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New Way Chemistry for Hong Kong A-Level Book 3A New Way Chemistry for Hong Kong A-Level
3A
1
Alkenes Alkenes and and
Electrophilic AdditioElectrophilic Additionn
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Preparation Preparation of Alkenesof Alkenes
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CrackinCrackingg• Prepared by the cracking of alkanes
of high molecular masses
• Give alkenes of low molecular masses
A. Industrial preparation
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CrackinCrackingg
e.g.
2CH3CH3 CH2 = CH2 + 2CH4600 oC
2CH3CH2CH3
CH3CH = CH2 + CH2 = CH2 + CH4 + H2
600 oC
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Elimination ReactionsElimination Reactions
• Involve removal of atoms or groups of atoms from adjacent carbon atoms in the reactant molecule
• Formation of a double bond between carbon atoms
B. Synthetic preparation
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1. Intramolecular Dehydration of Alcohols1. Intramolecular Dehydration of Alcohols• Removal of a water molecule from
a reactant molecule
• By heating the alcohols in the presence of a dehydrating agent.
E.g. Alumina(Al2O3), conc. H2SO4,
conc. H3PO4
• Give alkenes and water as the products
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1. Intramolecular Dehydration of Alcohols1. Intramolecular Dehydration of Alcohols
CH3CH2OHalumina
350oCC C
H
H
H
H
+ H2O
CH3CH2OH
conc. H2SO4
up to 200oCC C
H
H
H
H
+ H2O
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• Experimental conditions (i.e. temperature and concentration of concentrated sulphuric acid)
is closely related to the structure of the individual alcohol.
1. Intramolecular Dehydration of Alcohols1. Intramolecular Dehydration of Alcohols
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• Primary alcohols generally required concentrated sulphuric acid and a relatively high temperature
1. Intramolecular Dehydration of Alcohols1. Intramolecular Dehydration of Alcohols
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• Secondary alcohols are intermediate in reactivity
• Tertiary alcohols dehydrate under mild conditions (moderate temperature and dilute sulphuric acid)
1. Intramolecular Dehydration of Alcohols1. Intramolecular Dehydration of Alcohols
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• The relative ease of dehydration of alcohols generally decreases in the order:
Tertiary alcohol
Secondary alcohol
>
Primaryalcohol
>
1. Intramolecular Dehydration of Alcohols1. Intramolecular Dehydration of Alcohols
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• Intramolecular vs intermolecular
H C C OH
HH
H H
conc. H2SO4
170oCC C
H
H
H
H
+ H2O
Substitution
H C C OH
H
H
H
H
H O C C H
H
H
H
H
+
C2H5 O C2H5 + H2O
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Intramolecular dehydration is favoured at higher temperatures because it involves breaking of strong C – H bonds.
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Q.29(a)
C
CH3
H3C
CH3
OH
H3C C OH
CH H
H
CH3
C C
H3C
H3C
H
H
+ H2O
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Q.29(b)OH
OH
H
+ H2O
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Q.29(c)C C C CH3
OH
H
H
H
H
H
H
C C
H
H
H
C2H5
+ H2OC C C CH3
OH
H
H
H
H
H
H
C C C CH3
OH
H
H
H
H
H
H C C
H
H3C
H
CH3
+ H2O
C C
CH3
H
H
H3C
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• Secondary and tertiary alcohols may dehydrate to give a mixture of alkenes
• The more highly substituted alkene is formed as the major product
1. Intramolecular Dehydration of Alcohols1. Intramolecular Dehydration of Alcohols
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2. Dehydrohalogenation of haloalkanes2. Dehydrohalogenation of haloalkanes
• Elimination of a hydrogen halide molecule from a haloalkane
• By heating the haloalkane in an alcoholic solution of KOH
H C C H
H
X
H
H
OH-
C2H5OH, heatC C
H
H
H
H
+ H2O + X-
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H C C H
H
X
H
H
OH-
C2H5OH, heatC C
H
H
H
H
+ H2O + X-
C2H5OH is a co-solvent for both RX and OH
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e.g.
2. Dehyhalogenation of haloalkanes2. Dehyhalogenation of haloalkanes
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• Dehydrohalogenation of secondary or tertiary haloalkanes can take place in more than one way
• A mixture of alkenes is formed
2. Dehyhalogenation of haloalkanes2. Dehyhalogenation of haloalkanes
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Q.30(a)H2C C Br
CH3
CH3
H3C NaOH
C2H5OH, heat
C C Br
CH3
CH3
NaOH
C2H5OH, heat
H
H3C
H
C C
CH3
CH3
H
H3C
C C Br
CH3
C
NaOH
C2H5OH, heat
H
H3C
H
C C
H
H
H3C
C2H5
H
H H
major
minor
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Q.30(b)Cl
CH3
NaOH
C2H5OH, heat
Cl
CH3
NaOH
C2H5OH, heat
H
CH3
Cl
C
NaOH
C2H5OH, heat
H
H
H
C
H
H
major
minor
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• The ease of dehydrohalogenation of haloalkanes decreases in the order:
Tertiary haloalkane
Secondary haloalkane
>
Primaryhaloalkane
>
2. Dehyhalogenation of haloalkanes2. Dehyhalogenation of haloalkanes
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• The relative stabilities of alkenes decrease in the order:
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Relative Stability of Alkenes in Relative Stability of Alkenes in Terms of Enthalpy Changes of Terms of Enthalpy Changes of HydrogenationHydrogenation
• Hydrogenation of alkenes is exothermic
• From enthalpy changes of hydrogenation
predict the relative stabilities of alkenes
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Enthalpy changes of hydrogenation of but-1-ene, cis-but-2-ene and trans-but-2-ene
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Relative Stability of Alkenes in Relative Stability of Alkenes in Terms of Enthalpy Changes of Terms of Enthalpy Changes of HydrogenationHydrogenation
• The pattern of the relative stabilities of alkenes determined from the enthalpy changes of hydrogenation:
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Addition Addition ReactionsReactionsHydrogenation of alkynesHydrogenation of alkynes
• Alkenes can be prepared by hydrogenation of alkynes
Depend on the conditions and the catalyst employed
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HydrogenationHydrogenation
• Lindlar’s catalyst is metallic palladium(Pd) deposited on calcium carbonate
further hydrogenation of the alkenes formed can be prevented
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Reactions of Reactions of AlkenesAlkenes
An An IntroductionIntroduction
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• Alkenes are more reactive than alkanes
• Undergoes addition reaction rather than substitution
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• Presence of C=C double bond
• C=C double bond is made up of a bond and a bond
• Addition reactions only involve breaking of weaker bonds of alkenes
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• The electrons of the bond are
diffuse in shape
less firmly held by the bonding carbon
nuclei
Susceptible to the attack by electrophiles
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Electron-deficient species
Attack electron-rich center e.g. C=C bond
Examples :
Cations : H+, Br+, R+,… (lead to heterolysis)
Free radicals : H, Cl, R,…(lead to homolysis)
Electrophiles : -
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Electron-rich species
Attack electron-deficient site
e.g. carbonyl carbon, C=O
Examples :
anions : OH, Br, RO,…
molecules : H2O, ROH, NH3
Nucleophiles : -
All have lone pairs for donating to the reaction sitesAll lead to heterolytic fissions
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Reactions of Reactions of AlkenesAlkenes
ExamplesExamples
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Catalytic Catalytic HydrogenationHydrogenation• Alkenes react with hydrogen in the
presence of metal catalysts (e.g. Ni, Pd, Pt) to give alkanes
Lower temperatures can be used with Pd or Pt
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Catalytic Catalytic HydrogenationHydrogenation
e.g.
cis-addition, refer to notes on ‘chemical kinetics’, pp.36-37)
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Catalytic Catalytic HydrogenationHydrogenation
O
H3C CH3
CH2H2 / Pt
25oCCH3
CH3
H3C
O
Under mild conditions, C=O and benzene ring are unaffected.
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Q.31
C5H10
C5H10
H2 / Pt
H2 / Pt
C5H12
no reaction
BA
C
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A / B
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A / B
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C
** *
***
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Application : - hardening of plant oils
Plant oil (polyunsaturated liquid with low m.p.)
Margarine (soft unsat’d solid with higher m.p.)
Animal fat (hard sat’d solid with still higher m.p.)
Partial hydrogenation
Complete hydrogenation
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Catalytic Catalytic HydrogenationHydrogenation
• Fats and oils are organic compounds called triglycerides
triesters formed from glycerol and carboxylic acids of long carbon chains
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Catalytic Catalytic HydrogenationHydrogenation
• Saturated fats
solids at room temp
usually come from animal sources
long carbon chains are zig-zag and easily packed
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Catalytic Catalytic HydrogenationHydrogenation
• Unsaturated oils
liquids at room temp
usually come from plant sources
lower m.p. due to cis-arrangement (kinked shape)
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Catalytic Catalytic HydrogenationHydrogenation
• Fats are stable towards oxidation by air
• More convenient to handle and store
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Catalytic Catalytic HydrogenationHydrogenation• Advantages:
higher m.p. ideal for baking
turning rancid much less readily than unsaturated oils
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Application : - hardening of plant oils
Plant oil (polyunsaturated liquid with low m.p.)
Margarine (soft unsat’d solid with higher m.p.)
Animal fat (hard sat’d solid with still higher m.p.)
Partial hydrogenation
Complete hydrogenation
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H2C
CH
H2C
O
O
O
O
O
O
HC
O
OO
O
H2C
H2C
O
O
H2 / Ni150°C, 5 atm
trans-fat coronary heart disease
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Catalytic Catalytic HydrogenationHydrogenation
Hydrogenation of vegetable oils produces margarine
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Catalytic Catalytic HydrogenationHydrogenation
• Margarine and butter do not have sharp m.p. because they are NOT pure substances.
• They are mixtures containing different triesters.
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Electrophilic Addition Reactions(AdElectrophilic Addition Reactions(AdEE))
• Addition of electrophiles to the C=C double bond of alkenes
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• Electrophiles that attack the C=C double bond include
protons (H+)
neutral species in which the molecule is polarized, e.g. bromine
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(a) Addition of halogens in non-aqueous solvents(a) Addition of halogens in non-aqueous solvents
X = Cl, Br or I
CH3CCl
3
Occurs with or without light
Addition is preferred to substitution
Reaction mechanism is not required
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C C
Br
Br
+
bromonium ion
C C
Br
+ Br
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C C
Br
Br-
C C
Br
Br-
C C
Br
Br
C C
Br
Br
trans-addition
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(a) Addition of halogens in non-aqueous solvents(a) Addition of halogens in non-aqueous solvents
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e.g.
(a) Addition of halogens in non-aqueous solvents(a) Addition of halogens in non-aqueous solvents
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• The decolourization of bromine in 1,1,1-trichloroethane is a useful test for unsaturation
A drop of bromine
dissolved in 1,1,1-
trichloroethane is added to
an alkene
The reddish brown
colour of bromine is decolourize
d
(a) Addition of halogens in non-aqueous solvents(a) Addition of halogens in non-aqueous solvents
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(b) Addition of halogens in aqueous solutions(b) Addition of halogens in aqueous solutions
C C + X2(aq)
C C
OH
X
(major)
C C
X
X
(minor)
-OH comes from H-OH which is in excess.
Reaction mechanism is not required.
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e.g.
• The consequent decolourization of the reddish brown colour of bromine water is also a test for unsaturation
(b) Addition of halogens in aqueous solutions(b) Addition of halogens in aqueous solutions
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C C
Br
Br
+
bromonium ion
C C
Br
+ Br
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C C
Br
O
H H
C C
Br
O
H H
O
HH
C C
Br
OH
+ H3O+
bromohydrin
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Q.32
NaCl(aq)
C C + Br2
C C
Br
Br
C C
Br
OH
+ C C
Br
Cl
+
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C C
Br
Cl
C C
Br
Cl
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Q.32
NaI(aq)
C C + Br2
C C
Br
Br
C C
Br
OH
+ + C C
Br
I
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C C
Br
I
C C
Br
I
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Q.32
NaNO3(aq)
C C + Br2
C C
Br
Br
C C
Br
OH
+ + C C
Br
ONO2
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C C
Br
NO3
C C
Br
ONO2
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(c) Addition of H – X (c) Addition of H – X
X = Br, Cl, OSO3H, OH, etc.
Mechanism required
H-Br
conc. H-OSO3H
H-OH H3O+
C C
H
H
H
H
H C C H
HH
H Br
H C C H
HH
H OSO3H
H C C H
HH
H OH
Acid-catalyzed hydration
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Addition of Hydrogen BromideAddition of Hydrogen Bromide• A molecule of HBr adds to the C=C
double bond of an alkene
• Give a bromoalkane
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Reaction Mechanism: Electrophilic Reaction Mechanism: Electrophilic Addition Reactions of Hydrogen Addition Reactions of Hydrogen Bromide to AlkenesBromide to Alkenes
sp2 hybridized carbonium ion
Br is a nucleophile
rate-determining step
C
H
H
H
C Br-C C
Br
H
+C C
BrH
fast
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sp2 hybridized trigonal planar
C
H
H
H
C Br-C C
Br
H
+C C
BrH
fast
50%
50%
50%50%
If the resulting C is chiral
* *
racemic mixture
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Q.33
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(a)one bond and one bond are broken
(b) two bonds are formed
(c) Heat evolved during bond formation >Heat required during bond breaking
Addition reactions are usually exothermic
Q.33
view movie
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Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
CH2=CH2 & CH3CH=CHCH3 are symmetrical alkenes.
CH3CH=CH2 is an asymmetrical alkene.
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Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
• A hydrogen halide can add to an asymmetrical alkene in either of the two ways
• The reaction proceeds to give a major product preferentially
the reaction is said to exhibit “regioselectivity”
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the addition of HBr to ethene produces bromoethane as the only product
Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
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• When but-2-ene reacts with HBr
2-bromobutane is formed as the only product
Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
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• When propene reacts with HBr
the major product is 2-bromopropane
the minor product is 1-bromopropane
Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
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Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
H is given to the rich
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Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
Markovnikov’s rule states that in the addition of HX to an asymmetrical alkene, the hydrogen atom adds to the carbon atom of the carbon-carbon double bond that already has the greater number of hydrogen atoms
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Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
• The products formed according to this rule are known as Markovnikov products
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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• Carbocations are a chemical species that
contains a positively charged carbon
• Very unstable
• Exist transiently during the reaction
• Classified as primary, secondary or tertiary
according to the number of alkyl groups that are directly attached to the positively charged carbon
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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s RuleThe more stable the carbocation
the more stable the transition state
the lower the activation energy
the faster its formation
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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• The stability of the carbocations
increases in the order:
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Alkyl groups stabilize the positively charged carbocation by positive inductive effect
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• A greater number of alkyl groups
release more electrons to the positively charged carbon
increase the stability of the carbocation
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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• Consider the addition of HBr to
propene:
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Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• The hydrobromination of propene
involves two competing reactions:
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Since the formation of carbocation is the rate-determining step,
the overall reaction is faster if it involves the formation of a more stable carbocation.
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Q.34(a)
C C
H
H
H3C
H3CHX
H3C C C H
H
H
CH3
X
CH2H
H3C
H3C
> H3C C
CH3
H
C
H
H
X = Cl, Br, or I
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Q.34(b)
C C
H
H
H
H3C
H O S OH
O
O
++
cold
H C CH3
CH3
OSO3H
heatC CH3
H
H3C
> C2H5 C
H
H
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• On heating, alkyl hydrogensulphates form alkenes and sulphuric acid
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Separation of a mixture containing an alkane and an alkene.
Alkane / Alkene
conc. H2SO4 / cold
no reaction
Alkane
addition
C C
H OSO3H
dissolved in conc. H2SO4
insoluble in conc. H2SO4
Separated by separating funnel
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Separation of a mixture containing an alkane and an alkene.
Alkane / Alkene
conc. H2SO4 / cold
no reaction
Alkane
addition
C C
H OSO3Hheat
Alkene
dissolved in conc. H2SO4
insoluble in conc. H2SO4
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Alkyl hydrogensulphates can be easily hydrolyzed to alcohols by heating with water
conc. H2SO4 + H2O dilute H2SO4
acid-catalyzed hydration
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(c) Addition of H – X (c) Addition of H – X
X = Br, Cl, OSO3H, OH, etc.
Mechanism required
H-Br
conc. H-OSO3H
H-OH H3O+
C C
H
H
H
H
H C C H
HH
H Br
H C C H
HH
H OSO3H
H C C H
HH
H OH
Acid-catalyzed hydration
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Acid-catalyzed hydration
C C
H
H
H
H3C
H O S OH
O
O
++
C CH3
H
H3C
O
H
H
C
CH3
CH3
H
O
H
H
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C
CH3
CH3
H
O
H
H
O
H H
C
CH3
CH3
H
HO
+ H3O+The acid catalyst is regenerated
Acid-catalyzed hydration
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Q.34(d)
(d)H2O / H+
OH
H
>
3 2
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Q.34(c)
C C
H3C
H3C
H
H
I – Cl +
H3C C C H
H
I
CH3
Cl
C CH2I
H3C
H3C
> CC
H
HCH3
H3C
I
3 1EN : C = I = 2.5
electron-donating
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Q.34(e)
C CH3
H
F3C
< CC
H
HCF3
H
H
More destabilized by negative inductive effect
H C
H
CF3
C
H
H
ClH
H
H
F3CHCl
(e)H – Cl
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C CH3
H
Cl
Q.34(f)
H
H
H
ClHCl
(f)H – Cl
Cl C C
H
Cl
H
H
H
C CH3
H
Cl
The resonance effect more than compensates the negative inductive effect of Cl
C C
H
HCl
H
H
>
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Q.34(g)
H
H
H
HBr(g)
H – Br C C
H
Br
H
H
H
C C
H
HH
H
C CH3
H
>
21
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C CH3
H
C CH3
H
C CH3
H
C CH3
H
The +ve charge is shared by the benzene ring by resonance effect.
Stabilized by resonance effect as well as inductive effect(2)
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C CH3
H
benzylic carbocation
More stable than 3 carbocation
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Q.34(h)
H2C CH
CH
CH2
excess H – F
H C C C C
H
F
H
H
H
F
H
H
H
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CCH
H
CH2H
H2C > C C
H
HH
H
CH
H2C
21
C C
H
CH2H
H
H2C
Allylic carbocation is stabilized by resonance effect.
Stabilized by resonance effect as well as inductive effect
Benzylic > allylic > 3 > 2 > 1 > CH3
+
Stability : -
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CH
C
H
CH2H
H2C F- C FCH
H2C
CH2H
H
H – F
C C C C H
H
H
H
F
H
H
F
H
H
C C C C H
H
H
H
F
H
HH
H
> C C C C H
H H
F
H
H
H
H
H
less destabilized by –ve I-effect of F
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Effect of substituents on the reactivity of AdE
1. Electron-donating groups increase the reactivity by
(a) the electron density of C=C bond, thus making it more susceptible to electrophilic attack.
(b) Stabilizing the carbocation intermediate/T.S. by +ve I-effect and/or resonance effect, thus lowering the Ea for the rate-determining step.
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Effect of substituents on the reactivity of AdE
2. Resonance effect > inductive effect
3. Electron-withdrawing groups lower the reactivity by working in the opposite
ways.
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Q.35
C
C
H H
H
>C
C
H3C CH3
H3C CH3
>C
C
H CH3
H CH3
>C
C
H H
H H
>C
C
H CF3
H H
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Oxidation of alkenes
(a) Combustion
C C + O2 CO2 + CO + C + H2O
More sooty and luminous than that of corresponding alkanes due to higher carbon contents
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(b) Reaction with KMnO4
C O
R2
R1
O C
R4
R3
+C C
R4
R3
R2
R1
KMnO4, H+ or OH
heat
C C
R4
R3
R2
R1
KMnO4, H+ or OH
coldR2 C C R4
R1
OH
R3
OH
Used as a test for alkenes
carbonyl products
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C O
R2
R1
O C
R4
R3
+C C
R4
R3
R2
R1
KMnO4, H+ or OH
heat
If all R groups are alkyl groups,
ketones will be the final products.
C C
CH3
CH3
H3C
H3C
KMnO4, H+ or OH
heat
C O
H3C
H3C
2
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KMnO4, H+ or OH
heatNo reaction
C O
H3C
H3C
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C O
R2
R1
O C
R4
R3
+C C
R4
R3
R2
R1
KMnO4, H+ or OH
heat
If either R1 or R2 is H / either R3 or R4 is H,
further oxidation of aldehydes to carboxylic acid will occur.
C C
H
CH3
H
H3C
KMnO4, H+ or OH
heatC O
H
H3C
2
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KMnO4, H+ or OH
heatC O
H
H3C
C O
HO
H3C
aldehyde carboxylic acid
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C O
R2
R1
O C
R4
R3
+C C
R4
R3
R2
R1
KMnO4, H+ or OH
heat
If both R1 & R2 are H / both R3 & R4 are H,
further oxidation to first methanoic acid and then CO2 will occur.
C C
H
H
H
H
KMnO4, H+ or OH
heatC O
H
H
2
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C O
H
H
2KMnO4, H+ or OH
heat2CO2 + 2H2O
C O
HO
H
2
methanoic acid
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(c) Ozonolysis
+ O3
CH3CCl3
< 20oCO O
O
unstable ozonide
O O
O
+ H2OZn dust
CH3COOH2 O + H2O2
Further oxidation of aldehyde to carboxylic acid by H2O2 is inhibited using Zn dust and CH3COOH
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(c) Ozonolysis
1. O3
2. Zn dust / H2OC O
H
H3C
CO
H
CH3
+C C
H
CH3
H
H3C
C C
H
H
H3C
H3C
C O
H3C
H3C
CO
H
H
+
1. O3
2. Zn dust / H2O
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Oxidative cleavage can be used to locate C=C bond in an unknown sample
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Q.36
C C
H
CH3
H
C
CH3
H2C
C C
CH3
H
H
C
CH3
H2C
(3Z)-2-methylpenta-1,3-diene
(3E)-2-methylpenta-1,3-diene
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Q.36CH3
H
H3C
H
CH2
CH3H
H3C
H
CH2
(3E)-3-methylpenta-1,3-diene
(3Z)-3-methylpenta-1,3-diene
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Q.37
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Q.37
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Q.37
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The END
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28.4 Preparation of Alkenes (SB p.173)
Classify the following alcohols as primary, secondary or tertiary alcohols.
(a) CH3CHOHCH2CH3
(b) CH3CH2CH2OH
(c) (CH3)2COHCH2CH2CH3 Answer(a) It is a secondary alcohol.
(b) It is a primary alcohol.
(c) It is a tertiary alcohol. Back
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Classify the following haloalkanes as primary, secondary or tertiary haloalkanes.
(a) (c)
(b)
Answer
(a) A secondary haloalkane
(b) A primary haloalkane
(c) A tertiary haloalkane
Back28.4 Preparation of Alkenes (SB p.173)
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28.5 Reactions of Alkenes (SB p.177)
Of the isomeric C5H11+ carbocations, which one
is the most stable?Answer
The more stable C5H11+ carbocation is the tertiary
carbocation as shown below:
Back
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28.5 Reactions of Alkenes (SB p.179)
Both alkanes and alkenes undergo halogenation. The
halogenation of alkanes is a free radical substitution
reaction while the reaction of alkenes with halogens is
an electrophilic addition reaction. Can you tell two differences between the products formed by the
twodifferent types of halogenation?
Back
AnswerAlkenes give dihalogenated products while alkanes usually give
polysubstituted products. Another difference is the position of the
attachment of the halogen atom. For alkenes, the halogen atom is
fixed to the carbon atom of the carbon=carbon double bond. In the
substitution reaction of alkanes, the position of the halogen atom
varies.
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28.5 Reactions of Alkenes (SB p.183)
(a) What chemical tests would you use to distinguish between two unlabelled bottles containing hexane and hex-1-ene respectively? Answer
(a) We can perform either one of the following tests:
Hex-1-ene can decolourize bromine water or chlorine water in
the dark while hexane cannot.
Hex-1-ene can decolourize acidified potassium manganate(VII)
solution while hexane cannot.
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28.5 Reactions of Alkenes (SB p.183)
(b) What is the major product of each of the following reactions?
(i)
(ii)
Answer
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28.5 Reactions of Alkenes (SB p.183)
(b) (i)
(ii)
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(c) Give the products for the following reactions:
(i) CH3CH = CH2 + H2
(ii) CH3CH = CHCH3
(iii) CH3CH = CHCH3 + Br2
Ni
conc. H2SO4
28.5 Reactions of Alkenes (SB p.183)
Answer
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28.5 Reactions of Alkenes (SB p.183)
(c) (i) CH3CH2CH3
(ii)
(iii)
Back
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28.5 Reactions of Alkenes (SB p.184)
(a) Arrange the following carbocations in increasing order of stability. Explain your answer briefly.
Answer
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28.5 Reactions of Alkenes (SB p.184)
(a) The increasing order of the stability of carbocations is:
Tertiary carbocations are the most stable because the three alkyl
groups release electrons to the positive carbon atom and thereby
disperse its charge. Primary carbocations are the least stable as
there is only one alkyl group releasing electrons to the positive
carbon atom.
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28.5 Reactions of Alkenes (SB p.184)
(b) Based on your answer in (a), arrange the following molecules in the order of increasing rates of reaction with hydrogen chloride.
Answer
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28.5 Reactions of Alkenes (SB p.184)
(b) The reaction of these compounds with hydrogen chloride involves
the formation of carbocations. Therefore, the order of reaction
rates follows the order of the ease of the formation of
carbocations, i.e. the stability of carbocations:
Therefore, the rates of reactions of the three compounds with
hydrogen chloride increase in the order:
Back