Electrophilic Addition to Alkenes
E
X
+
E
X
+
E
X
Hydrohalogenation (addition of HX)
Markovnikov’s Rule of Electrophilic Addition:
Electrophile adds in such a way as to generate the
most stable cation intermediate. (Usually, such that
electrophile is bound to less substituted carbon.)
H
3
C
H C
H
H
H
Br
+
H
3
C
Br
CH
3
H
3
C
Br
+
H
3
C
H
H
3
C
selectively
not
observed
H
3
C
We say reaction is regioselective.
Anti-Markovnikov Addition w/
HBr + peroxide
HBr + peroxide
For most Markovnikov-rule electrophilic additions, there are
alternative conditions that generate anti-Markovnikov preference.
O
H
3
C
H
H
3
C
Br
O
O
O
O
ben o l per o ide
H
3
C
not
H
3
C
H
H
Br
+
H
3
C
benzoy l per oxide
H
3
C
Br
CH
3
selectively
not
observed
Markovnikov Addition of H
2
O
Hydration
H
2
SO
4
/H
2
O
Harsh and reversible
H C
H
H
3
C
H
3
C
OH
t
H
3
C
H
3
C
H
H
H
3
C
H
3
C
HO
CH
3
H
3
C
H
3
C
OH
selectively
not
observed
Hydroxymercuration-
D
ti
1. Hg(OAc)
2
, H
2
O
2. NaBH
4
Demercuration
Milder, higher yielding
Diastereoselectivity of
Hydroxymercuration-Demercuration
Ph
Ph
OH
1. Hg(OAc)
2
, H
2
O
2. NaBH
4
OH
Ph
+
(S)
(R)
H & OH add to
opposite faces
CH
3
H
CH
3
CH
3
H
(R)
(S)
of alkene
enantiomers
Ph
=
Reaction is diastereoselective: (R,R) or (S,S) diastereomers are
not produced
Markovnikov Addition of ROH
Alkoxymercuration-Demercuration
l
i
l
H
3
C
H
3
C
H
H
1. Hg(OAc)
2
, ROH
2. NaBH
4
H
3
C
H
3
C
RO
CH
3
selectively
H
3
C
H
H
3
C
Hg
OAc
δ+
M
i i
i
A
ith H O
H
3
C
H
H
3
C
H
δ+
O
H
Mercurinium ion
trapped by ROH
instead of H
2
O
As with H
2
O,
reaction is
stereospecific
O
R
H
Anti-Markovnikov Addition of H
2
O
Hydroboration
1 BH
3
·THF
H
3
C
H
3
C
OH
H
3
C
H
3
C
H
H
1. BH
3
THF
2. H
2
O
2
, OH
-
l
ti
l
H
3
C
H
3
C
HO
CH
3
selectively
not
observed
CH
3
CH
3
H
H
H
OH
CH
3
+
1. BH
3
·THF
2. H
2
O
2
, OH
-
H & OH add to
same face of
alkene
OH
H