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Chapter 23 759
Copyright © 2017 Pearson Education, Inc.
5.
The nitrogen that is the stronger base is the one most apt to be protonated. N-1 is a weaker base than
N-5 because the
p
electrons that belong to N-1 are delocalized onto an oxygen when a nucleophile
adds to C-10a. The
p
electrons that belong to N-5 end up on nitrogen when a nucleophile adds to
C-4a. Nucleophilic addition, therefore, occurs at the position (4a) that results in the stronger base
(N-5) being protonated.
_
R S
NH
N
N
5
1
O
O
10a
4a
N
N
NH
N
H
3
C
H
3
C
O
R
O
N
N
NH
N
H
3
C
H
3
C
O
R
O
S
−
HS BH
H S
HS B
−
R
R
6.
Solved in the text.
7.
When a proton is removed from the methyl group at C-8, the electrons that are left behind can be delocalized
onto the oxygen at the 2-position or onto the oxygen at the 4-position.
H
2
C
H
3
C
N
N
NH
N
O
O
R
H
C-8
B
_
When a proton is removed from the methyl group at C-7, the electrons that are left behind can be
delocalized only onto carbons that, being less electronegative than oxygen, are less able to accommodate
the electrons.
H
3
C
H
2
C
H
N
N
NH
N
O
O
R
C-7
B
_