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Chapter 9 319
Copyright © 2017 Pearson Education, Inc.
21.
The rate of an
S
N
1
reaction is not affected by increasing the concentration of the nucleophile, whereas the
rate of an
S
N
2
reaction is increased when the concentration of the nucleophile is increased. Therefore, we
first have to determine whether the reactions are
S
N
1
or
S
N
2
reactions.
A
is an
S
N
2
reaction because the reactant is a secondary alkyl halide (and the configuration of the
product is inverted compared to that of the reactant).
B
is an
S
N
2
reaction because the reactant is a primary alkyl halide.
C
is an
S
N
1
reaction because the reactant is a tertiary alkyl halide.
Because they are
S
N
2
reactions, the rate of
A
and
B
increases if the concentration of the nucleophile is increased.
Because it is an
S
N
1
reaction, the rate of
C
does not change if the concentration of the nucleophile is increased.
22.
a.
CH
3
C CHCH
3
CH
3
b.
CH
3
C CHCH
3
CH
3
c.
CH
3
CH
2
CCH
CH
3
CH
3
CH
2
23.
a.
CH
3
CH CHCH
3
Removal of a hydrogen from the more substituted
b
-carbon forms the
more stable “alkene-like” transition state.
b.
CH
3
C CHCH
2
CH
3
CH
3
Removal of a hydrogen from the more substituted
b
-carbon forms the
more stable “alkene-like” transition state.
c.
CH
3
CH CHCH CH
2
The hydrogen is removed from the
b
-
carbon that leads to a conjugated
alkene.
d.
CH
2
CHCH
2
CH
3
Removal of a hydrogen from the less substituted
b
-
carbon forms the
more stable “carbanion-like” transition state.
e.
The hydrogen is removed from the
b
-carbon that leads to a conjugated
alkene.
f.
CH
3
CHCH CHCH
3
CH
3
Removal of a hydrogen from the less substituted
b
-
carbon forms the
more stable “carbanion-like” transition state.
24.
a.
Br
The alkene (2-butene) that is formed is more stable than the alkene
(1-butene) that is formed from the other alkyl halide.
b.
Br
Br
-
is a better leaving group (weaker base) than
Cl
-
.
c.
Br
The alkene (2-methyl-2-pentene) that is formed is more stable than
the alkene (4-methyl-2-pentene) that is formed from the other alkyl
halide.
d.
Cl
The other alkyl halide cannot undergo an E2 reaction, because it does
not have any
b
-hydrogens.