Alkenes and Alkynes
Addition Reactions
© E.V. Blackburn, 2011
Reactions of alkenes catalytic hydrogenation
Pt, Pd
+ H2
or Ni
H C C H
Heat of hydrogenation - the heat liberated during this
reaction. H is ~125 kJ/mol for each double bond in the
compound.
© E.V. Blackburn, 2011
Mechanism of alkene
hydrogenation
H2
catalyst
H H
H H
H2 adsorbed
at surface
H H
complexation of
alkene to catalyst
hydrogen insertion
+
catalyst
H
H
alkane
© E.V. Blackburn, 2011
Mechanism of alkene
hydrogenation
Hydrogenation is stereospecific.
The two hydrogens add to the same side of the
double bond - a syn addition.
© E.V. Blackburn, 2011
Reactions of alkenes

C
C

The  electrons are less tightly held than the 
electrons. The double bond therefore acts as a
source of electrons - a base – a nucleophile.
It reacts with electron deficient compounds - acids electrophiles.
© E.V. Blackburn, 2011
Electrophilic addition
Electron seeking reagents are called electrophilic reagents.
The typical reaction of alkenes is one of electrophilic
addition - an acid - base reaction.
C C + YZ
C C
Y Z
Don’t forget that free radicals are electron deficient. They
undergo addition reactions with alkenes.
© E.V. Blackburn, 2011
Addition of hydrogen halides
+ HX
H C C X
HX = HCl, HBr, HI
© E.V. Blackburn, 2011
Addition of hydrogen halides
CH3CH2Cl
H2C=CH 2 + HCl
only one product is possible, chloroethane
but....
CH3-CH=CH 2
CH3-CH=CH 2
?
CH3CH2CH2Cl
H-Cl
Only 2-chloropropane is formed
© E.V. Blackburn, 2011
Markovnikov’s rule
H
CH3
H
CH3
HCl
CH3
H3C C CH3
Cl
In 1869, Markovnikov proposed that in the addition
of an acid to an alkene, the hydrogen of the acid
bonds to the carbon which is already bonded to the
greater number of hydrogens.
© E.V. Blackburn, 2011
Markovnikov’s rule
CH3CH2CH=CHCH 3 + HI
CH3CH2CHICH 2CH3 + CH3CH2CH2CHICH3
Each carbon of the double bond is bonded to one H
therefore both isomers are formed.
© E.V. Blackburn, 2011
Markovnikov addition - a
regioselective reaction
These reactions are said to be regioselective because
only one of the two possible directions of addition occurs.
Regioselectivity - the preferential formation of one isomer
in those situations where a choice is possible.
© E.V. Blackburn, 2011
HBr - the peroxide effect
1933, Kharasch and Mayo
absence of
CH3
HBr
CH3-C=CH 2
peroxides
(-O-O-)
presence of
peroxides
CH3
H3C C CH3
Br
CH3
H3C C CH2Br
H
© E.V. Blackburn, 2011
Addition of sulfuric acid
CH3CH=CH 2
cold
H 2SO4
80%
CH3CHCH 3
OSO3H
H2O

CH3CHCH 3
OH
© E.V. Blackburn, 2011
Hydration
H
H
CH3
+ H 2O
CH3
H+
H CH3
H C C CH3
H OH
a Markovnikov addition
© E.V. Blackburn, 2011
The mechanism of the
addition
1.
C C
H +
+ X-
H X
2.
C C
H +
+ X-
C C
H X
HX = HCl, HBr, HI, H 2SO4, H3O+
© E.V. Blackburn, 2011
An example
1.
H3C
H
H
H
H H
H3C C C H + Cl
+
H
H Cl
H H
2. H3C C C H + Cl
+
H
H H
H3C C C H
Cl H
© E.V. Blackburn, 2011
Orientation
+
CH3CHCH 3
Cl
-
Cl
CH3CHCH 3
HCl
CH3CH=CH 2
X
+
CH3CH2CH2
© E.V. Blackburn, 2011
Orientation
CH3
CH3
CH3CCH 3
+
HCl
CH3C=CH 2
CH3
X
CH3CH=C
+
CH3CHCH 2
CH3
CH3CH2CCH 3
+
CH3 HCl
CH3
Cl
CH3
CH3CCH 3
Cl
-
Cl
-
CH3
CH3CH2CCH 3
Cl
CH3
X
+
CH3CHCCH 3
H
© E.V. Blackburn, 2011
A more general “rule”
Electrophilic addition to a carbon - carbon double
bond involves the intermediate formation of the most
stable carbocation.
Why? Let’s look at the transition state:-
+ H+
C C
H +
+
C C
H +
© E.V. Blackburn, 2011
A more general “rule”
Ea > Ea
E
CH3CH2CH2+
CH3CHCH3
+
CH3CH=CH 2 + H+
© E.V. Blackburn, 2011
Carbocation rearrangements
CH3
H3C
CH =CH2
CH3
CH3
H3C +CH -CH3
CH3
HCl
CH3
H3C +CH -CH3
CH3
CH3 H
H3C
Cl-
+
CH3
CH3
CH3
CH3
CH3 H
CH3 H
H3C
+
H3C
Cl
CH3
CH3
© E.V. Blackburn, 2011
Oxymercuration
© E.V. Blackburn, 2011
Oxymercuration
An anti addition via a mercurinium ion:
Dissociation: Hg(OAc) 2
CH3CO2-
+
+ HgOCOCH 3
+
HgOCOCH 3
Electrophilic
attack:
CH3
+
HgOCOCH 3
+
HgOCOCH 3
Nucleophilic
opening:
CH3
H-O-H
CH3
CH3
OH
Hg OCOCH 3
H
© E.V. Blackburn, 2011
Oxymercuration
Why do we observe Markovnikov addition?
+
HgOAc


HgOAc
In the mercurinium ion, the positive charge is shared
between the more substituted carbon and the mercury
atom.
Only a small portion of the charge resides on this carbon
but it is sufficient to account for the orientation of the
addition but is insufficient to allow a rearrangement to
occur.
© E.V. Blackburn, 2011
Hydroboration
H.C. Brown and G. Zweifel, J. Am. Chem. Soc., 83, 2544
(1961)
H2O2
+ (BH3)2
diborane
H B
OH
-
+ B(OH) 3
H OH
Brown was co-winner of the 1979 Nobel Prize in
Chemistry.
© E.V. Blackburn, 2011
Hydroboration
1. (BH3)2
H3C
2. H2O2/OH-
3HC
H
syn
addition
H OH
trans-2-methylcyclopentanol
(CH3)3CCH=CH2
(CH3)3CCH2CH2OH
no rearrangement
no carbocation!
© E.V. Blackburn, 2011
Hydroboration
(BH3)2
H2C=CH 2
CH3CH2BH2
H2C=CH2
(CH3CH2)2BH
H2C=CH 2
3C2H5OH + B(OH) 3
H2O2
OH-
(CH3CH2)3B
© E.V. Blackburn, 2011
Hydroboration - the
mechanism
CH3CH=CH 2
CH3CH=CH2
HX
1. (BH3)2
-
2. H2O2/OH

CH3CH
CH3CH2CH2OH
CH2
+
CH3CHCH3 + X-
H
X 
© E.V. Blackburn, 2011
Hydroboration - the mechanism
CH3CH=CH 2
1. (BH3)2
-
2. H2O2/OH

CH3 > CH

CH3 > CH
H
CH3CH2CH2OH
CH2

H B H
H
CH2

B H
H
© E.V. Blackburn, 2011
Hydroboration - the mechanism
R
R B
O-OH
R
R
R -B O OH
R
RO
B OR
RO
R
R -B O OH
R
R
+
HO
B OR
R
HO -
3ROH + BO 33-
© E.V. Blackburn, 2011
Addition of halogens
+ X2
X2 = Cl2, Br2
CH3CH=CH 2
X C C X
usually iodine does
not react
Br2 in
CH3CHBrCH 2Br
CCl 4
1,2-dibromopropane
© E.V. Blackburn, 2011
Mechanism of X2 addition
C C
+
X
+ X-X
C C
+
X
+ X-
C C
X X
+ X-
+ X X
polarisation
© E.V. Blackburn, 2011
Mechanism of X2 addition
H
H
H
Br2
H
CH2BrCH2+
NaCl
no reaction
Br-
CH2BrCH2Br
NaCl
CH2BrCH 2Cl
© E.V. Blackburn, 2011
Stereospecific reactions
CH3CH=CHCH 3
Br2
*
*
CH3CHBrCHBrCH 3
(Z)-2-butene gives racemic 2,3-dibromobutane and no
meso compound is formed.
(E)-2-butene gives only meso-2,3-dibromobutane.
A reaction is stereospecific if a particular stereoisomer of
the reactant produces a specific stereoisomer of the
product.
© E.V. Blackburn, 2011
syn and anti addition
Y
syn
Z
Y-Z
anti
Y
Z
© E.V. Blackburn, 2011
Bromine addition - an anti
addition
I. Roberts and G.E. Kimball, J. Am. Chem. Soc., 59, 947
(1937)
+
1. Br Br
2.
+
Br
C C
Br
-
Br
C C
+
bromonium
ion
+ Br-
Br
C C
Br
© E.V. Blackburn, 2011
The bromonium ion
Br
H
+
CH3
H
CH3
H
Br-Br
H
CH3
CH3
H
Br
H
+
Br
Br-
H
(S,S)
H
H
Br
H
Br
H
+
Br
-
Br
Br
(R,R)
H
© E.V. Blackburn, 2011
Halohydrin formation
+ X 2 + H 2O
+ HX
C C
X OH
X2 = Cl2, Br2
© E.V. Blackburn, 2011
Halohydrin formation
greater
+ here
Br2
CH3-CH=CH 2
+
Br
H3CHC CH2
CH3CH-CH 2Br
O+
H H
H2O
CH3CH-CH 2Br
O+
H H
+
Br
H3CHC CH2
+
-H
1-bromo-2-propanol
© E.V. Blackburn, 2011
Unsymmetric electrophiles
In the electrophilic addition of an unsymmetric reagent,
the electrophilic part adds to the less substituted carbon
of the alkene unit:
CH3CH=CH 2

+ A-B
CH3CH-CH 2
B
A
© E.V. Blackburn, 2011
Free radical addition reactions
peroxides
+ Y-Z
1. peroxides
c C
Y Z
or h
Rad
chain
initiation
2. Rad + HBr
3. Br
RadH + Br
+
Br
+ HBr
4.
Br
propagation
+ Br
Br H
© E.V. Blackburn, 2011
ionic vs radical addition
+
CH3CHCH 3
HBr
CH3CH=CH 2
X
X
CH3CHCH 3
Br
+
CH3CH2CH2
CH3CHCH 2Br HBr
Br
CH3CH=CH 2
Br-
CH3CH2CH2Br
CH3CHBrCH 2
© E.V. Blackburn, 2011
Polymerization
A polymer is a long chain molecule made up of
structural units (monomers) joined together.
© E.V. Blackburn, 2011
Free radical polymerization of
alkenes
Initiation
peroxide
Rad + CH2=CH
G
Rad
Rad-CH 2-CH
G
© E.V. Blackburn, 2011
Free radical polymerization of
alkenes
chain propagation
Rad-CH 2-CH + CH2=CH
G
Rad-CH 2-CH-CH 2-CH
G
G
G
CH2=CHG
Rad-CH 2-CH-CH 2-CH-CH 2-CH
G
G
etc
G
© E.V. Blackburn, 2011
Free radical polymerization of
alkenes
Chain termination
combination
.
2 Rad(CH 2-CH) n-CH2-CH
G
G
Rad(CH 2CH) nCH2CHCHCH 2(CH-CH 2)nRad
G
G G
G
© E.V. Blackburn, 2011
Free radical polymerization of
alkenes
Chain termination
disproportionation
.
Rad(CH 2-CH) n-CH2-CH
G
Rad(CH 2CH) n CH2CH2
G
G
+
G
Rad(CH 2CH) nCH=C H
G
G
© E.V. Blackburn, 2011
Examples
G
Cl
CN
monomer
polymer
CH2=CHCl
-CH2CHCl-CH2CHCl-CH2-
vinyl chloride
polyvinyl chloride, PVC
CH2=CHCN
-CH2CHCN-CH2CHCN-
acrylonitrile
polyacrylonitrile
Orlon, Acrilon
© E.V. Blackburn, 2011
Examples
G
Ph
monomer
CH2=CHPh
styrene
polymer
-CH2CHPh-CH2CHPhpolystyrene
Ph =
CH3
C
CO2CH3
CH3
CH3
CH3
-CH2-C - CH2 - C-
CH2=C
CO2CH3
methyl methacrylate
CO2CH3 CO2CH3
poly(methyl methacrylate)
Plexiglas, Lucite
© E.V. Blackburn, 2011
Polymerization
The addition of other compounds can modify the
polymerization:
.
-CH2-CH + CCl4
. CCl
-CH2-CHCl + . CCl3
.
3
+ CH2=CH
Cl3C-CH2-CH
styrene
polymer
© E.V. Blackburn, 2011
Carbenes
- +
 or h
H 2C N N
diazomethane
RO- K+ + H-CCl 3
H2C: + N2
methylene
ROH + K + + Cl- + Cl2C:
dichlorocarbene
CH2I2 + Zn(Cu)
ICH2ZnI
a carbenoid
Carbenes (and carbenoids) add to alkenes in a
stereospecific manner to form cyclopropanes.
© E.V. Blackburn, 2011
Carbenes
KOC(CH 3)3
CHBr 3
H
H
Br
Br
H
CH2I2/Zn(Cu)
H
© E.V. Blackburn, 2011
Hydroxylation
KMnO 4,
OH
HCO 2OH
or OsO4
OH
a glycol
© E.V. Blackburn, 2011
Ozonolysis
+ O3
O
O
+O O-
O
O O
O
molozonide
H2O/Zn
O
O O
aldehydes and ketones
ozonide
© E.V. Blackburn, 2011
Ozonolysis
CH3CH2CH=CHCH 3
1. O3
2. Zn/H2O
H
H
CH3CH2C=O + O=CCH 3
H
CH3 1. O3
CH3CHO + O
C C
H3C
CH3 2. Zn/H2O
CH3
CH3
© E.V. Blackburn, 2011
KMnO4 oxidation
H
CH3
C C
H3C
CH3
KMnO 4
CH3CH2CH=CH 2
CH3CO2H + O
KMnO 4
CH3
CH3
CH3CH2CO2H + CO2
© E.V. Blackburn, 2011
Addition of halogen to alkynes
C C
X2
X
C C
X
X2
X X
X X
© E.V. Blackburn, 2011
Addition of HX to alkynes
Cl HI
HCl H
H3C C C H
C C
H
CH3
H I
H
CH3
H Cl
Does this seem reasonable?
H
Cl
C C
H
CH3
H
H
+
H Cl
H
+
H
v CH3
H Cl
This carbocation should be
destabilized by the inductive
effect of the Cl!
H
CH3
H
CH3
H Cl
+
The cation is stablized by
resonance! Remember
this in CHEM 263
© E.V. Blackburn, 2011
H2, Pd/CaCO3
1.
quinoline
Na, NH3, -78oC
H H
H
H
1. Hg(OAc)2/T HF-H2O
2. CH3CH=CH2
CH3CH(OH)CH3
2. NaBH4
CH3CH=CH2
1. B2H6
2. H2O2/OH-
3. CH3CH=CH2
CH3CH=CH2
HBr/-O-OHBr/-O-O-
CH3CH2CH2OH
CH3CH2CH2Br
CH3CHBrCH3
© E.V. Blackburn, 2011