urbaf2

(58)

(30% yield)

(59)

NITROUS ACID AS A NITRATION CATALYST

AND AS A NITRATING AGENT

The name “nitrous acid” is usually applied to substances which give HNO 2

when dissolved in water. Thus not only is N 2 O 3 a nitrous acid anhydride, but

so also is nitrogen dioxide, which, as is known, may be regarded as a mixed

nitrous-nitric anhydride.

Ingold and his co-workers [144] present the dissociation of nitrogen dioxide

and nitrous anhydride in the following way:

2N 2 O 4 + H 2 O <-> N 2 O 3 + 2HNO 3 (60)

NO + NO 2 <-> N 2 O 3 <-> NO+ + NO 2 - (61)

The nitrous ion NO 2 - attracts a proton, thus facilitating the nitrosating action

of the nitrosonium ion NO+.

In his work on the nitration kinetics, which has already been referred to, Mar-

tinsen [145] found that in the nitration of phenol with nitric acid, nitrous acid, if

present, acted as a catalyst on the nitration process. The presence of nitrous acid

was necessary for initiating the reaction, further quantities being formed in the reac-

tion of oxidation of phenol with nitric acid, which accompanied the nitration reac-

tion. Thus the reaction of nitrating phenol is an autocatalytic one.

Amall [146] inferred from his investigations on the nitration of phenol in an

alcoholic or acetic acid solution that nitrous acid was formed only initially as a

result of side reactions and then the following reactions took place:

HNO 3 + HNO 2 -> N 2 O 4 + H 2 O (62)

C 6 H 5 OH + N 2 O 4 -> NO 2 .C 6 H 4 .OH + HNO 2 (63)