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552 Chapter 16
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97.
a.
The negative
r
value obtained when hydrolysis is carried out in a basic solution indicates that electron-
donating substituents increase the rate of the reaction. This means that the rate-determining step must
be protonation of the carbon (the first step), because the more electron donating the substituent is, the
greater the partial negative charge on the carbon is and the easier it will be to protonate.
O
N
C CHCH
3
H OH
X
O
N
C CH
2
CH
3
HO
X
+
−
O
N
C CH
2
CH
3
X
OH
rate-determining
step
b.
The positive
r
value obtained when hydrolysis is carried out in an acidic solution indicates that
electron-withdrawing substituents increase the rate of the reaction. This means that the rate-determining
step must be nucleophilic addition of water to the iminium carbon to form the tetrahedral intermediate
(the second step), because electron withdrawal increases the electrophilicity of the iminium carbon,
making it more susceptible to nucleophilic addition.
N
O
C CHCH
3
H OH
2
N
O
C CH
2
CH
3
H
2
O
+
X
X
N
O
C CH
2
CH
3
X
OH
H
+
+
rate-determining
step