poor nucleophile because it will experience difficulty in approaching the carbon bearing the leaving group. Hence sterically bulky bases such as potassium t-butoxide (the potassium salt of t-butanol) favour

elimination over substitution:
C
16
H
33
Br
+ K +
-
O–C(CH
3
)
3
C
16
H
33
E2
88%
+
C
16
H
33
S
N
2
12%
OC(CH
3
)
3
Where more than one elimination product can be formed the more highly substituted ( i.e.
more stable) alkene is normally favoured
(Zaitsev's rule):

 '
H Br


H H H
H
H
K
+
-
OEt
EtOH
OEt
+ +
25% 14% cis-
41% trans-
55% overall
20%
However using a bulkier base favours the less substituted alkene:
CH
3
CH
3
 '

Br

H
H
+
H H H
KOC
2
H
5
in C
2
H
5
OH 71% 29%
KOC(CH
3
)
3
in (CH
3
)
3
COH 28% 72%
KOC(C
2
H
5
)
3 in (C
2
H
5
)
3
COH 11% 89%
Alkene formation may also proceed by an E1 (elimination, unimolecular ) mechanism which bears the same relationship to E2 that

S
N
1 bears to S
N
2:
E2 : Rate = k x [ RX ] x [ Base ]
E1 : Rate = k x [ RX ]
CH
3
C Br
CH
3
CH
3
Slow
Rate determining step
CH
3
C
+
CH
3
H
C
H
H
: B
Rapid
CH
3
C C
H
CH
3
H
+ H : B
+
Unlike the case of the E2 mechanism - there is no steric requirement for the conformation of the substrate in an E1 reaction.
Where more than one possible alkene can result from deprotonation of

the intermediate carbocation then Zaitzev's rule operates:
CH
3
C Br
CH
3
CH
2
CH
3
Slow
Rate determining step
CH
C
+
3
CH
3
H
C
H
CH
3
- H
+
, Rapid
H
C C
CH
3
H CH
2
CH
3
CH
3
C C
CH
3
+ H : B
+
CH
3
H
The same factors which promote S
N
1 substitution in alkyl halides also favour E1 elimination of HX - i.e.
a high degree of alkyl substitution at the carbon atom bearing the leaving group, a good leaving group and a polar solvent . Hence the S
N
1 and E1 mechanisms usually compete in the case of the reactions of nucleophiles with tertiary halides or tosylates. Using a sterically hindered (and therefore poorly nucleophilic) base will tend to promote E1 over S
N
1 .

Reactivity of Alkenes:
H
H
C C
H
H
Electron-rich  -cloud: site for reaction with electrophiles
Electrophilic addition of polar reagents to carbon-carbon double bonds:
 +  -
C C + X Y
X Y
C C
(1) Electrophilic addition of HX to alkenes - regiospecific Markovnikov addition to form alkyl halides :
CH
3
H X
C C
H H
H
H X CH
3
C
H
C
H
H
H
HX = HCl, HBr, HI
X
CH
3
X H
C C
H H
H
Why does this reaction produce only one of the two possible

structurally isomeric products?.

The reaction is a two step bimolecular process and proceeds via a carbocation intermediate:
 +
H X
 -
: X :
_
CH
3
C
H
C
H
H
CH
3
H
+
C C
H
H
H
CH
3
X H
C C
H H
H
: X :
_
H
CH
3
H
H
H
CH
3
H X
C C
H H
H
Long before anything was known about the mechanism of this reaction it was recognised that ' Addition of HX to an alkene will proceed in such a way as to attach hydrogen to the least substituted carbon and X to the most substituted carbon' .
This is known as Markovnikov's Rule after the Russian chemist who first put it forward.
CH
3
H
H
Br
HBr CH
3 HBr
CH
3
Br
H
H
Markovnikov's Rule can now be restated: Addition of HX (or any other polar species) to an alkene will take place in such a way as to produce the most stable - i.e. the most highly substituted - carbocation intermediate .

Slow
RDS
 G
1
‡
TS #1
TS #2
CH
3
+
CH CH
3
+ Br
-
Carbocation intermediate
 G
2
‡
Fast
CH
3
CH CH
2
+ H Br  G 0
CH
3
CH Br CH
3
Progress of reaction
Remember that the order of stability of carbocations is:
H
C
+
H H
Methyl <
CH
3
C
+
H
1°
H
<
H
CH
3
C
+
2°
CH
3
<
Increasing carbocation stability
CH
3
C
+
CH
3
3°
CH
3

Another feature of polar additions is structural rearrangement - a process in which a compound or intermediate changes its structure without changing its composition. The driving force is the formation of the more stable carbocation:
H H
CH
3
H
CH
3
CH
3
H
C
C
H
H
C
H
H Cl
CH
3
CH
3
CH
3
C C
C
Cl
H Cl
H
C
C
C
H
H
H
50%
50%
H H
H H
CH
3
CH
3
C
C
H
C
H
+
H
CH
3
CH
3
CH
3
CH
3
H H
H
C +
C
C
H
H
C
H
Cl
-
H
2°
H
C
C H
H Cl
50%
1,2 hydride
( i.e. H
-
) shift.
CH
3
CH
3
CH
3
CH
3
3°
H
+
C
Cl
H
C
C
H
Cl
-
H
C
C
H H
50%
C
H
H
H
H

1,2-alkyl ( i.e. CH
3
-
, carbanion) shifts to generate a more stable carbocation are also possible:
CH
3
CH
3
CH
3
C
C
H
H
C
H
+
H
CH
3
CH
3
CH
3
C +
C
H
Cl
-
H
C
2°
H
H
1,2 alkyl
( i.e. R
-
) shift.
CH
3
CH
3
3°
+
C
CH
3
C
H
H
C
Cl
-
H
H
CH
3
CH
3
CH
3
H
C
C
H Cl
C
H
H
CH
3
CH
3
Cl H
C
CH
3
C
H
C
H
H
Note that in these rearrangements the migrating atom (H) or group (R) carries a bonding electron-pair along with it when it moves, i.e.
the migrating species is to be regarded as either a hydride anion (H ) or as a carbanion (R ) .

(2) Electrophilic addition of H
2
O to alkenes - Markovnikov hydration to form alcohols :
R
OH
CH
2
CH
3
RCH CH
2
H
3
PO
4
High temperature
Dehydration
RCH CH
2
+ H
2
O
H
2
SO
4
Low temp.
xs. H
2
O
Hydration
+ H
2
O
OH
R CH
2
CH
3
This is not a useful laboratory preparation of alcohols but is used industrially for the preparation of tbutanol:
CH
3
C CH
2
CH
3
2-methylpropene
CH
3
C
CH
3
OH
CH
3 tertbutanol
H
2
SO
4
H
3
O
+
- H
3
O
+
:
:
OH
2
CH
3 +
C CH
3
CH
3
CH
CH
3
3
C
:
:
OH
2
+
O
H
CH
3
H
Markovnikov

In the laboratory the Markovnikov hydration of alkenes is usually carried out indirectly via oxymercuration with mercury(II) acetate:
OH
RCH CH
2
(i) Hg(OAc)
2
, H
2
O, THF
(ii) NaBH
4
AcO – = CH
3
CO
2
– , i.e.
acetate
RCHCH
3
Hg(OAc)
2
Mercury(II) Acetate
+
Hg(OAc) + AcO
-
Acetoxymercury(II) Cation
OAc
: Hg
+
RCH CH
2
OAc
+
: Hg
RCH CH
2
OAc
OAc
: Hg
+
H
:
O
:
RCH CH
2
Mercurinium cation
H
RCH CH
2
HgOAc
- H
+
RCH CH
2
HO
HgOAc
RCH CH
2
H
2
O +
Na
+
[BH
4
]
–
(Sodium borohydride, reducing agent)
RCH CH
3
HO
Overall reaction
H
2
O
Markovnikov
RCH CH
2

(3) Electrophilic addition of borane (BH
3
) to alkenes - Anti-
Markovnikov hydration to form alcohols :
RCH CH
2
(i) BH
3
, (ii) H
[H
2
O]
2
O
2
, OH
-
H OH
RCH – CH
2
This synthesis of alcohols by addition of BH
3
to alkenes
(hydroboration) followed by oxidation will be dealt with as part of
Module CM2005.
(4) Electrophilic addition of halogens to alkenes - Formation of 1,2dihaloalkanes :

RCH CH
2
Br
2
RCH Br CH
2
Br
:
Br Br
RHC CH
2
Addition of bromine is stereospecific and anti.
N.B. The terms synand antirefer to the stereochemistry of the reaction process while cis and trans refer to the stereochemistry of the product.
+
: Br
+ Br –
RHC CH
2
:
Br
Br
RHC CH
2
: Br
–
:
Br
2 H H
Br
+
RHC CH
Bromonium cation
2
: Br
–
:
S
N
2
Br
RHC CH
2
Br
H Br
+ Br Br H trans -1,2-dibromocyclopentane
In the transition state for the S
N
2 attack of a nucleophile on a cyclic mercurinium or bromonium cation bond-breaking of the C-Hg or C-Br bond is more advanced than bond-formation with the incoming nucleophile . Hence the S
N
2 transition state for this reaction has partial carbocationic ( i.e.
S
N
1) character and therefore nucleophilic attack is at the most substituted carbon atom.