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34 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

hydrate with water, originally present in the mixture and formed in the reaction.

Hence they proposed the following equilibrium equation for the nitrating mixture:

H 2 SO 4 , + H 3 O+ + NO 3 - <-> H 3 O+ + HSO 4 - + HNO 3 (43)

The more the equilibrium is shifted to the right the higher the reaction rate.

According to Hetherington and Masson undissociated nitric acid is a nitrating

agent and in its interaction with nitrobenzene a complex with the C 6 H 5 NO 2 H

+

cation is formed, which subsequently reacts with HNO 3 to yield dinitrobenzene.

In his experiments on the nitration of naphthalenesulphonic acid Lantz [85]

found that the rate of nitration with a nitric acid and sulphuric acid mixture was

highest when the concentration of sulphuric acid used was about 90% H 2 SO 4

From these experiments, and on the basis of the Brönsted acids and bases

theory, as well as Hantzsch’s theory, Lantz has concluded that in a nitrating mixture

the hydrated sulphuric acid H 2 SO 4 .H 2 O acts as a base towards anhydrous sulphuric

acid. Thus an equilibrium acid-base system is established:

H 2 SO 4 + nH 2 SO 4 *H 2 O <-> HSO 4 - + H 2 SO 4 .H 3 O+ (44)

The HSO 4 - ions formed participate in the reaction

2H 2 SO 4 + HNO 3 <-> 2HSO4- + H 3 NO 3 (31)

shifting the equilibrium of reaction (31) to the left, which results in a decrease in

concentration of H 3 NO 3

2+

cations and in an increase in concentration of HNO 3.

These substances-the cation and the undissociated acid-both have nitrating

properties.

In the case of higher concentrations of water, the following reaction might

take place :

HNO 3 + H 2 O <-> NO 3 - + H 3 O+ (35)

Then the nitrating action of nitric acid would stop.

Lantz also draws attention to the fact that the equilibrium of reactions (44)

and (31) are greatly affected by the nature of the substance, being nitrated. The

rate of the nitration reaction also depends on the structure of the nitrated substance.

The more basic groups it contains the more distinct the acidic character of the

nitrating agent and the more vigorous the nitration reaction.
Westheimer and Kharasch [86] also chose reaction kinetics as a method of

studying the actual composition of nitrating mixtures. They found, that the rate

of a nitration reaction depends chiefly on the acidity of the medium and not on

its dehydrating properties. Therefore, the reaction rate is highest when 90%

sulphuric acid is used as a solvent for nitric acid, while the reaction remains unaf-

fected when a substance of strong dehydrating properties such as phosphoric
anhydride is added to the mixture.
When 90% sulphuric acid is used as a solvent, increase in concentration
of HNO 3 in a nitrating mixture results in reducing the rate of nitration, since the

acidity of the mixture is reduced. In the case of more concentrated sulphuric acid