Chemistry
Properties of alkenes
Session objectives
1. Chemical reactions of alkene
2. Addition of hydrogen, halogen, hydrogen halides
3. Markownikoff’s rule
4. Anti-Markownikoff’s rule
5. Hydroboration-oxidation
6. Oxymercuration-demercuration
7. Oxidation of alkene
8. Ozonolysis
9. Conjugated dienes
Addition of Hydrogen
H /Ni
2
RCH= CH2 
 RCH2CH3
The ease of hydrogenation
CH2  CH2  RCH  CH2  RCH  CHR  R2C  CH2
Addition takes
predominantly cis manner.
Addition of Halogens
CH2 - CH2
CH2 = CH2 + Br2 
 |
|
Solvent Br
Br
CCl4
Brown
CH2  CH2  Br2
Colourless
H2O
Solvent
Test of unsaturation
Br — CH2 — CH2 — OH  HBr
Mechanism
Step I
C
:
C

Br
: ....... :
..

Br
:
C
C
..
:

Br
..
:
Br
Bromonium ion
Step II
B
r
C C
B
r
C C

: :
B
r
B
r
..
B
ro
m
o
n
iu
m
io
n
A
n
tia
d
d
itio
n
stereospecific reaction
–
Types of reaction
Stereospecific reaction
R1
R3
C
C
R1
CH2
C
R4 (Singlet)
R2
R3
R2
C
R4
Stereoselective reaction
Me
MeC
CEt
Et
Ni / H2
C
H
C
H
Cis-isomer (major)
Addition of hydrogen halides
CH2 = CH2 + HCI  CH3 CH2CI
CI
+
HCI
HI > HBr > HCI > HF
Markownikoff’s rule
Polar addition to an unsymmetrical
alkene takes place in such a manner
that the negative part of the addendum
attaches to that carbon atom which is
more highly substituted (or contains
the least number of H atoms)
Markownikoff’s rule
CH3
CH3
CH3
2
C
1
At Position
(1)
CH2
CH3

C
CH3
Br

3°
more stable cation
CH3
CH3
C
CH3
Br
Major Product
CH3
At Position
(2)
CH3
CH

Br 
CH2 

1°
less stable cation
CH3
CH3
CH
CH2 Br
Minor Product
Markownikoff’s rule
If HBr is added to a symmetrical
alkene, only one product is obtained.
CH3 CHCH2 CH3
CH3
CH
CH
CH3
HBr
Br
CH3 CH2 CHCH3
Br
Markownikoff’s addition product depends on the stability
of intermediate carbocation.
Markownikoff’s rule
CH3
CH
CH
NO2 + HB
CH3
CH2
CH
NO2
Br
(Minor)
CH3
CH
CH2
Br
(Major)
NO2
Markownikoff’s rule
Mechanism
+
CH3
CH
CH
NO2 + H+
CH3
CH
Major
I
+
+
CH3
CH2
CH
Minor
II
NO2 group at p – position withdraws electron
cloud from benzene ring.
NO2
CH
NO2
Markownikoff’s rule
+
CH3
CH
CH
NO2
Br–
CH3
Major
I
CH2
Minor
II
CH
NO2
Br
Major
+
CH3
CH
CH
NO2
Br–
CH3
CH2
CH
Br
Minor
NO2
Peroxide effect (Anti
Markownikoff’s addition)
CH3
CH
HBr
CH2
Peroxide
CH3
CH2
CH2 Br
Propyl bromide
Mechanism:
H2O2
h
2 OH
Hydroxide
free
OH + HBr
H2O + Br
Br + CH3 — CH
CH2
CH3 — CH — CH2Br
2° stable free radical
CH3 — CH — CH2Br
H
CH3CH2CH2Br
Major product
Addition of water (Hydration)
Acid catalyzed addition of water
CH3
CH
CH2
+ H2O
65-70% H2SO4
CH3
CH
OH
CH3
CH3
C CH2

H2 O/H
CH3
CH3
C CH3
OH
CH3
Hydroboration–oxidation
3CH2 = CH2
H2O2 /OH-
BH3
  CH3CH2 3 B 

Triethylborane
The addition is anti–Markownikoff
3CH3CH2OH
Ethyl alcohol
Hydroboration–oxidation
CH3
CH3
CH3
C
CH
CH2
C
CH3
(i) BH3
(ii) H2O2/OH
CH3
CH3
CH3
CH CH2 OH
(i) (BH3 )2

(ii) H2 O2 /OH
CH3
CH
CH
OH
CH3
CH3
In a substituted cyclic alkene, a trans alcohol is formed.
H3C
CH3
H
(i) BH3
(ii) H2O2/OH-
OH
H
trans - Methyl cyclo-pentanol
Oxymercuration–demercuration
H2C
CH2
Hg(OAC)2
THF, H2 O
CH2 CH2
Hg OAC
OH
NaBH4
OH

CH3 CH2 OH + Hg
Oxidation of alkenes
Test for unsaturation
Baeyer’s test
3
C
C
+ 2KMnO4 + 4H2O
Cold
3
C
C
OH OH
• This is also called Hydroxylation
• cis-diol
+ 2 MnO2 + 2 KOH
Oxidation of alkenes
Cis- hydroxylation
CH3
CH
CH2
(i) OsO4
(ii) NaHSO3
CH3
CH
CH2
OH
OH
Oxidative cleavage of alkenes
(i) Alk.KMnO4
CH3
CH
CH2
(ii) H+ 
CH CH
3
CH3 COOH + CO2 +
(i) Alk.KMnO4
CH CH3
(ii) H+ 
CH C
3
CH3
H2O
CH CH3
(i) Alk.KMnO4
(ii)
H+ 
CH3 COOH + CH3COOH
CH3
C
CH3
O + CH3COOH
Oxidation with peroxide
O
C
C
+
C6H5CO3H
CH2Cl2
Perbenzoic acid
OH
OH–
C
C
H2O
trans-diol
OH
Ozonolysis of alkenes
CH3CH2CH
CCH3
CH3
O3
CH3
O
CH3CH2CH
O
Zn/H2O
C
O
CH3
Ozonide
O
O
CH3CH2CH + CH3CCH3 + Zn(OH)2
Do you know
Write IUPAC name of alkene which on ozonolysis gives
one molecule of propanal and one molecule of propanone
Addition of methylenes
Cl
+ : CCl2
H
H
Cl
Dichlorocarbene
Cl
Cl
-
H
Base
H
Cl
- Cl 
Chlorobenzene
Conjugated dienes (alkadienes)
H
H
C
H
C
C
H
C
H
H
CH2
CH
CH
CH2
Br2
CH2
Br
CH
CH
Br
1,2-addition
CH2 + CH2
CH
CH
CH2
Br
Br
1,4-addition
Conjugated dienes
(alkadienes)
CH2
CH
CH
CH2
-80o
HBr
CH3 CH CH
CH2 + CH3 CH
Br
80 %
40o
CH3 CH CH
Br
20%
CH CH2Br
20%
CH2 + CH3 CH
CH CH2 Br
80 %
Class exercise
Class exercise 1
If singlet carbene is added to the
ethylene, the product formed will be
(a) propane
(b) butane
(c) cyclopropane
(d) cyclobutane
Solution:
Cyclopropane
Hence the answer is (c)
Class exercise 2
Hydroboration-oxidation of 1-methyl
cyclopentane will give
(a) trans-1-methyl cyclopentan-2-ol
(b) cis-1-methyl cyclopentan-2-ol
(c) Mixture of (a) and (b)
(d) 1-methylcyclopentan-1-ol
Solution:
cis-1-methyl cyclopentan-2-ol
Hence the answer is (b)
Class exercise 3
Anti-Markownikoff’s addition of HBr
is not observed in
(a) But-2-ene
(b) Propene
(c) But-1-ene
(d) Pent-2-ene
Solution:
As but-2-ene is a symmetrical structure,
we get same product while antiMarkownikoff’s rule applies.
Hence the answer is (a)
Class exercise 4
Electrophilic addition of Br2 to
ethylene proceeds through
(a) a transition state
(b) a cyclic bromonium ion

(c) CH2  CHBr ion

(d) CH2  CHBr ion
Solution:
A three-centred cyclic bromonium ion is formed.
Hence the answer is (a)
Class exercise 5
Predict the major product of each of
the following reaction.
CH3
+ HBr
Solution:
CH3
Br
Peroxide
Class exercise 6
Predict the major product of each
of the following reaction.
CH=CH2
(i) BH3
(ii) H2O2/OH-
Solution:
CH2 CH2OH
Class exercise 7
Predict the major product of each
of the following reaction.
CH3
(i) O3
(ii) Zn/H2O
Solution:
O
CHO
Class exercise 8
Write a mechanism for each of the
following reaction.
H
OH
O
Solution:
+
O—H
+
O
H
+
–H
O
CH3
Class exercise 9
A hydrocarbon (R) adds one mole of
hydrogen to form n-hexane . When R is
oxidized vigorously with hot alk. KMnO4
followed by acid treatment it yields a
single carboxylic acid containing
3 carbons. Give the structure of R
Solution:
R: CH3CH2CH = CHCH2CH3
Class exercise 10
An organic compound (A) having
molecular formula decolourizes
bromine water.On hydrogenation (A)
yields 2-methylbutane and reductive
ozonolysis forms ethanal and propanone.
What is A ?
Solution:
A:
Thank you