Alkenes
and
Electrophilic Addition
1
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Preparation of
Alkenes
2
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A. Industrial preparation
Cracking
3
•
Prepared by the cracking of alkanes of
high molecular masses
•
Give alkenes of low molecular masses
New Way Chemistry for Hong Kong A-Level 3A
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Cracking
e.g.
600 oC
2CH3CH3  CH2 = CH2 + 2CH4
2CH3CH2CH3
 CH3CH = CH2 + CH2 = CH2 + CH4 + H2
600 oC
4
New Way Chemistry for Hong Kong A-Level 3A
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B. Synthetic preparation
Elimination Reactions
5
•
Involve removal of atoms or groups of
atoms from adjacent carbon atoms in
the reactant molecule
•
Formation of a double bond between
carbon atoms
New Way Chemistry for Hong Kong A-Level 3A
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1. 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
•
6
Give alkenes and water as the
products
New Way Chemistry for Hong Kong A-Level 3A
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1. Intramolecular Dehydration of Alcohols
H
H
alumina
CH3CH2OH
C
350oC
H
conc. H 2SO4
CH3CH2OH
H2O
H
H
H
C
+
C
up to 200 oC
H
7
+
C
H
New Way Chemistry for Hong Kong A-Level 3A
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H2O
1. Intramolecular Dehydration of Alcohols
•
Experimental conditions (i.e. temperature
and concentration of concentrated
sulphuric acid)
is closely related to the structure of the
individual alcohol.
8
New Way Chemistry for Hong Kong A-Level 3A
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1. Intramolecular Dehydration of Alcohols
•
9
Primary alcohols generally required
concentrated sulphuric acid and a
relatively high temperature
New Way Chemistry for Hong Kong A-Level 3A
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1. Intramolecular Dehydration of Alcohols
10
•
Secondary alcohols are intermediate in
reactivity
•
Tertiary alcohols dehydrate under mild
conditions (moderate temperature and
dilute sulphuric acid)
New Way Chemistry for Hong Kong A-Level 3A
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1. Intramolecular Dehydration of Alcohols
•
The relative ease of dehydration of
alcohols generally decreases in the order:
>
>
Tertiary
alcohol
11
Secondary
alcohol
New Way Chemistry for Hong Kong A-Level 3A
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Primary
alcohol
•
Intramolecular vs intermolecular
H
H
conc. H 2SO4
H
C
C
OH
H
H
C
o
+
C
170 C
H
H
H
H
Substitution
12
New Way Chemistry for Hong Kong A-Level 3A
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H2O
Intramolecular dehydration is favoured at
higher temperatures because it involves
breaking of strong C – H bonds.
13
New Way Chemistry for Hong Kong A-Level 3A
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Q.29(a)
CH 3
H3C
C
OH
CH 3
H
H
C
H
H3C
C
OH
CH 3
14
H3C
H
C
+
C
H3C
New Way Chemistry for Hong Kong A-Level 3A
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H
H2O
Q.29(b)
OH
H
OH
+
15
New Way Chemistry for Hong Kong A-Level 3A
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H2O
Q.29(c)
H
H
H
H
OH
H
C
C
C
H
H
H
H
OH
H
C
C
C
H
H
H
H
OH
H
C
C
C
H
H
H
CH3
H
C2H5
C
CH 3
H
CH 3
C
H
H3C
H
H
New Way Chemistry for Hong Kong A-Level 3A
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
+
C
H
C
16
H2O
H
H3C
CH 3
+
C
H2O

C
CH 3
1. Intramolecular Dehydration of Alcohols
17
•
Secondary and tertiary alcohols may
dehydrate to give a mixture of alkenes
•
The more highly substituted alkene is
formed as the major product
New Way Chemistry for Hong Kong A-Level 3A
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2. Dehydrohalogenation of haloalkanes
•
Elimination of a hydrogen halide
molecule from a haloalkane
•
By heating the haloalkane in an
alcoholic solution of KOH
H
H
-
H
H
OH
H
C
C
H
C
C2H5OH, heat
H
18
X
H
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+ H2O + X-
C
H
H
H
-
H
H
OH
H
C
C
H
C
C2H5OH, heat
H
X
H
+ H2O + X-
C
H
C2H5OH is a co-solvent for both RX and OH
19
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2. Dehyhalogenation of haloalkanes
e.g.
20
New Way Chemistry for Hong Kong A-Level 3A
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2. Dehyhalogenation of haloalkanes
21
•
Dehydrohalogenation of secondary or
tertiary haloalkanes can take place in
more than one way
•
A mixture of alkenes is formed
New Way Chemistry for Hong Kong A-Level 3A
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Q.30(a)
CH3
H3C
NaOH
H2C
C
Br
C2H5OH, heat
CH3
H
CH3
NaOH
H3C
C
C
H3C
Br
CH3
C
C
C2H5OH, heat
H
CH3
H
CH 3
H
NaOH
H3C
C
C
major
C2H5
Br
CH3
H
C
C
C2H5OH, heat
H
C
H
22
H
H
New Way Chemistry for Hong Kong A-Level 3A
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H3C
H
minor
Q.30(b)
Cl
NaOH
CH 3
C2H5OH, heat
H
CH 3
Cl
NaOH
major
CH 3
C2H5OH, heat
H
Cl
C
H
NaOH
H
C2H5OH, heat
C
H
23
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
H
minor
2. Dehyhalogenation of haloalkanes
•
The ease of dehydrohalogenation of
haloalkanes decreases in the order:
>
>
Tertiary
haloalkane
24
Secondary
haloalkane
New Way Chemistry for Hong Kong A-Level 3A
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Primary
haloalkane
•
25
The relative stabilities of alkenes
decrease in the order:
New Way Chemistry for Hong Kong A-Level 3A
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Relative Stability of Alkenes in Terms
of Enthalpy Changes of Hydrogenation
•
Hydrogenation of alkenes is
exothermic
•
From enthalpy changes of
hydrogenation
 predict the relative stabilities of
alkenes
26
New Way Chemistry for Hong Kong A-Level 3A
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Enthalpy changes of hydrogenation of but-1-ene,
cis-but-2-ene and trans-but-2-ene
27
New Way Chemistry for Hong Kong A-Level 3A
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Relative Stability of Alkenes in Terms
of Enthalpy Changes of Hydrogenation
•
28
The pattern of the relative stabilities
of alkenes determined from the
enthalpy changes of hydrogenation:
New Way Chemistry for Hong Kong A-Level 3A
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Addition Reactions
Hydrogenation of alkynes
•
Alkenes can be prepared by
hydrogenation of alkynes
 Depend on the conditions and the
catalyst employed
29
New Way Chemistry for Hong Kong A-Level 3A
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Hydrogenation
•
Lindlar’s catalyst is metallic palladium(Pd)
deposited on calcium carbonate
 further hydrogenation of the alkenes
formed can be prevented
30
New Way Chemistry for Hong Kong A-Level 3A
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Reactions of
Alkenes
An Introduction
31
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•
Alkenes are more reactive than
alkanes
•
Undergoes addition reaction
rather than substitution
32
New Way Chemistry for Hong Kong A-Level 3A
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33
•
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
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
•
The electrons of the 
bond are
 diffuse in shape
 less firmly held by the
bonding carbon
nuclei
Susceptible to the attack by electrophiles
34
New Way Chemistry for Hong Kong A-Level 3A
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Electrophiles : 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)
35
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Nucleophiles : Electron-rich species
Attack electron-deficient site
e.g. carbonyl carbon, C=O
Examples :
anions : OH, Br, RO,…
molecules : H2O, ROH, NH3
All have lone pairs
for donating to
the reaction sites
All lead to heterolytic fissions
36
New Way Chemistry for Hong Kong A-Level 3A
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Reactions of
Alkenes
Examples
37
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Catalytic Hydrogenation
•
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
38
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Catalytic Hydrogenation
e.g.
cis-addition, refer to notes on ‘chemical
kinetics’, pp.36-37)
39
New Way Chemistry for Hong Kong A-Level 3A
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Catalytic Hydrogenation
O
H3C
CH2
CH3
H2 / Pt
O
CH3
H3C
CH3
25oC
Under mild conditions, C=O and benzene ring
are unaffected.
40
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.31
H2 / Pt
C5H10
C5H12
A
B
H2 / Pt
C5H10
no reaction
C
41
New Way Chemistry for Hong Kong A-Level 3A
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A / B
42
New Way Chemistry for Hong Kong A-Level 3A
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A / B
43
New Way Chemistry for Hong Kong A-Level 3A
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C
*
*
44
*
*
*
*
New Way Chemistry for Hong Kong A-Level 3A
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Application : - hardening of plant oils
Plant oil (polyunsaturated liquid with low m.p.)
Partial hydrogenation
Margarine (soft unsat’d solid with higher m.p.)
Complete hydrogenation
Animal fat (hard sat’d solid with still higher m.p.)
45
New Way Chemistry for Hong Kong A-Level 3A
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Catalytic Hydrogenation
•
Fats and oils are organic compounds
called triglycerides
 triesters formed from glycerol and
carboxylic acids of long carbon chains
46
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Catalytic Hydrogenation
•
Saturated fats
 solids at room temp
 usually come from animal sources
 long carbon chains are zig-zag and
easily packed
47
New Way Chemistry for Hong Kong A-Level 3A
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Catalytic Hydrogenation
•
Unsaturated oils
 liquids at room temp
 usually come from plant sources
 lower m.p. due to cis-arrangement
(kinked shape)
48
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Catalytic Hydrogenation
49
•
Fats are stable towards oxidation by air
•
More convenient to handle and store
New Way Chemistry for Hong Kong A-Level 3A
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Catalytic Hydrogenation
•
Advantages:
higher m.p.  ideal for baking
turning rancid much less readily than
unsaturated oils
50
New Way Chemistry for Hong Kong A-Level 3A
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Application : - hardening of plant oils
Plant oil (polyunsaturated liquid with low m.p.)
Partial hydrogenation
Margarine (soft unsat’d solid with higher m.p.)
Complete hydrogenation
Animal fat (hard sat’d solid with still higher m.p.)
51
New Way Chemistry for Hong Kong A-Level 3A
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O
H2C
O
O
HC
O
O
H2C
O
150°C,
H2 / Ni
5 atm
O
H2C
O O
CH
O
H2C
O
O
trans-fat  coronary heart disease
52
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Catalytic Hydrogenation
Hydrogenation of vegetable oils
produces margarine
53
New Way Chemistry for Hong Kong A-Level 3A
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Catalytic Hydrogenation
54
•
Margarine and butter do not have sharp
m.p. because they are NOT pure
substances.
•
They are mixtures containing different
triesters.
New Way Chemistry for Hong Kong A-Level 3A
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Electrophilic Addition Reactions(AdE)
•
55
Addition of electrophiles to the C=C
double bond of alkenes
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
•
Electrophiles that attack the C=C double
bond include
 protons (H+)
 neutral species in which the molecule
is polarized, e.g. bromine
56
New Way Chemistry for Hong Kong A-Level 3A
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(a) Addition of halogens in non-aqueous solvents
CH3CCl3
X = Cl, Br or I
Occurs with or without light
Addition is preferred to substitution
Reaction mechanism is not required
57
New Way Chemistry for Hong Kong A-Level 3A
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C
C
C
C
+ Br
Br
+ Br
 Br
58
bromonium
ion
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Br Br
C
C
C
Br
Br
C
Br
-
Br
C
C
Br
59
C
C
Br
New Way Chemistry for Hong Kong A-Level 3A
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transaddition
(a) Addition of halogens in non-aqueous solvents
60
New Way Chemistry for Hong Kong A-Level 3A
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(a) Addition of halogens in non-aqueous solvents
e.g.
61
New Way Chemistry for Hong Kong A-Level 3A
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(a) Addition of halogens in non-aqueous solvents
•
The decolourization of bromine in 1,1,1trichloroethane is a useful test for unsaturation
A drop of
bromine
dissolved in
1,1,1trichloroethane
is added to an
alkene
62
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
The reddish
brown colour
of bromine
is
decolourized
(b) Addition of halogens in aqueous solutions
OH
C
C
(major)
X
C
C
+
X2(aq)
X
C
C
(minor)
X
-OH comes from H-OH which is in excess.
Reaction mechanism is not required.
63
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
(b) Addition of halogens in aqueous solutions
e.g.
• The consequent decolourization of the
reddish brown colour of bromine water
is also a test for unsaturation
64
New Way Chemistry for Hong Kong A-Level 3A
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C
C
C
C
+ Br
Br
+ Br
 Br
65
bromonium
ion
New Way Chemistry for Hong Kong A-Level 3A
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H
H
O
H
H
O
H
H
O
C
C
C
Br
C
Br
OH
C
bromohydrin
C
+
H3O+
Br
66
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.32
C
+
C
Br2
NaCl(aq)
Br
C
OH
C
Br
67
+
C
Cl
C
+
Br
New Way Chemistry for Hong Kong A-Level 3A
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C
C
Br
Cl
C
C
Cl
C
C
Br
Br
68
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.32
C
+
C
Br2
NaI(aq)
Br
C
C
Br
69
I
OH
+
C
C
+
Br
New Way Chemistry for Hong Kong A-Level 3A
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C
C
Br
I
C
C
Br
70
I
C
Br
New Way Chemistry for Hong Kong A-Level 3A
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C
Q.32
C
+
C
Br2
NaNO3(aq)
Br
C
C
Br
71
ONO 2
OH
+
C
C
+
Br
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
C
C
Br
NO3
C
C
ONO 2
C
Br
Br
72
New Way Chemistry for Hong Kong A-Level 3A
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C
(c) Addition of H – X
X = Br, Cl, OSO3H, OH, etc.
H
H
H
C
C
H
Br
H-Br
H
H
C
H
conc. H-OSO3H
C
H-OH
H
+
H3O
H
73
H
Mechanism
required
H
H
H
C
C
H
OSO3H
H
H
C
C
H
OH
H
New Way Chemistry for Hong Kong A-Level 3A
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H
Acid-catalyzed
hydration
Addition of Hydrogen Bromide
74
•
A molecule of HBr adds to the C=C
double bond of an alkene
•
Give a bromoalkane
New Way Chemistry for Hong Kong A-Level 3A
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Reaction Mechanism: Electrophilic Addition
Reactions of Hydrogen Bromide to Alkenes
rate-determining step
sp2 hybridized
carbonium ion
H
H
C
C
-
Br
fast
H
Br
C
C
H
+
C
C
H
Br
Br is a nucleophile
75
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
sp2 hybridized
trigonal planar
50%
H
H
C
H
racemic mixture
-
C
Br
fast
H
Br
C
C
*
H
+
50%
*
C
Br
50%
If the resulting C is chiral
76
C
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
50%
Q.33
77
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.33
(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
view movie
78
New Way Chemistry for Hong Kong A-Level 3A
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Regioselectivity of Hydrogen Halide Addition:
Markovnikov’s Rule
CH2=CH2 & CH3CH=CHCH3 are
symmetrical alkenes.
CH3CH=CH2 is an asymmetrical alkene.
79
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Regioselectivity of Hydrogen Halide Addition:
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”
80
New Way Chemistry for Hong Kong A-Level 3A
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Regioselectivity of Hydrogen Halide Addition:
Markovnikov’s Rule
the addition of HBr to ethene
produces bromoethane as the only
product
81
New Way Chemistry for Hong Kong A-Level 3A
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Regioselectivity of Hydrogen Halide Addition:
Markovnikov’s Rule
•
When but-2-ene reacts with HBr
 2-bromobutane is formed as the only
product
82
New Way Chemistry for Hong Kong A-Level 3A
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Regioselectivity of Hydrogen Halide Addition:
Markovnikov’s Rule
•
When propene reacts with HBr
 the major product is 2-bromopropane
 the minor product is 1-bromopropane
83
New Way Chemistry for Hong Kong A-Level 3A
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Regioselectivity of Hydrogen Halide Addition:
Markovnikov’s Rule
H is given to the rich
84
New Way Chemistry for Hong Kong A-Level 3A
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Regioselectivity of Hydrogen Halide Addition:
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
85
New Way Chemistry for Hong Kong A-Level 3A
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Regioselectivity of Hydrogen Halide Addition:
Markovnikov’s Rule
•
86
The products formed according to this
rule are known as Markovnikov products
New Way Chemistry for Hong Kong A-Level 3A
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Stability of Carbocation and Mechanistic
Explanation of the Markovnikov’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
87
New Way Chemistry for Hong Kong A-Level 3A
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Stability of Carbocation and Mechanistic
Explanation of the Markovnikov’s Rule
The more stable the carbocation
 the more stable the transition state
 the lower the activation energy
 the faster its formation
88
New Way Chemistry for Hong Kong A-Level 3A
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Stability of Carbocation and Mechanistic
Explanation of the Markovnikov’s Rule
•
89
The stability of the carbocations
increases in the order:
New Way Chemistry for Hong Kong A-Level 3A
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Alkyl groups stabilize the positively charged
carbocation by positive inductive effect
90
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•
A greater number of alkyl groups
 release more electrons to the
positively charged carbon
 increase the stability of the carbocation
91
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Stability of Carbocation and Mechanistic
Explanation of the Markovnikov’s Rule
•
92
Consider the addition of HBr to propene:
New Way Chemistry for Hong Kong A-Level 3A
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Stability of Carbocation and Mechanistic
Explanation of the Markovnikov’s Rule
•
93
The hydrobromination of propene involves
two competing reactions:
New Way Chemistry for Hong Kong A-Level 3A
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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.
94
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Q.34(a)
X = Cl, Br, or I
H3C
H
C
H3C
H
CH2H
95
C
C
X
H
H
CH3
H3C
H3C
H
HX
C
H3C
CH 3
>
H3C
C
H
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H
C
H
Q.34(b)
H3C
C
H
O
H
+
C
H
+
O
S

H
OH
O
heat
cold
H3C
H
C
CH3
> C2H5
C
CH 3
H
H
C
CH 3
OSO3H
96
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H
•
97
On heating, alkyl hydrogensulphates
form alkenes and sulphuric acid
New Way Chemistry for Hong Kong A-Level 3A
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Separation of a mixture containing an alkane
and an alkene.
Alkane / Alkene
conc. H2SO4 / cold
no reaction
addition
Alkane
insoluble in
conc. H2SO4
Separated by separating
funnel
98
C
C
H
OSO3H
dissolved in
conc. H2SO4
New Way Chemistry for Hong Kong A-Level 3A
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Separation of a mixture containing an alkane
and an alkene.
Alkane / Alkene
conc. H2SO4 / cold
no reaction
Alkane
insoluble in
conc. H2SO4
Alkene
99
addition
heat
C
C
H
OSO3H
dissolved 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
100
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(c) Addition of H – X
X = Br, Cl, OSO3H, OH, etc.
H
H
H
C
C
H
Br
H-Br
H
H
C
H
conc. H-OSO3H
C
H-OH
H
+
H3O
H
101
H
Mechanism
required
H
H
H
C
C
H
OSO3H
H
H
C
C
H
OH
H
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H
Acid-catalyzed
hydration
Acid-catalyzed hydration
H3C
O
H
C
H
+
C

S
OH
O
H3C
O
102
+
O
H
H
H
H
C
H
CH3
H
CH3
O
H
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C
H
CH3
Acid-catalyzed hydration
H
H
O
CH3
H
O
H
C
CH3
CH3
HO
H
C
H
The acid catalyst is + H3O+
regenerated
103
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CH3
Q.34(d)
OH
H2O / H+
(d)
H
>
3
104
2
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.34(c)
H3C
H
C
+
H
H3C
C
C
Cl
I
H
H3C
electron-donating
C
CH2I
>
H3C
H3C
3
EN : C = I = 2.5
105
CH 3
I – Cl
C
H3C

CH3
C
H
C
H
I
1
New Way Chemistry for Hong Kong A-Level 3A
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H
Q.34(e)
F3C
H
H
C
C
H
Cl
H HCl
– Cl
(e)
H
H
CF 3
H
CF3
F3C
C
CH3
<
H
H
More destabilized by negative
inductive effect
106
H
C
H
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H
C
H
Q.34(f)
Cl
H
(f)
H HCl
– Cl
Cl
H
H
H
H
C
C
Cl
H
H
Cl
C
H
C
CH3
CH3
>
H
C
H
H
The resonance effect more than compensates the
negative inductive effect of Cl
107
H
Cl
Cl
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C
H
Q.34(g)
H
H –HBr
Br
(g)
H
H
H
H
C
C
Br
H
H
>
H
2
C
CH3
H
108
C
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1
H
H
C
H
C
H
CH3
C
H
The +ve charge is shared by the
benzene ring by resonance effect.
Stabilized by resonance effect as
well as inductive effect(2)
109
C
CH3
New Way Chemistry for Hong Kong A-Level 3A
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CH3
H
C
H
CH3
benzylic carbocation
C
CH3
H
More stable than 3 carbocation
110
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Q.34(h)
H2C
C
H
C
H
CH2
excess H – F
H
111
H
H
H
H
C
C
C
C
F
H
F
H
H
New Way Chemistry for Hong Kong A-Level 3A
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CH2H
H2C
C
H
2
H2C
C
CH2H
C
H
H
H
>
C
H2C
C
H
H
Allylic carbocation is stabilized by
resonance effect.
H
C
C
H
H
1
Stabilized by resonance effect as well as
inductive effect
Stability : Benzylic > allylic > 3 > 2 > 1 > CH3+
112
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CH 2H
CH2H
H2C
C
H
-
C
H2C
F
C
H
C
H
H
H–F
H
H
C
H
H
H
H
H
C
C
C
C
F
H
F
H
H
H
H
C
C
C
H
F
H
H
> H
H
H
H
H
H
C
C
C
C
F
H
H
less destabilized by –ve I-effect of F
113
F
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H
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.
114
New Way Chemistry for Hong Kong A-Level 3A
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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.
115
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.35
H
H
H3C
C
116
H3C
>
H
C
C
C
CH 3
CH 3
CF 3
C
>
C
H
H
C
>
C
H
CH 3
C
C
>
H
H
CH 3
H
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
H
H
H
Oxidation of alkenes
(a)
C
Combustion
C
+
O2
CO2 + CO + C + H2O
More sooty and luminous than that of
corresponding alkanes due to higher carbon
contents
117
New Way Chemistry for Hong Kong A-Level 3A
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(b)
Reaction with KMnO4
R1
R3
C
C
R2
R4
KMnO4, H+ or OH
cold
R2
R1
R3
C
C
OH
OH
R4
Used as a test for alkenes
R1
R3
C
R2
118
C
R4
KMnO4, H+ or OH
heat
R1
R3
C
R2
O
+
O
carbonyl
products
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C
R4
R1
R3
C
C
R2
KMnO4, H+ or OH
heat
R4
R1
R3
C
O
+
O
C
R2
R4
If all R groups are alkyl groups,
ketones will be the final products.
H3C
CH 3
C
H3C
119
C
CH 3
H3C
KMnO4, H+ or OH
heat
C
2
H3C
New Way Chemistry for Hong Kong A-Level 3A
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O
H3C
C
H3C
120
O
KMnO4, H+ or OH
heat
No reaction
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R1
R3
C
R2
C
KMnO4, H+ or OH
heat
R4
R1
R3
C
O
+
O
R2
C
R4
If either R1 or R2 is H / either R3 or R4 is H,
further oxidation of aldehydes to carboxylic
acid will occur.
H3C
C
H
121
H 3C
CH 3
KMnO4, H+ or OH
C
H
heat
2
C
H
New Way Chemistry for Hong Kong A-Level 3A
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O
H3C
H3C
C
O
KMnO4, H+ or OH
C
heat
H
HO
aldehyde
122
O
carboxylic
acid
New Way Chemistry for Hong Kong A-Level 3A
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R1
R3
C
C
R2
R4
KMnO4, H+ or OH
heat
R1
R3
C
O
O
+
C
R2
R4
If both R1 & R2 are H / both R3 & R4 are H,
further oxidation to first methanoic acid
and then CO2 will occur.
H
C
H
123
H
H
KMnO4, H+ or OH
C
heat
H
C
2
H
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
O
H
C
2
O
KMnO4, H+ or OH
heat
2CO2 + 2H2O
H
H
2
C
O
HO
methanoic acid
124
New Way Chemistry for Hong Kong A-Level 3A
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(c)
Ozonolysis
O
+
CH3CCl3
O3
< 20oC
O
O
unstable ozonide
O
+
O
O
H2O
Zn dust
2
CH3COOH
O
+
H2O2
Further oxidation of aldehyde to carboxylic
acid by H2O2 is inhibited using Zn dust and
CH3COOH
125
New Way Chemistry for Hong Kong A-Level 3A
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(c)
Ozonolysis
H3C
CH 3
C
C
H
H
H3C
H
C
H3C
126
C
H
1. O3
2. Zn dust / H2O
1. O3
CH 3
H3C
C
O
+ O
C
H
H
H3C
H
C
2. Zn dust / H2O
H3C
New Way Chemistry for Hong Kong A-Level 3A
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O
+ O
C
H
Oxidative cleavage can be used to locate C=C
bond in an unknown sample
127
New Way Chemistry for Hong Kong A-Level 3A
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CH 3
Q.36
H2C
C
CH 3
C
C
H
H
(3Z)-2-methylpenta-1,3-diene
CH 3
H2C
C
H
C
H
C
CH 3
(3E)-2-methylpenta-1,3-diene
128
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.36
H3C
CH 3
H
CH 2
H
(3E)-3-methylpenta-1,3-diene
H
H3C
H
CH 2
CH 3
(3Z)-3-methylpenta-1,3-diene
129
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.37
130
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.37
131
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Q.37
132
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The END
133
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New Way Chemistry for Hong Kong A-Level Book 3A
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
(a) It is a secondary alcohol.
(b) It is a primary alcohol.
(c) It is a tertiary alcohol.
134
Answer
Back
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
28.4 Preparation of Alkenes (SB p.173)
Back
Classify the following haloalkanes as primary, secondary
or tertiary haloalkanes.
(a)
(b)
(c)
(a) A secondary haloalkane
(b) A primary haloalkane
(c) A tertiary haloalkane
Answer
135
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28.5 Reactions of Alkenes (SB p.177)
Of the isomeric C5H11+ carbocations, which one is the
most stable?
+
The more stable C5H11 carbocation is the tertiary
carbocation as shown below:
Back
136
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Answer
28.5 Reactions of Alkenes (SB p.179)
Back
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 two
different types of halogenation?
Answer
Alkenes 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
137
New Way Chemistry for Hong Kong A-Level 3A
varies.
New Way Chemistry for Hong Kong A-Level Book 3A
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)
138
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.
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
28.5 Reactions of Alkenes (SB p.183)
(b) What is the major product of each of the following
reactions?
(i)
(ii)
Answer
139
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
28.5 Reactions of Alkenes (SB p.183)
(b)
(i)
(ii)
140
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28.5 Reactions of Alkenes (SB p.183)
(c)
Give the products for the following reactions:
Ni
(i) CH3CH = CH2 + H2 
conc. H2SO4
(ii) CH3CH = CHCH3 
(iii) CH3CH = CHCH3 + Br2 
141
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Answer
28.5 Reactions of Alkenes (SB p.183)
Back
(c)
(i)
CH3CH2CH3
(ii)
(iii)
142
New Way Chemistry for Hong Kong A-Level 3A
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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
143
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
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.
144
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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
145
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28.5 Reactions of Alkenes (SB p.184)
Back
(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:
146
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