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NITRATION AND NITRATING AGENTS 39

and have inferred that both nitration and oxygen exchange involve formation (b)

and destruction (c, d) of the nitronium ion. Reactions (c) and (d) are competitive

with preference for reaction (d). The nitracidium ion is formed in reaction (d).

In 1948, Titov [35] also studied the problem of the existence of a maximum

in the nitration rate at certain concentration of sulphuric acid. He assumed, like

Hetherington and Masson, (see p. 33), that cations might be formed in nitration

by the attachment of protons to nitro compounds, for example:

C 6 H 5 NO 2 + H+ -> C 6 H 5 NO 2 H+

d[C 6 H 4 (NO 2 ) 2 ]

= km [NO 2 +] [C 6 H 5 NO 2 +] kk [NO 2 +] [C 6 H 5 NO 2 H+]

dt

The C 6 H 5 NO 2 H+ cation undergoes a substitution reaction with much more

difficulty than the nitro compound C 6 H 5 NO 2 itself, since the addition of a proton

gives rise to a sharp drop of prototropic behaviour of the compound.

The rate of nitration of nitrobenzene to dinitrobenzene may be expressed,

according to Titov, by the equation:

It is only the first factor in the right side of the equation which affects the nitration

rate, since the second factor is very small because of the low rate of nitration of

the C 6 H 5 NO 2 H+ ion.

An increase in sulphuric acid concentration results in increase in the concen-

tration of the C 6 H 5 NO 2 H

+

cation, hence in a decrease in the C 6 H 5 NO 2 concen-

tration which in turn brings about a decrease in the nitration rate. On the other

hand the concentration of the NO 2 + ion increases with increase in sulphuric acid

concentration.

Titov believes, as in Bennett’s interpretation, that summing up the two effects

results in a maximum in the reaction rate at a certain concentration of H 2 SO 4.

Ingold, Hughes and Reed [39] studied the kinetics of nitration of aromatic

compounds with nitric acid only and found that it was a first order reaction. Its

rate, VN, may be expressed by an approximate equation:

VN = k 1 [ArH]

at [HNO 3 ] = const.

Ingold and his co-workers [36,37,39] inferred from the results of their studies

that the nitration rate, VS, in the presence of sulphuric acid might be expressed

roughly by the equation:

VS = k 1 [ArH][HNO 3 ]

In the nitration of numerous compounds, and particularly those containing

many nitro groups, e.g. nitration of dinitro- to trinitro compounds, or nitro de-

rivatives of naphthalene, which are sparingly soluble in a nitrating mixture, nitra-

tion takes place in a two-phase system: acid-organic compound, the acid phase

being a saturated solution of the organic compound. If the compound is liquid
at nitration temperature, then the organic liquid phase is a saturated solution of

acid in the nitrated substance. In this case the reaction rate depends among other