CH 6-8: Regioselectivity in E2 Reactions
•A reaction is regioselective if only “select” products are
formed from all possible constitutional isomers.
•How many alkene products can be formed in an E2
reaction with the alkyl halide below?
CH2
b2
CH3
b1
a
C
CH3
H
+
NaOH
E2
Br
•Locate the “a” carbon
•Locate ALL “b” carbons and “b” hydrogens
•A double bond may form between the “a” and “b” carbons,
with “elimination” of a b-hydrogen and the leaving group.
How many alkene products?
CH2
b2
a
C
CH3
CH3
C
b1
H
+
E2
NaOH
Br
H
CH2
CH3
b1
=3
H
H
C
C
H
H
CH3
C
CH3
b2 - trans
H
C
CH3
b2 - cis
H
C
CH3
Major Product
•Which is the “major” product? The most stable alkene!
This is the most highly substituted alkene (fewest sp2 H’s),
with the largest sp2 groups trans (least steric hindrance).
Alkyl Halide Reactivity in E2 Reactions
•If the most highly substituted alkene is the most stable
(major product), which of the following alkyl halides is
the most reactive in an E2 elimination?
CH3
CH3 C
CH3
CH3
Br
CH
3o
CH3
•The order of reactivity is:
Br
CH3 CH2
o
2
3o > 2o > 1o
•Note that this is opposite to the order of alkyl halide
reactivity for SN2 reactions.
Br
1o
Structure of the Base in E2 Reactions
CH2
b2
CH3
b1
a
C
CH3
+
small,
strong base
Br
H
Na OH
vs
bulky (large),
strong base
H
HO-
gives:
CH3
O K
C
b2
CH3
C
CH3
H
E2
O K
gives:
H
CH2
C
C
b1
H
H
•WHY? ……..Steric Hinderance
•A large, bulky base will grab the most accessible bhydrogen (usually a methyl b-hydrogen). A small base grabs
the b-hydrogen that gives the most stable alkene.