both rings. The electron-donating imidazole ring makes the protonated
pyrimidine part less acidic (pK a ¼ 2.5) than unsubstituted protonated pyrimidine (pK a ¼ 1.0). On the other hand, the electron-withdrawing pyrimidine ring makes hydrogen on N-9 (pK a ¼ 8.9) more acidic than the
corresponding N-1 hydrogen of imidazole (pK a ¼ 14.4).
N
N
N
N
H
H
N
N
N
N
H
N
N
N
N
+
+ H +
_
+ H +
..
Reactions of purine
Nucleophilic substitutions Aminopurines react with dilute nitrous acid to
yield the corresponding hydroxy compounds.
N
N
N
N
H
NH 2
N
N
N
N
H
OH
NaNO 2
AcOH, H 2 O
AcONa
0 o C
Adenine
Deamination of aminopurines Adenine undergoes deamination to produce hypoxanthine, and guanine is deaminated to xanthine.
N
N
N
N
H
N
H 2
O
N
N
N
N
H
O
O
H
H
N
N
N
N
H
NH 2
N
N
N
N
H
H
H
O
H +
H 2 O
Guanine
Xanthine
H +
H 2 O
Adenine
Hypoxanthine
Oxidation of xanthine and hypoxanthine Xanthine and hypoxanthine
can be oxidized enzymatically with xanthine oxidase to produce uric
acid.
N
N
N
N
H
O
O
H
H
N
N
N
N
H
H
H
O
N
N
N
N
H
H
H
O
O
O
H
Xanthine
Hypoxanthine
Xanthine oxidase
Uric acid
Xanthine oxidase
164
CH4 ORGANIC FUNCTIONAL GROUPS