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750 Chapter 22
Copyright © 2017 Pearson Education, Inc.
39.
− + +
+
+ +
+
+
−
+
+ +
+
+
+
H
N
H
H
N
H
N
N
N
N Zn O
O
O
C
H
N
H
N
H
N
H
N
H
H
N
H
N
N
N
N
N
N
N
Zn
Zn
O
O
O
C
−
+
HO
H
2
O
O
OC
H
O H
Water exchanges
for bicarbonate.
40.
At pH
=
12, the nucleophile is hydroxide ion. Addition of hydroxide ion to the carbonyl group is faster in
A
because the negative charge on the oxygen that is created in the tetrahedral intermediate is stabilized by the
positively charged nitrogen.
HO
−
O
O
N
HO
−
+
O
O
N
A
B
faster
At pH
=
8, the nucleophile is water because the concentration of hydroxide ion is only 1
*
10
-
6
M.
Addition of water to the carbonyl group is faster in
B
because the amino group can act as a general-base
catalyst to make water a better nucleophile.
O
O
+
N
A
B
O
H H
O
O
N
H H
O
faster