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Chapter 18 637
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86.
a.
The hydroxy-substituted carbocation intermediate is more stable because the positive charge can be
stabilized by resonance electron donation from the OH group.
OH
H NO
2
+
OH
H NO
2
+
OH
H NO
2
+
b.
The carbanion with the negative charge meta to the nitro group is more stable because a negative
charge in this position can be delocalized onto the nitro group but a negative charge ortho to the nitro
group position cannot.
N
HO Cl
+
N
HO Cl
+
N
HO Cl
−
O O
_
−
O O
_
−
O O
+
−
87.
a.
The chloro substituent primarily withdraws electrons inductively. (It only minimally donates electrons
by resonance.) The closer it is to the COOH group, the more it withdraws electrons from the OH bond
and the stronger the acid. Therefore, the ortho isomer is the strongest acid and the para isomer is the
weakest acid.
b.
The nitro substituent withdraws electrons inductively. It also withdraws electrons by resonance, if it is
ortho or para to the COOH group. Therefore, the ortho and para isomers are the strongest acids, and the
ortho isomer is a stronger acid than the para isomer because of greater inductive electron withdrawal
from the closer position.
c.
The amino substituent primarily donates electrons by resonance, but it can donate electrons by reso-
nance to the COOH group only if it is ortho or para to it. Therefore, the meta isomer is the strongest
acid. Seeing that the ortho isomer is the weakest acid tells us that resonance electron donation to the
COOH group is more efficient from the ortho position.