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NITRATION AND NITRATING AGENTS 9

accordingly and finally gives way to an oxidizing action. The more dilute the nitric

acid the more vigorous (up to a limit) is the oxidizing action. Extensive research

into the physical properties of nitric acid, especially that which has been carried

out recently, has been aimed at finding an explanation of this double function of

nitric acid.

The limiting concentration of the nitric acid in a nitration process depends

largely on the nitrated compound itself. Some phenols, for example, can be nitra-

ted with nitric acid in concentrations of below 5%, while benzene does not nitrate

at those concentrations. Aliphatic hydrocarbons can be nitrated even with 13 %

nitric acid.

Knowing how a nitration process is likely to proceed is possible only when the

structure of the nitrating agent itself is known. This is why a great deal of research

work has been devoted lately to the elucidation of the structure of nitric acid when

pure and in mixtures with other mineral acids.

The most frequently used nitrating mixture is the one comprising nitric plus

sulphuric acids. The role of the sulphuric acid has been the subject of many in-

vestigations. These are described in the following section.

The most recent review of the physicochemical properties of nitric acid is that

of Stern, Mullhaupt and Kay published in 1960 [12a].

NITRIC ACID IN ADMIXTURE WITH OTHER MINERAL ACIDS

EARLIER WORKS

Muspratt and A. W. Hofmann [13] were the first to use a mixture of nitric and

sulphuric acids for nitrating nitrobenzene to dinitrobenzene.

Initially sulphuric acid was considered to act as a “dehydrating agent” in a

nitrating mixture with nitric acid. This view was expressed in the literature for

the first time by Spindler [14]. The first attempt to elucidate the form of nitric acid

in admixture with sulphuric acid was a hypothesis formulated by Markovnikov

[15]. It assumed that the two acids form a mixed anhydride, i.e. nitrosulphuric
acid :

Later Sapozhnikov [16] developed a theory, based on density, electrical con-

ductivity and partial vapour pressure measurements. He assumed a state of equi-

librium between the hydrates of the two acids in their mixtures:

HNO 3. nH 2 O + xH 2 SO 4 <—> HNO 3. (n-x)H 2 O + xH 2 SO 4. H 2 O (3)

Since the affinity of water for sulphuric acid is higher than that for nitric acid,