Organic Chemistry

906 CHAPTER 21 More About Amines • Heterocyclic Compounds

Substituted pyridines undergo many of the side-chain reactions that substituted

benzenes undergo, such as bromination and oxidation.

When 2- or 4-aminopyridine is diazotized, or is formed.

Apparently, the diazonium salt reacts immediately with water to form a hydroxypyri-

dine (Section 16.10). The product of the reaction is a pyridone because the keto form

of a hydroxypyridine is more stable than the enol form. (The mechanism for the con-

version of a primary amino group into a diazonium group is shown in Section 16.12).

The electron-withdrawing nitrogen causes the of alkyl groups at-

tached to the 2- and 4-positions of the pyridine ring to have about the same acidity as

the of ketones (Section 19.1).

Consequently, the of alkyl substituents can be removed by base, and the

resulting carbanions can react as nucleophiles.

+ H 2 O

+ Br−

N

−

CH 3

N

CH 3

N

CH CH

N

CH 2

CH

O

HO−

CH 3 N

CH 3

N CH 2 CH 3

CH 3

CH− 2

−NH 2 CH 3 Br

an SN2 reaction

an aldol condensation

a-hydrogens

N

−

CH 3

N

CH 2

HO−

N

CH 2

−

−

−

−

N

CH 2

N

CH 2

N

CH 2

a-hydrogens

a-hydrogens

a-pyridone g-pyridone

NBS

∆/peroxide

N CHCH 3

Br

N

Na 2 Cr 2 O 7

H 2 SO 4

∆

CH 2 CH 3

N

CH 3

N

COOH

O

N

NH 2

N

N NH 2 N N N Cl− N OH

+

NaNO 2 , HCl

0 °C

NaNO 2 , HCl

0 °C

N

N

N Cl− OH

+

4-hydroxypyridine

enol form

N

H

-pyridone

keto form

N

H

-pyridone

keto form

2-hydroxypyridine

enol form

O

H 2 O

H 2 O

2-aminopyridine

4-aminopyridine