ORGANIC CHEMISTRY · Web viewIntroduction It was originally thought that the formation of organic...

ORGANIC CHEMISTRY

IntroductionIt was originally thought that the formation of organic compounds could only be achieved by the influence of a ‘vital force’ which was present in nature, and the word organic applied only to those substances that were produced by living organisms. However, in 1828, Woehler found that when the inorganic compound, ammonium cyanate was heated, the organic compound, urea was obtained.

Woehler

Later, in 1845, Kolbe synthesised ethanoic acid from carbon, hydrogen and oxygen.

Today, the distinction between organic and inorganic chemistry is ill-defined, but usually organic chemistry refers to the chemistry of carbon compounds other than oxides, metal carbonates and related compounds.

The bewildering variety of forms in which the 5 million known carbon compounds exist arises from a relatively small number of structural features, which include:

carbon atoms linked by strong single, double and triple bonds strong bonds to other elements, particularly H, O and N straight carbon chains of varying length branched carbon chains of varying length aliphatic homocyclic rings of varying size aliphatic heterocyclic rings of varying size aromatic homocyclic rings of varying size aromatic heterocyclic rings of varying size functional groups fused ring systems

TOPIC 12.11: ORGANIC CHEMISTRY 1

NH4CNO CO(NH2)2

Some of these structural features are illustrated in the following compounds:

methane cyclohexane

pyrrole

coumarin


C H

H

H H

CH3(CH2)7CH=CH(CH2)7COOH

octadec-9-enoic acid (oleic acid)

O ON

H

CH2.CO.NH C C

C N

OCHCOOH

S CH H

CH3CH3

Penicillin G

cholesterol

carotene

OHO

HOCH2

OH

OHHO H

H

H

H

H

D-glucoseHO


NH

HN

N

N

porphyrinO

OOH

HO

OH

OHOH

quercetin

O

HO

HO

N CH3

morphine

Empirical & Molecular FormulaeMicroanalysis of an organic compound gives the percentage by mass of each element present in the compound. From these data, the empirical formula of the compound can be calculated. (See Topic 12.1: Amount of Substance)

The two are related by the expression:

Molecular formula = Empirical formula x n

where n is a whole number.

For example:The molecular formulae of methane and ethane are CH4 and C2H6 respectively.The empirical formula of methane is CH4; in the above expression n = 1.The empirical formula of ethane is CH3, which is the simplest whole number ratio of carbon to hydrogen atoms. In the above expression n = 2.

Structural Formulae

CH3CH2CH2CH3 is the structural formula of butane whereas

CH3CH(CH3)CH3 is the isomer 2-methylpropane

The displayed formulae of the first few alkanes are shown below. Although shown in two dimensions, these structures are in reality three-dimensional. The formulae shown in brackets are partial structural formulae. Partial structural formulae are acceptable as long as they are unambiguous and can only represent one isomer.


The EMPIRICAL FORMULA is the simplest WHOLE number ratio of the atoms of each element present in a compound.

The MOLECULAR FORMULA is the actual number of atoms of each element present in one molecule of the compound.

H C C H

H H

H H

H C C C H

H H H

H H H

H C H

H

H

The STRUCTURAL FORMULA of a molecule shows the number and type of each atom present and how they are joined together.

The DISPLAYED FORMULA of a molecule shows all how of the atoms are arranged and ALL of the bonds between them.

methane (CH4) ethane (CH3.CH3)

propane (CH3.CH2.CH3)

butane (CH3.CH2.CH2.CH3)

pentane (CH3.CH2.CH2.CH2.CH3)

Skeletal formulaeIt can quickly become very time-consuming to draw displayed formulae and, to a lesser extent, structural formulae. We can use skeletal formulae as an alternative. In this we use straight lines to represent C-C bonds and we assume there is a carbon atom where the lines meet. We don’t draw C-H bonds but they are assumed to be there in full (ie so each C atom has 4 bonds). We do, however, show the presence of all other atoms (including H’s bonded to atoms other than carbon) and also indicate multiple bonds. If other atoms like oxygen are present (see ethanol below) then the covalent bond between the oxygen and the adjacent carbon is drawn.

butane 2-methylpropane ethanol but-1-eneFunctional Groups


H C C C C H

H H H H

H H H H

H C C C C C H

H H H H H

H H H H H

Note that each carbon atom has formed four single bonds and is joined to four other atoms. Each hydrogen atom has formed one bond.

The functional group determines the chemical properties of a compound. Some compounds contain more than one functional group.

C-H alkane-OH hydroxyl group (alcohols)C=O carbonyl group (aldehydes & ketones)C=C alkene-COOH carboxyl group (carboxylic acids)-OR alkoxy (ethers)-COOR ester-NH2 amino (amines)-Cl chloro (haloalkanes)-CONH2 amide-CN nitrile

Homologous SeriesTo simplify the study of the huge number of organic compounds, they are divided into homologous series.

The similarity of chemical properties arises because members of a series contain the same functional group. Since the number of carbon atoms increases steadily down a homologous series, there is a gradual change in physical properties such as boiling point and density. Structural IsomerismFor a given molecular formula, there may be more than one possible structure, giving rise to isomerism.

Structural isomers must have at least one bond in their molecules which is different. Structural isomers are sometimes sub-divided into chain isomers, position isomers and functional group isomers.The number of structural isomers rises rapidly as the number of carbon atoms increases.

Chain IsomersThese isomers differ only in the arrangement of the carbon skeleton of the molecule.


STRUCTURAL ISOMERS are compounds with the same molecular formula but with different structural formulae.

A FUNCTIONAL GROUP is an atom or group of atoms which, when present in different molecules, gives them similar chemical properties.

A HOMOLOGOUS SERIES is a group of compounds with the same general formula and similar chemical properties.

Examples of chain isomers (of C4H10) are butane and methyl propane.

butane methylpropane

For the molecular formula C5H12, there are three chain isomers:

pentane (b.p. 36oC)

2-methylbutane (b.p. 28oC)

2,2-dimethylpropane(b.p. 10oC)

Position IsomersThese isomers have the same carbon skeleton and the same functional group(s) but differ in the position in which the functional group is attached to the carbon skeleton.

Examples of position isomers are propan-1-ol and propan-2-ol.

propan-1-ol propan-2-ol

Functional Group IsomersThese isomers have different functional groups and therefore have different chemical properties.


H C C C C H

H H H H

H H H H

H C C C H

H H H

H CH3 H

H C C C H

H H H

H OH H

H C C C H

H H H

H H OH

H C C C C C H

H H H H H

H H H H H

H C C C C H

H H C H

H H H H

H H H

H C C C H

H C H

H C H

H H H

H H H

Examples of functional group isomers are ethanol and methoxymethane.

ethanol methoxymethane


H C O C H

H H

H H

H C C H

H H

H OH

Stereoisomerism

There are two types of stereoisomerism: geometrical isomerism and optical isomerism. Only geometrical isomerism will be considered in this module.

Geometrical IsomerismGeometrical isomerism arises when there is restricted rotation about a bond, for example around the C=C double bond in alkenes and around C-C single bonds in cycloalkanes.

AlkenesThere is always restricted rotation around a C=C double bond. The double bond is a -orbital formed by the overlap of two 2p-orbitals on the two carbon atoms. If rotation about a double bond were to take place, it would require the -bond to be broken This requires an amount of energy not possessed by molecules at room temperature.

Restricted rotation gives rise to geometrical isomers only if there are two different atoms or groups attached to both of the double bond carbon atoms.

For example, but-2-ene exhibits geometrical isomerism, but but-1-ene does not.

E-but-2-ene Z-but-2-ene

These two molecules are identical apart from their orientation in space. Both of the double bond carbon atoms are attached to two different groups. The prefixes E and Z indicate that the two methyl groups are on opposite sides or on the same side of the double bond respectively.

but-1-eneCycloalkanes


C C

H CH3

CH3 HC C

CH3 CH3

H H

C C

H CH2CH3

H H

twoidentical atoms

STEREOISOMERS have the same structural formula. They have the same number and types of bonds and differ only in their orientation in space.

C C rotation C C-orbital 2p

2p

-bond broken

two different groups

two different

groups

Geometrical isomerism is not possible because one of the double bond carbon atoms is attached to two identical atoms.

There is always restricted rotation around a C-C single bond in a ring, but this gives rise to geometrical isomers only if there are two different atoms or groups attached to at least two different carbon atoms.

For example, 1,2-dimethylcyclopentane exhibits geometrical isomerism but 1,1-dimethylcyclopentane does not.

Z-1,2-dimethylcyclopentane E-1,2-dimethylcyclopentane

1,1-dimethylcyclopentane


CH3

H

HCH3

twoidentical

groups

CH3

HH

CH3

CH3

HH

CH3

twodifferent

groups

twodifferent groups

NomenclatureOrganic compounds are named according to IUPAC rules. Some of the simpler rules are given below:

1. The naming of a compound is based on the longest straight carbon chain present in the molecule, and the first step in naming is to select this longest chain. Where a functional group is present, the longest straight carbon chain containing or attached to the functional group is selected.

2. The chain is named according to the number of carbon atoms and functional groups it contains.

No. of C atoms Prefix Functional group Suffix1 meth- C-H -ane2 eth- C=C -ene3 prop- -OH -ol4 but- C=O -one5 pent- H-C=O -al6 hex- -COOH -oic acid7 hept- -CONH2 -amide8 oct- -COCl -oyl chloride

3. The carbon atoms of this longest chain are numbered from one end to the other, starting from the end which gives the functional group the lower possible number. If there is no functional group, the chain is numbered so as to give the alkyl substituents the lower possible number(s).

4. Carboxyl groups (-COOH), aldehydes –(CHO) and nitriles (-CN) can only ever appear at the end of a chain, i.e. at carbon number 1. This number is usually omitted.

5. The position of the functional group and any chain branches are indicated by the number of the carbon atom to which the functional group or the substituent is attached followed by the name of the functional group or substituent.

Formula Name-CH3 methyl

-CH2CH3 ethyl-CH2CH2CH3 propyl

6. If two or more of the same substituent are present in a molecule, the number of them is indicated by multipliers:

Number of identical substituents Multiplier2 di-3 tri-4 tetra-5 penta-6 hexa-


7. In unsaturated compounds which contain one double bond (alkenes), the double bond is formed between two numbered carbon atoms. The position of the double bond is indicated by the lower of these two numbers.

8. Most names are written as a single word, with commas separating numbers and hyphens separating numbers and letters.

9. When there is more than one functional group present in a molecule, the groups have an order of priority; the more important appear as suffixes, the less important as prefixes. If functional groups appear as prefixes, they have the following names:

Functional group Prefix-Cl chloro-Br bromo-I iodo

-OH hydroxy-OR alkoxyC=O oxo-NH2 amino

10.When there is more than one prefix, the prefixes (ignoring the multiplier) are listed in alphabetical not numerical order. For example, tribromo- appears before dichloro-.

Example 1:

There are four substituent groups attached to this chain:

a methyl group is attached to carbon no. 4 4-methyl

three bromines are attached, at carbons no. 3, 4 and 5 3,4,5-tribromo

The name of this compound is therefore 3,4,5-tribromo-4-methylpentan-2-ol

Note the following: commas between numbershyphens between numbers and letterstribromo is alphabetically before methyl


The longest carbon chain has five carbons.

The chain is numbered to give the most important functional group (OH) the lower possible number.

This gives ......pentan-2-ol

H C C C C C H

H CH3 H OH H

Br Br Br H H5 4 3 2 1

Example 2:

There are three substituent groups attached to this chain:

a methyl group is attached to carbon no. 3 3-methyl

a chlorine atom is attached to carbon no. 3 3-chloro

an ethyl group is attached to carbon no. 2 2-ethyl

The name of this compound is therefore 3-chloro-2-ethyl-3-methylpentanoic acid

Example 3:

A carboxyl group takes precedence over an aldehyde or ketone, which take precedence over an alcohol.

CH3CH(OH)COOH is 2-hydroxypropanoic acid; the less important group (OH) appears as a prefix.

CH3COCH2COOH is 3-oxobutanoic acid; the less important group (C=O) appears as a prefix.

CH3CH(OH)CHO is 2-hydroxypropanal; the less important group (OH) appears as a prefix.

CH3CHClCH2OH is 2-chloropropan-1-ol. The OH group is now the more important group and appears as a suffix.

HOCH2CH=CHCH2COOH is 5-hydroxypent-3-enoic acid. OH again appears as a prefix, but the alkene, as is usual, appears as a suffix in front of other suffixes.


The longest carbon chain in the molecule has six carbons, but the longest carbon chain of which the most important functional group (COOH) forms part has five carbons.

The chain is numbered to give this functional group the lower possible number.

This gives ......pentanoic acid

2 3

4 5

H C C C C C H

H H C Cl H

H H H H

HO

H C C

H H

O

H H

1

PETROLEUM & ALKANES

PETROLEUMPetroleum (crude oil) was formed over millions of years from the accumulated remains of sea creatures which became buried on the ocean bed. The conditions required for the formation of petroleum (and natural gas) are:

high temperature high pressure (compression by overlying sediments) absence of oxygen

Petroleum is a complex mixture of hydrocarbons, mostly alkanes.

ALKANESThe alkanes are a homologous series of saturated hydrocarbons which all have the general formula CnH2n+2.

Carbon atoms form the spine of hydrocarbon molecules. Each carbon atom forms four covalent bonds; each hydrogen forms one covalent bond.

When the carbon atoms are joined only by single covalent bonds, the molecule contains the maximum possible number of hydrogen atoms for its particular number of carbon atoms. This is why the molecule is said to be saturated.

Physical PropertiesThe alkanes have simple molecular structures. The carbon and hydrogen atoms within each molecule are joined by strong covalent bonds; there are weak van der Waals’ forces between molecules. The strength of the van der Waals’ forces increases as the surface area of the molecule increases.

Down a homologous series, the molecular mass and therefore the boiling point increases. Thus the lower alkanes are gases at room temperature; the higher members are liquids and solids.


A HOMOLOGOUS SERIES is a group of compounds which have: the same general formula similar chemical properties

HYDROCARBONS are compounds which are made from ONLY carbon and hydrogen atoms.

In a SATURATED compound, there are only single bonds between carbon atoms.

Straight chain alkanes

Name Formula m.p. /oC b.p. /oC Density /g.cm-3

methane CH4 -182 -162ethane C2H6 -183 -89propane C3H8 -188 -42butane C4H10 -138 -0.5pentane C5H12 -130 36 0.626hexane C6H14 -95 69 0.659heptane C7H16 -91 98 0.684octane C8H18 -57 126 0.703nonane C9H20 -54 151 0.718decane C10H22 -30 174 0.730undecane C11H24 -26 196 0.740dodecane C12H26 -10 216 0.749tridecane C13H28 -5.5 235 0.756tetradecane C14H30 6 254 0.763pentadecane C15H32 10 271 0.769hexadecane C16H34 18 287 0.773heptadecane C17H36 22 302 0.778octadecane C18H38 28 316 0.782nonadecane C19H40 32 330 0.786eicosane C20H42 37 343 0.789

Isomeric alkanesIsomers have different boiling points because these depend on the strength of the intermolecular forces. The strength of the intermolecular forces, and therefore the boiling point, decreases as the amount of chain branching increases. Straight chain alkanes are approximately sausage shaped, but as the amount of branching increases, the shape becomes more spherical. This can be seen in the diagrams below:

pentane 2-methylbutane 2,2-dimethylpropane b.p. 36oC b.p. 28oC b.p. 10oC

The more spherical the structure, the smaller the surface area is and so the weaker the van der Waals’ forces are. Therefore the boiling point decreases.

Chemical PropertiesAlkanes contain only C-C and C-H bonds, which are strong and non-polar. Alkanes are, therefore, unreactive towards acids, alkalis, electrophiles and nucleophiles. They do, however, readily undergo combustion and are important as fuels.


FRACTIONAL DISTILLATIONThe properties of each substance in a mixture are unchanged. This makes it possible to separate substances in a mixture by physical methods including distillation. The complex mixture of hydrocarbons in crude oil can be separated into simpler mixtures or fractions by fractional distillation. Fractions contain molecules with a similar number of carbon atoms and have a narrow boiling point range.

The crude oil is heated to about 400oC and the liquid/vapour mixture is then pumped into a tall tower called a fractionating column. Most of the hydrocarbons have been converted to vapour by the heating and start to rise up the column. The lower the boiling point of a hydrocarbon, the further it will rise up the column before it cools enough to condense. In this way, the different fractions are collected at different points up the column. The number of different fractions which are collected and the amount of each which is produced depends on the source of the crude oil.

Most of the fractions from crude oil are burned as fuels.

The residue from this primary distillation contains useful materials such as lubricating oil and waxes. If these were distilled at atmospheric pressure, the temperature needed to vaporise them would be so high that thermal decomposition would occur. Therefore, the residue is distilled in a separate column under reduced pressure; reducing the pressure lowers the boiling point and prevents decomposition.

The quantities of the different fractions produced by fractional distillation do not usually match up with the market requirements for each fraction. There is a shortage of light fractions, especially gasoline and an excess of the heavier fractions. To resolve this problem, some of the heavier fractions (larger molecules) are converted into lighter, higher value fractions (smaller molecules) by cracking.

CRACKINGIn the process of cracking, large hydrocarbon molecules are broken down ("cracked") to produce smaller, more useful molecules. Molecules may break down in more than one way and will give a mixture of products which can be separated by a further distillation process. During cracking carbon-carbon bonds are broken; in addition to smaller alkane molecules, alkenes and hydrogen are produced. For example:

C14H30 C7H16 + C3H6 + 2C2H4

alkane alkane alkenes

C14H30 C12H24 + C2H4 + H2

alkane alkenes

There are two main types of cracking: thermal crackingcatalytic cracking


FRACTIONALDISTILLATION


CRUDE OIL

VAPOURliquids

vapour

360oC

300oC

200oC

100oC

PETROLEUM GASES

GASOLINE

KEROSINE

NAPHTHA

GAS OIL

LUBRICATINGOIL & WAXES

FUEL OIL

BITUMEN

(fuel for cars)

(feedstock for petrochemicals)

(fuel for jet aircraft)

(diesel: fuel for cars & large vehicles)

(fuel for ships & industrial heating)

(road surfacing)

(bottled gases)

b.p.decreases

Mr decreases

size of molecule decreases

viscosity decreases

volatility increases

easier to ignite

Thermal CrackingIn this process, the bonds are broken by heating the hydrocarbon vapour to a high temperature under a high pressure for a few seconds.

Temperature: 400 – 900oCPressure: 7MPa

The higher the temperature at which the cracking is carried out, the closer to the end of the chain the C-C bond breaks.

Homolytic fission of the carbon-carbon bond takes place, forming two alkyl radicals.

Thermal cracking produces a high percentage of alkenes.

Catalytic CrackingIn this process, the bonds are broken by heating the hydrocarbon vapour to a high temperature under a high pressure for a few seconds.

Temperature: 450oCPressure: slightCatalyst: zeolite (crystalline aluminosilicates)

Catalytic cracking proceeds by a carbocation (C+) mechanism; heterolytic fission of the carbon-carbon bond takes place, forming two ions.

Catalytic cracking is used mainly to produce motor fuels (branched-chain alkanes) and aromatic hydrocarbons.

Economics of CrackingThe lower Mr branched-chain alkanes produced by the cracking of heavy fractions are more useful as fuels and are therefore of higher value.

The alkenes produced by cracking can be used to make plastics (polymers) such as poly(ethene) and poly(propene).


HOMOLYTIC FISSIONWhen a bond breaks homolytically, each of the bonded atoms takes one electron from the shared pair, forming two particles with unpaired electrons called (free) radicals.e.g. CH3Cl CH3

+ Cl . .

HETEROLYTIC FISSIONWhen a bond breaks heterolytically, one of the bonded atoms takes both electrons from the shared pair, forming a positive ion and a negative ion.e.g. (CH3)3CCl (CH3)3C+ + Cl-

COMBUSTION OF ALKANESMost of the hydrocarbon fractions obtained from petroleum are used as fuels, because their combustion reactions are very exothermic. The products of combustion depend on whether the combustion is complete or incomplete.

Complete CombustionWhen alkanes burn in a plentiful supply of air or oxygen, complete combustion takes place, forming carbon dioxide and water.

CH4 + 2O2 CO2 + 2H2O H = -890 kJ.mol-1

C8H18 + 121/2O2 8CO2 + 9H2O H = -5512 kJ.mol-1

A graph of enthalpy of combustion against no. of carbon atoms for straight chain alkanes is a straight line.

Incomplete CombustionWhen the supply of air or oxygen is restricted, incomplete combustion of alkanes takes place, forming water together with carbon monoxide or carbon. The design of the burner affects the product of incomplete combustion.

Bunsen burners, which are intended for use in open laboratories, produce carbon when combustion is incomplete (the luminous flame obtained when the air hole is closed is sooty).

CH4 + O2 C + 2H2O

The design of gas fires is such that if the flue becomes blocked, restricting the air supply, incomplete combustion takes place to form carbon monoxide. Carbon monoxide is toxic. Every year there are a number of accidental deaths caused by carbon monoxide from poorly maintained gas fires and central heating boilers.

CH4 + 11/2O2 CO + 2H2O

Pollutants from Combustion The principal products of the internal combustion engine are carbon dioxide and water. Carbon dioxide is a greenhouse gas and contributes to global warming.

Sulphur-containing compounds are often present as impurities in alkanes obtained by the fractional distillation of petroleum. When these hydrocarbons are burned in air or oxygen, the sulphur is oxidised to sulphur(IV) oxide, SO2, and possibly to sulphur(VI) oxide, SO3. Both these oxides are toxic and also dissolve in atmospheric moisture, causing acid rain. This happens on a massive scale when power stations burn fossil fuels to produce electricity. Flue Gas Desulphurisation is a process used to prevent SO2 escaping into the atmosphere. Waste gases containing SO2 are passed through a flue (chimney)


containing calcium oxide (CaO) which absorbs the SO2 producing calcium sulphite (CaSO3).

CaO + SO2 CaSO3

This can easily be oxidised to to make hydrated calcium sulphate (CaSO4), also known as gypsum, which is used to make plasterboard for the building industry.

Carbon monoxide (petrol engine) and carbon (diesel engine) are formed as a result of incomplete combustion. Carbon monoxide is toxic; carbon particles are irritant.

Unburned hydrocarbons pass through the engine and enter the exhaust gases.At the high temperatures produced in the engine (up to 2500oC), the nitrogen and oxygen molecules in air have enough energy to combine to form nitrogen oxide.

N2 + O2 2NO

On cooling and in the presence of more oxygen, nitrogen oxide reacts to form other oxides of nitrogen (NOx), especially nitrogen dioxide, NO2. With water and more oxygen, nitrogen dioxide reacts to form nitric acid, which contributes to acid rain.

2NO + O2 2NO2

4NO2 + 2H2O + O2 4HNO3

Oxides of nitrogen are irritant, toxic gases. They combine with unburned hydrocarbons in the presence of sunlight to form photochemical smog. This is a particular problem in Los Angeles.

Catalytic ConvertersCatalytic converters are fitted to the exhaust systems of cars to remove pollutant gases. They consist of a honeycomb of ceramic material which is coated with a thin layer of a catalyst containing platinum (Pt) and rhodium (Rh). Up to 90% of pollutant gases are removed.

The catalyst system catalyses two important reactions: the reaction between carbon monoxide and nitrogen oxide, forming carbon

dioxide and nitrogen

2NO + 2CO N2 + 2CO2


the reaction between nitrogen oxide and unburned hydrocarbon fuel, forming carbon dioxide and nitrogen

C8H18 + 25NO 8CO2 + 121/2N2 + 9H2O

The principal exhaust gases are therefore carbon dioxide, nitrogen and water vapour. These gases are harmless, but carbon dioxide causes environmental problems.


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