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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:
Br
C16 H33
-
+ K+ O–C(CH3)3
+
C16 H33
E2
88%
C16H33
OC(CH3 )3
SN 2
12%
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

HH

H
K+ OEt
EtOH
H
H
OEt
+
25%
+
20%
14% cis41% trans55% overall
However using a bulkier base favours the less substituted alkene:
CH3 Br
CH3
'
HH


H
H
H
+
KOC2H5 in C2H5OH
71%
29%
KOC(CH3)3 in (CH3)3COH
28%
72%
KOC(C2H5)3 in (C2H5)3COH
11%
89%
Alkene formation may also proceed by an E1 (elimination,
unimolecular) mechanism which bears the same relationship to E2 that
SN1 bears to SN2:
E2: Rate = k x [RX] x [Base]
E1: Rate = k x [RX]
CH3
C
CH3
CH3
Slow
Br
CH3
+
C
Rate determining
step
:B
CH3 C
H
H H
Rapid
CH3
C
CH3
H
C
+
+ H:B
H
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:
CH3
Slow
C
CH3
Br
CH2CH3
CH3
+
C
Rate determining
step
CH3 C
H H
CH3
- H+ , Rapid
H
CH3
C
H
C
CH3
C
CH2CH3
CH3
CH3
+
+ H:B
C
H
The same factors which promote SN1 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 SN1 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 SN1.
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:
CH3
H
X
C
C
H
H
HX CH3
C
H
H
H
HX
C
H
CH3
X
H
C
C
H
H
H
HX = HCl, HBr, HI
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:
CH3
C
_
:X:
H
CH3
C
H
H
H
+
C C
H
: :
H
X
-
: :
+
H
CH3
H
X
H
C
C
H
H
H
_
:X:
H
CH3
C
H
+ H
C
H
CH3
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.
CH3
H
Br
H
HBr
CH3
HBr
CH3
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.
TS #1
TS #2
G2‡ Fast
+
CH3CH CH3
+ BrCarbocation
intermediate
Energy
Slow G ‡
1
RDS
CH3CH CH2
+ HBr
G0
CH3CHBrCH3
Progress of reaction
Remember that the order of stability of carbocations is:
H
H
CH3
CH3
C+
C+
C
H
Methyl
H
<
H
1°
H
<
CH3
+
C
CH3
2°
Increasing carbocation stability
+
CH3 CH3
<
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:
CH3
CH3
CH3
H
H
C
C
C
CH3
HCl
H
CH3
H
CH3
H
H
C
C
C
H
Cl
C
H
CH3
CH3
H
H
C
C
C
H+
H
H
Cl
H
C
C
H
H
H
50%
50%
H
H
3°
CH3
CH3
H
H
C
+ C
C
H
CH3
H
Cl
H
C
C
C
H
Cl
50%
1,2 hydride
(i.e. H )
CH3
CH3
+
C
shift.
2°
H
CH3
H
H
C
H
H
CH3
H
CH3
C
Cl
C
C
H
H
H
Cl
H
C
H
H
50%
H
H
1,2-alkyl (i.e. CH3-, carbanion) shifts to generate a more stable
carbocation are also possible:
CH3
CH3
CH3
H
C
C
C
H
H
H
+
3°
CH3
CH3
CH3
C
H
+ C
C
H
Cl
CH3
H
C
C
H
CH3
CH3
+
C
Cl
C
C
H
H
CH3 H
-
CH3
C
1,2 alkyl
(i.e. R )
shift.
2°
H
CH3
H
H
Cl
H
CH3
H
CH3
-
Cl
H
C
C
C
H
H
CH3 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 H2O to alkenes - Markovnikov hydration to
form alcohols:
OH
RCH2CH3
H3PO4
RCH
CH2 + H2 O
High temperature
Dehydration
RCH
CH2 + H2 O
OH
H2SO4
RCH2CH3
Low temp.
xs. H2O
Hydration
This is not a useful laboratory preparation of alcohols but is used
industrially for the preparation of t-butanol:
OH2
:
:
CH3
C
CH2
CH3
H2SO4
H3O
+
CH3
+
C
CH3
Markovnikov
CH3
2-methylpropene
OH2
:
:
CH3
C
CH3
CH3
OH
CH3
tert-butanol
+
- H3 O
C
CH3
+
O
H
H
CH3
In the laboratory the Markovnikov hydration of alkenes is usually
carried out indirectly via oxymercuration with mercury(II) acetate:
RCH
OH
(i) Hg(OAc)2 , H2 O, THF
CH2
RCHCH3
(ii) NaBH4
AcO– = CH3 CO2– , i.e. acetate
Hg(OAc)2
+
Hg(OAc)
Mercury(II) Acetate
Acetoxymercury(II) Cation
+ AcO
-
Electrophile
OAc
: Hg+
RCH
+
CH2
OAc
OAc
:Hg
Hg
RCH
+
CH2
RCH
OAc
:
:
RCH
CH2
HO
H
H
CH2
HgOAc
RCH
Mercurinium
cation
O
: Hg+
CH2
-H
HgOAc
+
RCH
CH2
H2O +
Na+ [BH4 ]– (Sodium borohydride, reducing agent)
RCH
HO
CH3
Overall reaction
H 2O
Markovnikov
RCH
CH2
(3) Electrophilic addition of borane (BH3) to alkenes - AntiMarkovnikov hydration to form alcohols:
-
RCH
CH2
(i) BH3 , (ii) H2 O2 , OH
[H2O]
H
OH
RCH–CH2
This synthesis of alcohols by addition of BH3 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
Br2
CH2
: Br Br
RHC
CH2
RHC
CH2Br
+ Br–
: Br
+
CH2
+
Br
Bromonium
CH2 cation
RHC
–
: Br :
:
Addition of bromine is
stereospecific and anti-.
N.B. The terms synand anti- refer to the
stereochemistry of the
reaction process while
cis- and trans- refer to
the stereochemistry of
the product.
:
Br
: Br
RHC
SN2
Br
CH2
RHC
–
: Br :
:
CH2
Br
:
Br2
RCHBr
H
H
+ Br
H
Br
Br
H
trans-1,2-dibromocyclopentane
In the transition state for the SN2 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 SN2 transition state for this reaction has partial
carbocationic (i.e. SN1) character and therefore nucleophilic attack is at
the most substituted carbon atom.