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Chapter 17 559
Copyright © 2017 Pearson Education, Inc.
4.
Electron delocalization of the lone pair on nitrogen or oxygen competes with electron delocalization of the
electrons left behind on the
a
-carbon when it loses a proton. The lone pair on nitrogen is more delocalized
than the lone pair on oxygen, because nitrogen is better able to accommodate a positive charge since it is
less electronegative than oxygen. Therefore, the amide competes better with the carbanion for electron
delocalization, so the
a
-hydrogen is less acidic.
CH
3
NCH
3
C
O
CH
3
CH
3
NCH
3
C
O
CH
3
−
+
CH
3
OCH
3
C
O
CH
3
OCH
3
C
O
−
+
5.
a.
> HC CH > CH
2
CH
2
> CH
3
CH
3
H CH
3
C
O
b.
>
CH
3
CH
2
C
O
CH
3
C
O
>
CH
3
CH
2
C
O
OCH
3
C
O
>
CH
3
O CH
2
C
O
OCH
3
C
O
CH
3
CH
3
C
O
c.
O
>
O
O
Problem 4 explains why
the ester is more acidic
than the amide.
>
NCH
3
O
The ketone is a stronger acid than the ester or the
N
-alkylated amide, because there is no competition for
delocalization of the electrons that are left behind when the
a
-hydrogen is removed.
6.
Both the keto and enol tautomers of 2,4-pentanedione can form hydrogen bonds with water. Neither the
keto nor the enol tautomer can form hydrogen bonds with hexane, but the enol tautomer can form an
intramolecular hydrogen bond, which stabilizes it. Therefore, the enol tautomer is more stable relative to the
keto tautomer in hexane than in water and, therefore, more of the enol tautomer is present in hexane than
in water.
O O
CH
3
CH
3
O O
CH
3
CH
3
H
keto tautomer
enol tautomer
2,4-pentanedione