl2. It reacts with primary and secondary alcohols to produce chloroalkanes. Sulphur dioxide and hydrogen chloride are given off.
Fig. 31.6 A summary of the substitution reactions of alcohols with halides
31.5 Studying the reactions of alcohols with halides
Predict the product of each of the following reactions:
a)
b)
c)
SOCl2
reflux?OH
PBr3
reflux?CH2OH
HBr
reflux?CH3CH2CH2CH2OH
refluxCH3CH2CH2OH(l) + SOCl2(l) CH3CH2CH2Cl(l) + SO2(g) + HCl(g)
Fig. 31.6 summarizes the substitution reactions of alcohols with halides.
R OHalcohol
R Clchloroalkane
R Brbromoalkane
R Iiodoalkane
• reflux with conc. HCl + ZnCl2 catalyst; or• mix with PCl5; or• reflux with SOCl2
• reflux with NaI + conc. H3PO4; or• reflux with red P + I2
• reflux with NaBr + conc. H2SO4; or• reflux with red P + Br2
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Topic 8 Chemistry of Carbon Compounds
Solution
a)
b)
c)
31.6 Deducing the structure of a compound
Compound X has the molecular formula C4H8O.
• When phosphorus pentachloride, PCl5, was added to a dry sample of X, steamy fumes were observed.
• When aqueous bromine was shaken with a sample of X, the aqueous bromine turned colourless.
• Compound X has a geometrical isomer.
Use the information above to draw the structural formulae of the two isomers which could be compound X.
Solution
• X reacted with PCl5 to give steamy fumes (HCl). This shows the presence of a hydroxyl group in X.
• X decolorized aqueous bromine. This shows the presence of a carbon-carbon multiple bond in X.
The structural formulae of the two isomers which could be X are as follows:
SOCl2
refluxOH Cl
CH2OHPBr3
refluxCH2Br
HBr
refluxCH3CH2CH2CH2BrCH3CH2CH2CH2OH
CC
CH2OHCH3
HH
CC
CH2OHH
HCH3
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Unit 31 Typical reactions of selected functional groups
Elimination reactions: dehydration of alcohols to form alkenes
Most alcohols dehydrate and form alkenes when they are heated in the presence of a strong acid.
1 Predict the products of each of the following reactions:
a) reflux pentan-2-ol with sodium bromide and concentrated sulphuric acid;
b) reflux cyclohexanol with red phosphorus and iodine;
c) reflux propan-1-ol with concentrated hydrochloric acid and zinc chloride catalyst.
2 How would you carry out each of the following conversions?
a)
b)
CH3CH2CH2CHCH3
OH
?CH3CH2CH2CHCH3
Br
CH3CH2CH2CHCHCH3
CH3
OH
?CH3CH2CH2CHCHCH3
CH3
l
C
OH
C
H
CCstrong acid
heat+ H2O
These reactions are described as dehydration since they involve the removal of a water molecule from a molecule of the reactant. Dehydration is an example of elimination reaction.
In an elimination reaction, atoms or groups of atoms are removed from two adjacent atoms (usually carbon atoms) of the reactant molecule.
dehydration 脫水作用 elimination reaction 消去反應
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Topic 8 Chemistry of Carbon Compounds
The experimental conditions — temperature and acid concentration — that are required to bring about dehydration are closely related to the structure of individual alcohols.
Dehydration of primary alochols
Primary alcohols are the most difficult to dehydrate. For example, we can carry out the dehydration of ethanol in the laboratory by heating ethanol at 180 °C with excess concentrated sulphuric acid. Ethene is evolved and can be collected over water (Fig. 31.7).
+ H2O(l)conc. H2SO4C
OH
C
H
H H(l)
excessH H
H H
(g)
ethanol (1° alcohol)
HH
C C180 °C
ethene
We can regard the concentrated sulphuric acid as a dehydrating agent, removing water from the ethanol.
Fig. 31.7 Experimental set-up for the dehydration of ethanol using concentrated sulphuric acid
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Unit 31 Typical reactions of selected functional groups
Aluminium oxide at 300 °C can also act as a dehydrating agent (Fig. 31.8).
Dehydration of secondary alcohols
Secondary alcohols usually dehydrate under milder conditions.Cyclohexanol, for example, dehydrates in 85% phosphoric acid at 165 – 170 °C.
Fig. 31.8 Experimental set-up for the dehydration of ethanol using hot aluminium oxide
(l) + H2O(l)85% H3PO4
OH(l)
165 – 170 ˚C
cyclohexenecyclohexanol(2˚ alcohol)
Dehydration of tertiary alcohols
Tertiary alcohols are usually so easily dehydrated that extremely mild conditions can be used. For example, methylpropan-2-ol dehydrates in 20% sulphuric acid at a temperature of 85 °C.
H(g) + H2O(l)
CH3CH3
C
H
C
H
H H(l)85 °C
C
OH
HH
20% H2SO4 CC
H
H C
HH
methylpropan-2-ol(3° alcohol)
methylpropene
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Topic 8 Chemistry of Carbon Compounds
The relative ease with which alcohols undergo dehydration shows the following order:
Ease of dehydration: 3° > 2° > 1° alcohol
With some alcohols, several alkenes may be produced, but normally the most-substituted alkene is the major product.
Consider the dehydration of butan-2-ol. But-1-ene and but-2-ene are formed.
But-2-ene is the major product of the dehydration.
But-1-ene has only one alkyl group attached to the carbon atoms of the C=C bond while but-2-ene has two. But-2-ene is the more substituted alkene and hence the major product of the reaction.
OH
H C
H
H
C
H
C
H
H
C
H
H
H C
H
H
C
H
C
H
H
C
H
H
H
but-1-ene(minor product)
remove
dehydration
OH
H C
H
H
C
H
C
H
H
C
H
H
H H C
H
H
C
H
C
H
C
H
H
H
but-2-ene*(major product)
remove
dehydration
Remember that but-2-ene has two geometrical isomers. We have discussed this in Unit 30.
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Unit 31 Typical reactions of selected functional groups
31.7 Testing an unknown gas
A mixture of ethanol and excess substance X was heated to 180 °C in flask A as shown below. The gas G liberated passed through tube B containing sodium hydroxide solution and then tube C containing acidified dilute potassium permanganate solution. The excess gas liberated at D was ignited. The purple colour of the solution in tube C faded when gas G passed through it.
a) i) Identify substance X.
ii) Name gas G formed in flask A.
iii) Write an equation for the formation of gas G.
b) What was the purpose of passing gas G through the sodium hydroxide solution in tube B?
c) Explain the observable change for acidified dilute potassium permanganate solution in tube C when gas G passed through it.
Solution
a) i) Concentrated sulphuric acid
ii) Ethene
iii)
b) To remove any traces of acidic vapours.
c) Ethene underwent an addition reaction with acidified dilute potassium permanganate solution. The permanganate solution became colourless.
excess conc. H2SO4CH3CH2OH(l) C2H4(g) + H2O(l)
180 °C
118
Topic 8 Chemistry of Carbon Compounds
31.8 Dehydration of alcohol to alkene
a) Draw the structural formulae of alkenes that can be made by dehydration of 2-methylpentan- 2-ol.
b) Which of the alkenes drawn in (a) is likely to be the major product? Explain your answer.
Solution
a)
b) 2-methylpent-1-ene has two alkyl groups attached to the carbon atoms of the C=C bond while 2-methylpent-2-ene has three. 2-methylpent-2-ene is the more highly substituted alkene and hence the major product.
conc. H2SO4 or H3PO4
C3H7
CH3
C2H5
CH3
C
H
C
H
H
H
H
C
HH
OH
remove
2-methylpent-1-ene
CCdehydration
heat
C2H5
conc. H2SO4 or H3PO4
C2H5
CH3
C
H
C
H
Hdehydration
heat
H
C
HH
OH2-methylpent-2-ene
remove
CH3
CH3
1 Draw and name the major dehydration product of the following compound:
Tetrahydrofuran (THF) is a common solvent.OH
CH3?
H2SO4
THF*, 50 °C
O
119
Unit 31 Typical reactions of selected functional groups
Oxidation of alcohols
Consider the treatment of the following three alcohols with acidified potassium dichromate solution.
2 Alcohol A has the following structure:
When A was treated with hot concentrated sulphuric acid, three substances X, Y and Z were produced. X, Y and Z were isomers of each other. Only a small amount of Z was produced. Both X and Y reacted with bromine to give 1,2-dibromo-1-phenylpropane.
a) Draw the structure of 1,2-dibromo-1-phenylpropane.
b) Suggest structures of X, Y and Z.
c) What is the isomeric relationship between X and Y?
CH3CH2CH2OH CH3CHCH3
CH3CH3
CH3
OH
C
methylpropan-2-ol(3° alcohol)
OH
propan-2-ol(2° alcohol)
propan-1-ol(1° alcohol)
Fig. 31.9 shows the results when the three alcohols are heated with acidified potassium dichromate solution. Propan-1-ol and propan-2-ol are oxidized by the dichromate solution. The orange dichromate solution turns green as the dichromate ions are reduced to green chromium(III) ions. Methylpropan-2-ol does not react with acidified potassium dichromate solution.
Oxidation of the molecule of a carbon compound usually corresponds to increasing its oxygen content or decreasing its hydrogen content*.
Fig. 31.9 Results obtained when propan-1-ol, propan-2-ol and methylpropan-2-ol are heated with acidified potassium dichromate solution
methyl-propan-2-ol(3° alcohol)
propan-2-ol(2° alcohol)
propan-1-ol(1° alcohol)
We have discussed this in Topic 5 Redox Reaction, Chemical Cells and Electrolysis.
OH
C
H
H
C
H
C
H
H
H
120
Topic 8 Chemistry of Carbon Compounds
Table 31.1 Reactions between alcohols and oxidizing agents
Alcohol Reaction
Primary
Secondary
Tertiary
We can oxidize an alcohol to yield a carbonyl compound — the opposite of the reduction of a carbonyl compound to yield an alcohol:
Notice that in this reaction, two hydrogen atoms are being removed: one from the oxygen atom and one from the carbon atom. Hence oxidation of the –OH group will not take place unless there is a hydrogen atom on the carbon atom bearing the –OH group.
Upon oxidation, primary alcohols yield aldehydes or carboxylic acids, secondary alcohols yield ketones. Tertiary alcohols do not normally react with most oxidizing agents.
C
OH
an alcohol
H
reduction
oxidation
a carbonyl compound
O
C
C
H
OH
R H CR H[O]
O
an aldehyde
[O]
a carboxylic acid
C
O
R OH
R1
R1
C
OH
R H CR[O]
O
a ketone
R1
R2C
OH
R[O]
no reaction
A number of oxidizing agents can be used in the oxidation process, such as acidified potassium dichromate solution, acidified or alkaline potassium permanganate solution.
121
Unit 31 Typical reactions of selected functional groups
Oxidation of primary alcohols to aldehydes
Acidified potassium dichromate solution can oxidize propan-1-ol to propanal.
To prepare propanal, we must choose the reaction conditions to prevent any subsequent oxidation of propanal to propanoic acid. We can either
• carry out the reaction at room temperature and add the oxidizing agent to the alcohol so that the oxidizing agent is never in excess; or
• heat the reaction mixture to distil off the propanal once formed so that it will not be further oxidized (Fig. 31.10).
[O]
propan-1-ol(boiling point 97 °C)
propanal(boiling point 49 °C)
+ +C H(aq)
OH
H
CH3CH2 CH3CH2 C
O
H(l) H2O(l)
where [O] represents an oxidizing agent
Fig. 31.10 An experimental set-up for the oxidation of propan-1-ol to propanal
122
Topic 8 Chemistry of Carbon Compounds
Oxidation of primary alcohols to carboxylic acids
We can oxidize propan-1-ol to propanoic acid by refluxing it with acidified potassium dichromate solution (Fig. 31.11a).
In the reflux condenser, all the vapour is condensed back into the flask. This prevents any loss of the reaction mixture and favours the oxidation of propan-1-ol to propanoic acid rather than to propanal.
2[O]
propan-1-ol(boiling point 97 °C)
propanoic acid(boiling point 141 °C)
+ +C H(aq)
OH
H
CH3CH2 CH3CH2 C
O
OH(l) H2O(l)
Fig. 31.11a An experimental set-up for the oxidation of propan-1-ol to propanoic acid
To separate propanoic acid from the reaction mixture, transfer the mixture to the apparatus as shown in Fig. 31.11b. On heating, propanoic acid and water distil out to give an aqueous solution of propanoic acid.
123
Unit 31 Typical reactions of selected functional groups
Acidified potassium permanganate solution is too powerful an oxidizing agent to stop at the aldehyde; it oxidizes primary alcohols to acids and oxidizes secondary alcohols to ketones.
In the following activity, you are going to oxidize ethanol to ethanoic acid and test the ethanoic acid produced.
Fig. 31.11b An experimental set-up for separating propanoic acid from the reaction mixture
Oxidizing ethanol to ethanoic acid and testing the ethanoic acid produced.
31.1
Oxidation of secondary alcohols to ketones
Secondary alcohols can be oxidized to give ketones. For example, we can oxidize propan-2-ol to propanone by refluxing it with acidified potassium dichromate solution.
[O]
propan-2-ol(boiling point 82 °C)
propanone(boiling point 56 °C)
+C H(aq)
OH
CH3
CH3 CH3 C
O
CH3(l) H2O(l)+
124
Topic 8 Chemistry of Carbon Compounds
Fig. 31.12 summarizes the common reactions of propan-1-ol.
We will discuss this reaction later in this unit.
Fig. 31.12 A summary of the common reactions of propan-1-ol
31.9 Studying the oxidation of alcohols
What alcohol would give each of the following products on oxidation with acidified potassium dichromate solution?
a)
b) O
CH3
OHH C
H
H
C
H
H
C
H
H
C
H
H
C
H
C
O
• reflux with conc. HCl +
ZnCl2 catalyst; or
• mix with PCl5; or
• reflux with SOCl2
• reflux with NaBr + conc. H2SO4; or• reflux with red P + Br2
• reflux with NaI +
conc. H3PO4; or
• reflux with red P + I2
CH3COOH +conc. H2SO4*
• K2Cr2O
7 / H3O +, reflux; or
• KMnO4 / H
3O +, reflux
K2Cr2O7
/ H3O+
distil off th
e propanal • excess conc. H2SO4, 180 °C; or • Al2O3, 300 °C
C3H7Cl1-chloropropane
C3H7Br1-bromopropane
C3H7I1-iodopropane
CH3CH CH2
propene
CH3COOC3H7
propyl ethanoate
CH3CH2CHOpropanal
CH3CH2COOHpropanoic acid
C3H7OHpropan-1-ol
125
Unit 31 Typical reactions of selected functional groups
Solution
a) The product is a carboxylic acid. It is formed from the oxidation of a primary alcohol.
b) The product is a ketone. It is formed from the oxidation of a secondary alcohol.
31.10 Converting an alkene to an alcohol
Propene can be converted into propan-2-ol by the following route:
a) Give the reagents and conditions for reaction 1.
b) Name compound A.
c) The propan-2-ol can be converted back to propene.
Choose the type of reaction that is involved from the following terms:
addition elimination reduction substitution
d) Reaction 2 is a slow reaction. Give the reagent(s) and condition(s) for reaction 2.
CH3
OHH C
H
H
C
H
H
C
H
H
C
H
H
C
H
C
O
2-methylhexanoic acid
K2Cr2O7 / H3O+
reflux
OHCH3
H C
H
H
C
H
H
C
H
H
C
H
H
C
H
C
H
H
2-methylhexan-1-ol
K2Cr2O7 / H3O+
refluxcyclopentanol cyclopentanone
OHH
O
Br
CH3CH3 CH3CH3
OH
C
H
propan-2-ol
reaction 2C
H
compound A
reaction 1propene
126
Topic 8 Chemistry of Carbon Compounds
Solution
a) React with hydrogen bromide at room temperature.
b) 2-bromopropane
c) Elimination reaction
d) Reflux with sodium hydroxide solution.
31.11 Deducing structures of compounds
Suggest a possible structure for each of the compounds X, Y and Z. Explain briefly your deductions.
a) X (C4H6O) gives Y (C4H6O2) upon oxidation.
b) Z (C4H10O) can exist as a pair of enantiomers, and reacts with phosphorus pentachloride to give hydrogen chloride.
Solution
a) The oxidation of X adds one more oxygen atom to each molecule of X. Thus, the reaction is an oxidation of an aldehyde to carboxylic acid.
or
X
C
O
HC C
HH
H
CH
H
Y
C
O
OHC C
HH
H
CH
H
X
C
O
HC C
HH
H
CH
H
Y
C
O
OHC C
HH
H
CH
H
127
Unit 31 Typical reactions of selected functional groups
b) Z contains an –OH group as it reacts with phosphorus pentachloride to give hydrogen chloride.
The chiral carbon is marked above. Z has a chiral carbon so that it can exist as a pair of enantiomers.
PCl5 C CC C
HH
H
H H H H
Cl H
H HCl+C* CC C
HH
H
H H H H
OH H
H
1 Give the carbon compounds obtained from butan-1-ol in the following reaction scheme.
2 What alcohol would give each of the following products on oxidation?
a) b)
3 Explain why each of the following methods of preparation are NOT appropriate. In each case, suggest an appropriate method for the preparation.
a) Prepare CH3CH2I by reacting CH3CH2OH with a mixture of NaI(s) and concentrated H2SO4.
b) Prepare CH3CH2CHO by heating CH3CH2CH2OH with acidified Na2Cr2O7(aq) under reflux.
A
C
D
conc. H2SO4
heat
reflux with red P + Br2
K2Cr2O7 / H3O+
reflux
CH3CH2CH2CH2OH
BHBr
CH3
CH3CHCHOCH3
C
O
128
Topic 8 Chemistry of Carbon Compounds
Principles and applications of an alcohol breathalyzer*
Task
To know whether a driver has committed drink driving, a policeman can test the breath of a driver on the spot using a breathalyzer.
The oxidation of ethanol forms the basis of a breathalyzer. Search for information about the principles and applications of an alcohol breathalyzer. Then write a short essay of not more than 1 000 words on the topic.
Reference websites
1 You may start your search from the following websites:
• Essay titled ‘How Breathalyzers Work’ http://www.howstuffworks.com/breathalyzer3.htm
2 Extend your search using search engines and appropriate keywords.
• http://www.metacrawler.com • http://www.google.com • http://www.yahoo.com • http://www.altavista.com
breathalyzer 呼氣分析儀
We will discuss this further in Topic 16 Analytical Chemistry.
31.9 Reactions of aldehydes and ketones
Many reactions of aldehydes and ketones are similar because they
both contain the carbonyl group C
O
. However, they differ in their reactions with oxidizing and reducing agents.
129
Unit 31 Typical reactions of selected functional groups
Oxidation of aldehydes and ketones
Aldehydes are easily oxidized to yield carboxylic acids, but ketones are generally inert towards oxidation. This difference is a consequence of structure: aldehydes contain a hydrogen atom next to the carbonyl group. This hydrogen atom is readily oxidized to –OH. Hence aldehydes are readily oxidized.
Aldehydes can be oxidized by acidified potassium dichromate solution. For example, when we warm acidified potassium dichromate solution with ethanal, ethanal is oxidized to ethanoic acid. The solution changes from orange to green as the dichromate ions are reduced to chromium(III) ions.
On the other hand, ketones are not oxidized by acidified potassium dichromate solution.
Reduction of aldehydes and ketones
Reduction of the molecule of a carbon compound usually corresponds to increasing its hydrogen content or decreasing its oxygen content. For example, converting a carboxylic acid to an aldehyde is a reduction because the oxygen content of the carboxylic acid molecule is decreased.
CR
O
H
an aldehyde
hydrogen atom here
C
O
R OH
a carboxylic acid
[O]R1CR
O
a ketone
alkyl or aryl group here
CH3 CH3
K2Cr2O7 / H3O+
O
C C
O
OHheat
H + [O]
O
C
O
C R H[H]*
reductionR OH
a carboxylic acid an aldehyde
oxygen content decreases
Fig. 31.13 Left: Acidified potassium dichromate solution
Right: After the oxidation of ethanal, the solution becomes green
We use [H] to represent a reducing agent.
130
Topic 8 Chemistry of Carbon Compounds
Converting an aldehyde to an alcohol is also a reduction.
OH
H
HC
O
C RreductionR H
[H]
an alcohol
an aldehyde
hydrogen content increases
Sometimes, when devising routes of synthesis of carbon compounds, it is necessary to reduce aldehydes and ketones back to alcohols. This requires a powerful reducing agent. Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols.
OH OH
R1
R1R C
O
H[H]
an aldehyde
R C
H
H
a primary alcohol
R C
O
[H]
a ketone
R C H
a secondary alcohol
In the laboratory, reduction is usually effected by the use of a metal hydride. Lithium tetrahydridoaluminate (lithium aluminium hydride) (LiAlH4)* is used in anhydrous ethoxyethane*. The reaction with aldehydes is very vigorous.
For example,
CH3CH2CH2CH
O
CH3CH2CH2CH
OH
H
butan-1-ol(a primary alcohol)
butanal(an aldehyde)
1 LiAlH4 / ethoxyethane
2 H3O+
CH3CH2CH2CCH3
O
CH3CH2CH2CCH3
OH
H
pentan-2-ol(a secondary alcohol)
pentan-2-one(a ketone)
1 LiAlH4 / ethoxyethane
2 H3O+
Reduction of aldehydes and ketones can also be carried out with milder reducing agents such as sodium tetrahydridoborate (sodium borohydride) (NaBH4).
Lithium tetrahydridoaluminate reacts violently with water. Therefore reductions with this reagent must be carried out in anhydrous condition.
131
Unit 31 Typical reactions of selected functional groups
The reduction reactions of aldehydes and ketones are essentially the reverse of the oxidation reactions of primary and secondary alcohols.
The hydride reagent LiAlH4 does not affect carbon-carbon double bonds. Hence it can convert unsaturated aldehydes and ketones into unsaturated alcohols.
For example,
O OHH
cyclohex-2-enol(a secondary alcohol)
cyclohex-2-enone(a ketone)
1 LiAlH4 / ethoxyethane
2 H3O+
31.12 Question concerning three compounds of the same molecular formula
Compound X, Y and Z have the same molecular formula, C4H8O.
a) Suggest how X can be converted into Y. Give the reagent(s) used for each step and the structure of the intermediate compound(s).
b) Suggest a chemical test to distinguish between Y and Z.
Solution
a)
b) Warm Y and Z with acidified potassium dichromate solution separately. Y is oxidized by the dichromate solution. The solution changes from orange to green as the dichromate ions are reduced to chromium(III) ions. Z shows a negative result.
CH2 CHCH2CH2OH CH3CH2CCH3CH3CH2CH2CHO
O
ZYX
H2 / PtCHCH2CH2OHCH2 CH3CH2CH2CH2OH
K2Cr2O7 / H3O+
distil off the productCH3CH2CH2CHO
132
Topic 8 Chemistry of Carbon Compounds
1 Compound A can be converted into two different carbon compounds as shown below. Give the structural formulae of the new compounds B and C.
2 Draw the structural formulae of compounds X and Y in the following synthesis.
3 For each of the following reactions, suggest a possible reactant.
a)
b)
c)
CH3CH2CHO
A
B
C
1 LiAlH4 / ethoxyethane2 H3O
+
K2Cr2O7 / H3O+
heat
CHCH3CH3CH2CCH1 LiAlH4 / ethoxyethane
2 H3O+
X YH2
Pt catalyst
O
A CH2Clconc. HCl
ZnCl2 catalyst
CH2CH3 CH2CH3+Bconc. H2SO4
heat
OH
CH3
1 LiAlH4 / ethoxyethane
2 H3O+
C
133
Unit 31 Typical reactions of selected functional groups
A condensation reaction is a reaction in which two or more molecules react together to form a larger molecule with the elimination of a small molecule such as water.
31.10 Reactions of carboxylic acids
Esterification
Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid to form esters. The acid and alcohol are boiled under reflux with the concentrated acid.
R1 +R1
+
C O H + H O C O
+
R R
O O
alcoholcarboxylic acid ester water
H2O
Because water is produced, just like when we breathe on a cold surface, the process is named ‘condensation’.
The reversible reaction of a carboxylic acid with an alcohol to form an ester through a condensation reaction is known as esterification. Esterification reactions are acid catalyzed. They proceed very slowly in the absence of strong acids.
For example,
The trivial name for ethyl ethanoate is ethyl acetate.
condensation reaction 縮合反應 esterification 酯化作用
conc. H2SO4
CH2CH3 + H2OCH2CH3CH3 CH3
ethanoic acid
C O H + H O C O
ethanol ethyl ethanoate*
O O
CH3 + H2OCH3
conc. H2SO4O H + H
benzoic acid
C
O
O O
methanol methyl benzoate
C
O
134
Topic 8 Chemistry of Carbon Compounds
Fig. 31.14 shows an experiment* in which ethanol and ethanoic acid are warmed with concentrated sulphuric acid as a catalyst. An insoluble layer of ethyl ethanoate forms on the water.
An ester is also formed by heating an alkanol with an alkanoic acid and concentrated sulphuric acid under reflux.
Fig. 31.14 Ethyl ethanoate is formed when ethanol and ethanoic acid undergo esterification
In the following activity, you are going to study the reaction between ethanol and ethanoic acid in the presence of a catalyst.
Studying the reaction between ethanol and ethanoic acid.
31.2
Uses of esters
Esters have pleasant, sweet and fruity smells. The flavours and fragrances of many fruits and flowers are due to a mixture of natural esters.
Esters have many uses.
1 Perfumes, cosmetics and the artificial flavourings present in foods and drinks contain esters made by chemists.
2 Esters are good solvents for many carbon compounds. For example, nail varnish remover and whiteboard marker pens may contain ethyl ethanoate as a solvent.
Fig. 31.15 The fragrances of many flowers are due to a mixture of natural esters
artificial flavouring 人造香料
135
Unit 31 Typical reactions of selected functional groups
For example,
CH3
CH3
CH3
2 H3O+
1 LiAlH4 / ethoxyethane
methylpropanoic acid methylpropan-1-ol
CH2OHCH3C COOH C
H H
COOH
benzoic acid
1 LiAlH4 / ethoxyethane
2 H3O+
CH2OH
phenylmethanol
2 H3O+
1 LiAlH4 / ethoxyethaneC
O
OHR CR OH
H
H
Reduction
Reduction of carboxylic acids can be accomplished with the powerful reducing agent lithium tetrahydridoaluminate (LiAlH4). It reduces carboxylic acids to primary alcohols in excellent yields.
Fig. 31.17 Nail varnish remover and food flavouring contain esters
Fig. 31.16 Chewing gum contains esters as flavourings
136
Topic 8 Chemistry of Carbon Compounds
Amides from carboxylic acids
Carboxylic acids react with aqueous ammonia to form ammonium salts.
an ammonium carboxylate
O– NH4+CR
O
OH + NH3CR
O
If we evaporate the water and subsequently heat the dry salt, dehydration produces an amide.
an amide
O– NH4+CR
O
NH2 + H2OCR
O
heat
2CH3COH(l) + (NH4)2CO3(s)
O
2CH3CO– NH4+(s) + CO2(g) + H2O(l)
O
We can also produce ethanamide from ethanoic acid by adding solid ammonium carbonate to an excess of concentrated ethanoic acid.
When the reaction is complete, the mixture is heated under reflux. The ammonium salt dehydrates and produces ethanamide.
The excess ethanoic acid is to prevent the dissociation of the ammonium salt* before it dehydrates.
The mixture is distilled at about 170 °C to remove the excess ethanoic acid and water — leaving almost pure ethanamide in the flask.
O O
CH3CO– NH4+(s) CH3CNH2(s) + H2O(l)
ethanamide
The ammonium salt tends to split into ammonia and the parent acid on heating, and recombining on cooling.
CH3COONH4(s) CH3COOH(l) + NH3(g)
The presence of the excess ethanoic acid helps to prevent this from happening by shifting the position of equilibrium to the left.
We will discuss chemical equilibrium in Topic 11 Chemical Equilibrium.
137
Unit 31 Typical reactions of selected functional groups
31.13 Deducing structural formulae of alcohols and acids from which esters are derived
Esters have strong sweet smells which are often floral or fruity. Two examples are given below.
a) Write down the names of the alcohols and carboxylic acids from which they are derived.
b) Draw the structural formulae of these alcohols, carboxylic acids and esters.
Solution
Ester Fragrance
Ethyl 2-methylbutanoate pear
Phenylmethyl ethanoate jasmine
Ester Alcohol Carboxylic acid
Ethyl 2-methylbutanoate
CH3
CH3CH2CH CH2CH3C
O
O
ethanol
OHH C
H
H
C
H
H
2-methylbutanoic acid
OH
CH3
CH3CH2CH C
O
Phenylmethyl ethanoate
CH2 CH3O
O
C
phenylmethanol
OHCH2
ethanoic acid
OHCH3 C
O
138
Topic 8 Chemistry of Carbon Compounds
31.14 Predicting products of reactions of a compound
The structure of compound A is shown below:
From the structure of the compound, predict the reaction that occurs when A reacts with each of the following reagents.
a) Br2 (in organic solvent);
b) H2 (in the presence of a Pt catalyst); and
c) CH3COOH (in the presence of a concentrated H2SO4 catalyst).
Draw the structural formula of the product in each case.
Solution
There are two functional groups in compound A: the –OH group and the C=C bond.
a) The reaction with bromine results in the addition of bromine to the C=C bond.
b) The reaction with hydrogen also results in the addition of hydrogen to the C=C bond.
c) The ethanoic acid reacts with the –OH group to form an ester and water.
compound A
CH3
H3C
HO
C8H17
CH3
H3C
HOBr
Br
C8H17
CH3
H3C
HO
C8H17
H3C C
O
O
CH3
H3C
C8H17
139
Unit 31 Typical reactions of selected functional groups
1 Write balanced equations for the reactions that occur when the following pairs of compounds are heated under reflux with a few drops of concentrated sulphuric acid as a catalyst.
a) Propan-2-ol and propanoic acid
b) Ethanoic acid and ethane-1,2-diol
2 An ester A is used as a solvent for paints and varnishes. The structural formula of A is shown below.
CH3COOCH(CH3)CH2CH3
Ester A
a) Ester A can be manufactured by heating an alcohol under reflux with ethanoic acid and a catalyst.
i) Suggest a suitable catalyst for this reaction.
ii) Explain why the reaction is carried out under reflux.
b) Draw the structural formula of the alcohol used to make ester A.
c) Apart from being a good solvent, suggest another use of ester A.
3 This question is about a compound X and two of its reactions.
a) When X reacts with ethanol under suitable conditions, ester A is formed. Write a balanced equation for the reaction which occurs.
b) X can be reduced to compound B by LiAlH4 under suitable conditions. Draw the structural formula of B.
CH2OH
COOHethanol
LiAIH4
ester A
compound Bcompound X
140
Topic 8 Chemistry of Carbon Compounds
31.11 Hydrolysis of esters
Esterification is a reversible reaction. Therefore an ester can react with water to produce alcohol and carboxylic acid. This reaction is called hydrolysis of ester.
OHR1R1R C
O
O R C
O
O H +hydrolysis
For example, hydrolysis of methyl ethanoate produces ethanoic acid and methanol. However, the reaction of the ester with water is very slow by itself. A catalyst is needed to speed up the reaction. The catalyst can be either a dilute mineral acid (such as sulphuric acid) or a dilute sodium hydroxide solution.
In acidic solution
OH(aq)H(aq) + CH3CH3 CH3(l) + H2O(l)C O CH3 C
O O
methyl ethanoate ethanoic acid methanol
H3O+
O
In alkaline solution
When dilute sodium hydroxide solution is used, the ethanoic acid formed reacts with it to give a sodium salt, sodium ethanoate.
CH3 H(aq) + NaOH(aq)C O O– Na+(aq) + H2O(l)CH3 C
O O
ethanoic acid sodium ethanoate
The overall reaction thus becomes:
CH3 CH3(l) + NaOH(aq)C O O– Na+(aq) + CH3OH(aq)CH3 C
O O
methyl ethanoate sodium ethanoate methanol
hydrolysis 水解作用
141
Unit 31 Typical reactions of selected functional groups
Salt formation removes the ethanoic acid from the reaction mixture. The hydrolysis reaction therefore goes to completion and so alkaline hydrolysis is usually preferred.
In the laboratory, first heat a mixture of an ester and an aqueous alkali under reflux (Fig. 31.18a). Then separate the products by fractional distillation (Fig. 31.18b). The alcohol is obtained as distillate. The salt of carboxylic acid remains in the solution as it is less volatile. We can obtain the free carboxylic acid by adding excess mineral acid. For example:
Fig. 31.18a Alkaline hydrolysis of an ester in the laboratory
CH3COO–(aq) + H+(aq) CH3COOH(aq)
ethanoate ion(from salt)
hydrogen ion(from mineral acid)
ethanoic acid
Fig. 31.18b Separating alcohol from the products formed in the hydrolysis of ester
142
Topic 8 Chemistry of Carbon Compounds
Heat the amide under reflux with an aqueous acid or alkali. Moderately concentrated solution of the acid or alkali is usually used.
The products depend on whether an acid or alkali is used in the hydrolysis. If an acid is used in the hydrolysis, the product contains ammonium ions. If an alkali is used, the carboxylic acid loses H+ ions and the product contains carboxylate ions.
For example, when boiling hydrochloric acid is used for hydrolysis, ethanamide forms ethanoic acid and ammonium chloride.
31.12 Hydrolysis of amides
Just as esters, the reaction of an amide with water is extremely slow, but the reaction can be catalyzed by an acid or an alkali. In the presence of a strong acid or alkali, amides are hydrolyzed to carboxylic acids or their salts respectively.
In acidic solution
NH2 + H2O + H+CR
O
OH + NH4+CR
O
In alkaline solution
NH2 + OH–CR
O
O– + NH3CR
O
CH3COOH(aq) + NH4Cl(aq)CH3CONH2(aq) + H2O(l) + HCl(aq)heat
When ethanamide is refluxed with sodium hydroxide solution, ammonia gas is given off and you are left with a solution containing sodium ethanoate.
CH3COO– Na+(aq) + NH3(g)CH3CONH2(aq) + NaOH(aq)heat
We can obtain ethanoic acid by adding excess mineral acid to the solution containing sodium ethanoate.
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Unit 31 Typical reactions of selected functional groups
31.15 Predicting the hydrolysis products of a compound
The structure of compound X is shown below:
a) How many functional groups are there in X? Name all of these functional groups.
b) Write the structures of carbon compounds formed when X is heated with excess sodium hydroxide solution.
Solution
a) There are 4 functional groups in X:
• carbon-carbon double bond;
• amine group;
• amide group; and
• ester group.
b)
CH3COO– CH3CH2OH
CH3
NH
NH2
C CH2CH3
O O
C
O
O–
NH2
NH2
C
O
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Topic 8 Chemistry of Carbon Compounds
31.16 Distinguishing different compounds using test tube reactions
Describe, by giving reagent(s) and stating observations, how you could distinguish between compounds in each pair using a simple test tube reaction.
a) CH3CH2CH2OH and CH3CH2CHO A B
b)
and
C D
c) CH3CONH2 and CH3COOH E F
Solution
a) Warm each compound with ethanoic acid in the presence of concentrated sulphuric acid.
A reacts with ethanoic acid to form an ester with a sweet smell while there is no observable change for B.
b) Add aqueous bromine to each compound.
D decolorizes the aqueous bromine immediately while there is no observable change for C.
c) Add sodium hydroxide solution to each compound and heat.
E gives a gas that turns moist red litmus paper blue (ammonia) while there is no gas evolved for F.
Another possible test is to add sodium hydrogen carbonate solution to each compound. F gives a gas that turns limewater milky (carbon dioxide) while there is no gas evolved for E.
CH3
CH3
CH3
CH3
C C
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Unit 31 Typical reactions of selected functional groups
1 Draw the structural formulae of A, B, C and D in the following reactions.
a)
b)
c)
2 The structure of compound A is shown below:
Draw structural formulae to show the two carbon compounds formed by the alkaline hydrolysis of A.
C
O
OHCH3CH2 NH3+ A B H2O+heat
C
O
NH2CH3(CH2)16 NaOH+ C NH3+
C
O
NH2CH2 H2O+ D NH4Cl+HCl+
CH2
CH3
CH3
H
C
O
O
compound A
C C
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Topic 8 Chemistry of Carbon Compounds
146
1 Reactions of alkanes
a) Combustion
CH4 + 2O2 CO2 + 2H2O
b) Substitution reactions with halogens
Cl2 CH4 CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl UV light or heat
2 Addition reactions of alkenes
CH3
CH3
CH3
CH3
CH3
CH3
H C
H
H
C
H
CH3 hydrogenation
H C
H
X
C
X
CH3
H C
H
H
C
X
CH3
major product
H+ C
H
X
C
H
CH3
Markovnikov’s rule is followed
minor product
H2 / Pt catalyst
X2
X2 = Cl2(g) orBr2 (in organic solvent)
HX
X = Cl, Br or I
C
H
H
C
3 Markovnikov’s rule for addition reaction of an asymmetric alkene:
When a molecule HA adds to an asymmetric alkene, the major product is the one in which the hydrogen atom attaches itself to the carbon atom already carrying the larger number of hydrogen atoms.
4 Substitution reactions of haloalkanes — alkaline hydrolysis of haloalkanes
R OHR XNaOH(aq)
reflux
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Unit 31 Typical reactions of selected functional groups
147
5 Substitution reactions of alcohols with halides
a) Order of reactivity of alcohols is 3° > 2° > 1°
b) Reactions with halidesH
H C
H
H
C
H
Cl
H
H C
H
H
C
H
Br
H
H C
H
H
C
H
OH
H
H C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
I
NaBr, conc. H2SO4
reflux
conc. HCl
ZnCl2 catalyst, reflux
Nal, conc. H3PO4
reflux
c) Reactions with phosphorus halidesH
H C
H
H
C
H
Cl
H
H C
H
H
C
H
Br
H
H C
H
H
C
H
OH
H
H C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
I
red P + Br2
reflux
PCl5
room temperature
red P + I2
reflux
d) Reactions with sulphur dichloride oxide
H
H C
H
H
C
H
Cl
H
H C
H
H
C
H
OH C
H
H
C
H
H
SOCl2
reflux
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Topic 8 Chemistry of Carbon Compounds
148
6 Dehydration of alcohols to form alkenes
CH3
CH3
CH3
CH3
CH3
CH3
CH3
OH
H
H C
H
H
C
H
OHexcess conc. H2SO4
180 °C
(1° alcohol)
C
H
H
C
H
H
H
H C
H
H
Cconc. H3PO4
165 – 170 °C
(2° alcohol)
C
H
H
C
H
C OH20% H2SO4
85 °C
(3° alcohol)
C
H
H
C
mild
er cond
itions
7 Oxidation of alcohols
OH
CH3
CH3
CH3
H
OH
H
H C
H
H
C
H
OHK2Cr2O7 / H3O
+
gentle heat, distil
(1° alcohol)
H
H C
H
H
C
H
H
CK2Cr2O7 / H3O
+
reflux
(2° alcohol)
C OH no reactionK2Cr2O7 / H3O
+
reflux
(3° alcohol)
H C
H
H
C
O
Hreflux
further oxidationH C
H
H
C
O
H C
H
H
C
O
C
H
H
H
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Unit 31 Typical reactions of selected functional groups
149
8 Oxidation and reduction of aldehydes and ketones
a) Oxidation of aldehydes
OHK2Cr2O7 / H3O
+
heatR C
O
R C
O
H
R1
R1
OH
R C H
2° alcohol
OH
R C
H
H
1° alcohol
ketone
aldehyde
1 LiAlH4 / ethoxyethane
2 H3O+
1 LiAlH4 / ethoxyethane
2 H3O+
R C
O
H
R C
O
b) Reduction of aldehydes and ketones
9 Reactions of carboxylic acids
a) Esterification
b) Reduction
c) Amide synthesis
H2OR1OR1H+ OOHconc. H2SO4
R C
O
+R C
O
OHOHR C
O
R C
H
H
1 LiAlH4 / ethoxyethane
2 H3O+
O– NH4+NH3+ H2O+OHR C
O
R C
O
NH2R C
O
heat
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Topic 8 Chemistry of Carbon Compounds
150
10 Hydrolysis of esters
a) Acid hydrolysis
b) Alkaline hydrolysis
R1 OHO H +R C
O
H3O+
H2OR1O +R C
O
O– Na+ R1 OH+R C
O
NaOHR1O +R C
O
11 Hydrolysis of amides
a) Acid hydrolysis
b) Alkaline hydrolysis
NH2 OH NH4++R C
O
H2O+ H++R C
O
NH2 OH–+R C
O
O– NH3+R C
O
12 The following table summarizes typical reactions of members of some homologous series.
Homologous series
Reaction Reagent(s) and condition(s) Products
Alkanes
combustion good supply of oxygen carbon dioxide and water
substitution reactions with
halogens
halogens
UV light or heathaloalkanes
Continued on next page
151
Unit 31 Typical reactions of selected functional groups
151
Homologous series
Reaction Reagent(s) and condition(s) Products
Alkenes
hydrogenationhydrogen
Pt as catalystalkanes
addition of halogens
chlorine or bromine haloalkanes
addition of hydrogen
halidesHCl / HBr / HI haloalkanes
Haloalkanes hydrolysis reflux with NaOH(aq) alcohols
Alcohols
substitution reactions with
halides
reflux with conc. HCl + ZnCl2 catalyst; or mix with PCl5; or reflux with SOCl2
chloroalkanes
reflux with NaBr + conc. H2SO4; or reflux with red P + Br2
bromoalkanes
reflux with NaI + conc. H3PO4; or reflux with red P + I2
iodoalkanes
dehydration
excess conc. H2SO4, 180 °C for 1° alcohols;
conc. H3PO4, 165 – 170 °C for 2° alcohols;
20% H2SO4 85 °C for 3° alcohols
alkenes
oxidation K2Cr2O7 / H3O+
1° alcohol aldehyde carboxylic acid
2° alcohol ketone
3° alcohol no reaction
Aldehydesand ketones
oxidation K2Cr2O7 / H3O+ aldehyde carboxylic acid
reduction1 LiAlH4 / ethoxyethane
2 H3O+
aldehyde 1° alcohol
ketone 2° alcohol
Carboxylic acids
esterification conc. H2SO4 as catalyst esters
reduction1 LiAlH4 / ethoxyethane
2 H3O+
alcohols
amide synthesis
reaction with NH3(aq), heat amides
Esters hydrolysis reflux with acid or alkalialcohols and carboxylic acids
(or their salts)
Amides hydrolysis reflux with acid or alkalicarboxylic acids(or their salts)
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Topic 8 Chemistry of Carbon Compounds
152
Note: The symbol indicates the level of difficulty of a question.
Part I Knowledge and understanding
1 Complete the flow chart that includes reactions among the following carbon compounds: alkane, alkene, haloalkane, alcohol, aldehyde, ketone, carboxylic acid, ester and amide.
alkene
alcohol
aldehyde
amide
ester
R: H2 / Pt catalystT: addition
Key:R ReagentT Reaction type
R:
T:
R: HCl / HBr / HI
T:
R: HCl / HBr / HI
T:
R: OH–(aq)
T:
R:
T:
R:
T:
R: K2Cr2O7 / H3O+
T:
then to
R:
T:
R: K2Cr2O7 / H3O+
T:
R:
T:
R:
T:
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Unit 31 Typical reactions of selected functional groups
153
2 From the information given, draw the structural formula of each carbon compound. All of the compounds consist of molecules which have four carbon atoms.
a) A hydrocarbon that rapidly decolorizes aqueous bromine.
b) A compound that is oxidized to a ketone.
c) An ester that is formed from ethanoic acid.
d) An aldehyde that gives butan-1-ol upon reduction.
e) An amide.
3 Draw structural formulae of the products formed from the reactions of but-2-ene.
Br2 (in organic solvent) Br2(aq)
H2 / Ni catalyst MnO4– / OH–
CC
CH3H3C
HH
HBr(g)
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Topic 8 Chemistry of Carbon Compounds
154
4 Draw structural formulae of the products formed from the reactions of propan-1-ol.
NaBr + conc. H2SO4
conc. H2SO4
as catalyst
CH3CH2CH2OH
excess conc. H2SO4HCl / ZnCl2 as catalyst
NaI + conc. H3PO4 K2Cr2O7 / H3O+
then to
Structural type Structural formula of the isomerStructural formula for the
compound (if any) formed by complete oxidation of the alcohol
Primary
Secondary
Tertiary
5 a) A primary alcohol, a secondary alcohol and a tertiary alcohol are isomers with molecular formula C4H9OH. Each was heated under reflux with potassium dichromate in dilute sulphuric acid. Complete the table below.
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Unit 31 Typical reactions of selected functional groups
155
b) Propan-1-ol, CH3CH2CH2OH, can be converted to CH3CH2CH2I using red phosphorus and iodine.
i) Name the compound CH3CH2CH2I.
ii) State the conditions needed to react propan-1-ol with red phosphorus and iodine.
iii) This halogenation of propan-1-ol is brought about by an intermediate compound produced from the reaction between red phosphorus and iodine. Suggest the formula of this intermediate.
(Edexcel Advanced Subsidiary GCE, Unit Test 2, Jun. 2006)
Part II Multiple choice questions
6 What type of reaction occurs when the following compound reacts with sodium hydroxide solution?
CH2CH2Cl
A Addition B Dehydration C Elimination D Substitution
7 Many alcohols are oxidized by warming with acidified potassium dichromate solution.
Which of the following alcohols resists this oxidation?
A B
C
C OH
CH3
CH3
CH3
D
C OH
CH3
H
CH3CH2
CH2OHCH2OH
C H
CH3
CH2OH
CH3 C H
CH3
CH2OH
CH3
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Topic 8 Chemistry of Carbon Compounds
156
8 Refer to the reaction scheme shown below.
Step 1 Step 2 CH3CH2CH2CHO CH3CH2CH2CH2OH CH3CH2CH2CH2Br
Which of the following reagents can bring about the reaction indicated?
Step 1 Step 2
A LiAlH4 Br2(aq) B LiAlH4 NaBr / conc. H2SO4
C K2Cr2O7 / H3O+ Br2(aq)
D K2Cr2O7 / H3O+ NaBr / conc. H2SO4
9 An alcohol and a carboxylic acid are heated in the presence of concentrated sulphuric acid under reflux. The following compound is obtained.
C O
H
H
CH
H
C H
H
C
HH
H H
O
C
Which of the following combinations is correct?
Alcohol Carboxylic acid
A Ethanol ethanoic acid B Ethanol propanoic acid C Propanol ethanoic acid D Propanol propanoic acid
10 The compound CH3CH(OH)CH2COOCH3 is found in marshmallows. Which of the following statements concerning the compound are correct?
(1) Its systematic name is ethyl 2-hydroxybutanoate. (2) It has one chiral carbon. (3) It turns warm acidified K2Cr2O7 (aq) from orange to green.
A (1) and (2) only B (1) and (3) only C (2) and (3) only D (1), (2) and (3)
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Unit 31 Typical reactions of selected functional groups
157
Part III Structured questions
11 Give the structural formula(e) of the major product(s) expected from each of the following reactions.
CH2CH2OH
Na2Cr2O7 / H3O+
heat
COOHH3C1 LiAIH4 / ethoxyethane
2 H3O+
CH3CH2 CH2CH3OC
O
NaOH(aq)
CH3
NaOH(aq)NH2C
O
CH CHCH
O
1 LiAIH4 / ethoxyethane
2 H3O+
CH3
Cl2
UV light
OCCH3
O
COOH
OH–(aq)
heat
CH3CH2CH2OHexcess conc. H2SO4
180 °C
a)
b)
c)
d)
e)
f)
g)
h)
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Topic 8 Chemistry of Carbon Compounds
158
12 Hydrocarbon G reacts with HBr to give J as the major product.
CC
CH2CH3H3CH2C
HH3CG
+ HBr J
Give the structure of J and its systematic name. (HKALE 2004)
13 Consider the following reaction of 1-bromobutane.
C
H
H
C
H
H
C
H
H
H C Br
H
H
OH–(aq)
heatA
a) Draw the structural formula of A.
b) i) State the reagent(s) and condition(s) required to convert A back to 1-bromobutane.
ii) Name the type of reaction that takes place.
c) A student attempted to prepare A using the same reagents and conditions but using 1-fluorobutane in place of 1-bromobutane.
Suggest why the reaction would proceed at a different rate.
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Unit 31 Typical reactions of selected functional groups
159
14 Ethanol, C2H5OH, can be produced by the fermentation of glucose, C6H12O6.
a) Ethanol has a relatively high boiling point. This can be explained in terms of intermolecular hydrogen bonds.
Draw a second molecule of ethanol alongside the one drawn below and show how a hydrogen bond could be formed. Clearly show any relevant dipoles and lone pairs of electrons.
O
CH2CH3
H
b) When ethanol is heated with acidified potassium dichromate solution, it can be oxidized to form either ethanal, CH3CHO (Fig. A), or ethanoic acid, CH3COOH (Fig. B).
The boiling points of ethanol, ethanal and ethanoic acid are given in the table below.
CH3CH2OH CH3CHO CH3COOH
Boiling point (°C) 78 21 118
Use this table of boiling points to explain
i) why the organic product is likely to be ethanal if the apparatus shown in Fig. A is used;
ii) why the organic product is likely to be ethanoic acid if the apparatus shown in Fig. B is used.
c) Write a balanced equation for the oxidation of ethanol to ethanoic acid. Use [O] to represent the oxidizing agent.
(OCR Advanced Subsidiary GCE, Chains and Rings, Jan. 2005)
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Topic 8 Chemistry of Carbon Compounds
160
15 X, Y and Z are carbon compounds. The flow diagram below shows the conversion of X to Z.
acidified K2Cr2O7(aq)
heatconc. H2SO4
heat
X Y
methanol
Z
a) Z has a pleasant smell and its molecular formula is C4H8O2. Draw the structure of Z.
b) To which homologous series does Y belong?
c) Give the systematic name of X.
d) State the expected observation when X reacts with acidified potassium dichromate solution.
e) State the function of concentrated sulphuric acid in the reaction of Y with methanol. (HKCEE 2006)
16 The following reaction scheme shows some of the reactions of butan-2-ol.
solid XCH3CH(OH)CH2CH3
butan-2-oI CH2 CHCH2CH3
A
B
Na2Cr2O7(aq) and H2SO4(aq)
a) Why is butan-2-ol classified as a secondary alcohol?
b) Compound A can be prepared from butan-2-ol by passing its vapour over a heated solid, X.
i) Give the name of the carbon compound A.
ii) Name the solid X.
iii) What type of reaction is taking place?
iv) Draw a labelled diagram of the apparatus you would use to prepare and collect gas A from butan-2-ol.
c) Give the structural formula and the name of compound B.
d) Butan-2-ol can be used to clean plastic materials, such as CDs and DVDs. Suggest ONE precaution which should be taken when using butan-2-ol in this way.
(Edexcel GCE (Nuffield) Unit Test 1, Jun. 2005)
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Unit 31 Typical reactions of selected functional groups
161
17 Ethanol, C2H5OH, can be converted into ethanal, CH3CHO, if it is heated with an acid and sodium dichromate solution, provided that the ethanal is immediately distilled off.
A possible arrangement of apparatus for this experiment is shown below. However, it is incompletely labelled and the diagram contains some errors. You may assume that the apparatus is correctly clamped.
a) What are the names of the three items labelled A, B and C?
b) List THREE errors in this diagram.
c) Which acid should be used?
d) What type of reaction is the conversion of ethanol to ethanal? Justify your answer by considering their formulae.
e) i) What is the formula of the dichromate ion in sodium dichromate?
ii) What colour change would you expect to see as the reaction proceeded?
f) If the mixture is refluxed first before being distilled, what is the name and formula of the carbon compound formed?
(Edexcel Advanced Subsidiary GCE (Nuffield), Unit Test 1, Jan. 2007)
162
Topic 8 Chemistry of Carbon Compounds
162
18 Geraniol (C9H15CH2OH) is a naturally occurring compound that contributes to the smell of roses. The skeletal formula of geraniol is shown below.
OH
a) Mild oxidation of geraniol gives an aldehyde X.
i) Draw the skeletal formula of aldehyde X.
ii) Complete the equation for the oxidation of geraniol to aldehyde X.
C9H15CH2OH + [O]
b) Reaction of geraniol with ethanoic acid can be used to make ester Y, which is used in chewing gum and desserts.
i) Suggest why esters are used in the manufacture of foods.
ii) State the conditions needed to make ester Y from geraniol and ethanoic acid.
iii) Complete the equation for the formation of ester Y.
+ C9H15CH2OH
(OCR Advanced GCE, Chains, Rings and Spectroscopy, Jan. 2005)
19 a) A carbon compound, W, with molecular formula C4H10O reacts with phosphorus pentachloride to give compound X, C4H9Cl.
When W is heated with potassium dichromate solution and dilute sulphuric acid there is no colour change.
i) Identify the functional group present in W.
ii) Draw the structural formulae of W and X.
iii) When a structural isomer of W is heated under reflux with acidified potassium dichromate solution, it produces compound Y, C4H8O2.
Suggest a possible identity for Y.
b) Propene, C3H6, reacts with hydrogen bromide, HBr.
Draw the structures of the two possible products and indicate which is the major product.
(Edexcel Advanced Subsidiary GCE, Unit Test 2, Jan. 2007)
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Unit 31 Typical reactions of selected functional groups
163
20 In a certain experiment, a mixture of methyl propanoate and sodium hydroxide solution was heated under reflux for some time. The resulting mixture M was then transferred to flask Y and heated as shown below.
a) i) Write a chemical equation for the reaction between methyl propanoate and sodium hydroxide solution.
ii) Name the type of reaction taking place.
b) i) Name apparatus X.
ii) What was the function of apparatus X?
c) Name the distillate collected in flask Z.
d) What was the purpose of using anti-bumping granules?
e) Which one of the following hazard warning labels should be displayed on a bottle of methyl propanoate?
A B C D
f) Draw the structure of another ester which has the same molecular formula as methyl propanoate, and give its systematic name.
164
Topic 8 Chemistry of Carbon Compounds
164
21 Describe, by giving reagent(s) and stating observations, how you could distinguish between compounds in each pair using a simple test tube reaction.
a)
CH2CH2CHO CCH2CH3
O
and
b)
CH3CCH3
Oand CH2=CHCH2OH
c) CH3CH2COOH and CH3COCH2OH
22 Compound A is used to add the flavour of mushrooms to foods.
CH2
CC
OCH3C
O
CH3H
compound A
a) i) Apart from the benzene ring, name the TWO functional groups in compound A.
ii) Draw the skeletal formula of compound A.
iii) Deduce the molecular formula of compound A.
b) Compound B is a stereoisomer of compound A.
Explain what is meant by the term ‘stereoisomerism’. Use compounds A and B to illustrate your answer.
c) If the food is cooked for a long time, naturally occurring acids catalyze the hydrolysis of compound A.
Draw structures to show the TWO carbon compounds formed by the acid hydrolysis of compound A.
(OCR Advanced GCE, Chains, Rings and Spectroscopy, Jan. 2006)
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Unit 31 Typical reactions of selected functional groups
165
23 Oseltamivir is an antiviral drug against the avian virus H5N1. It is also known by the brand name Tamiflu®.
a) Mark each chiral centre with an asterisk ‘*’ on the structure of oseltamivir shown below.
CH2CH3
CH3
CH3CH2
CH2CH3
CHNH2
C
NH
O
OC
OO
oseltamivir
b) Besides the ether linkage, how many functional groups are there in oseltamivir? Name TWO of these functional groups.
c) Given that ether linkages are NOT affected by alkalis, write the structure of the products formed when oseltamivir is heated with excess NaOH(aq).
(HKASLE 2007)
24 Compound X is an aldehyde with the molecular formula C5H8O. It is known that X contains the following functional group as well.
CC
a) X shows no geometrical isomerism. Give TWO possible structures of X, neither of which is chiral.
b) Select ONE of your structures for X. This reacts with HBr to form two products, one of which is chiral.
i) Give the structures of the two products, indicating which one is chiral.
ii) Indicate which of the two products is likely to be in a greater yield.
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Topic 8 Chemistry of Carbon Compounds
166
25 Aromatic compounds P, Q and R are esters with the same molecular formula C8H8O2.
a) A mixture of P and aqueous NaOH was heated under reflux for an hour. Excess dilute H2SO4 was then added to the resulting mixture and a white precipitate (C7H6O2) was formed. Suggest the structure of P and write an equation for the reaction of P with aqueous NaOH.
b) A mixture of Q and aqueous NaOH was heated under reflux for an hour. Excess dilute H2SO4 was then added to the resulting mixture. Upon warming, a smell of vinegar was detected. Deduce the structure of Q with the help of chemical equations.
c) Propose one possible structure of R. (HKASLE 2006)
26 The rates of hydrolysis of chloroethane, bromoethane and iodoethane are different.
• Describe how you would monitor the reaction rates.
• Explain why chloroethane, bromoethane and iodoethane react at different rates.
(For this question, you are required to give answers in paragraph form. Use equations, diagrams and examples where appropriate.)
(OCR Advanced Subsidiary GCE, Chains and Rings, Jun. 2005)