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NITRO DERIVATIVES OF TOLUENE 313

centration of sulphuric acid from 100 to 92% may be explained by the fact that

the concentration of HSO 4

• ions increases considerably, whereas the concentration
  of NO 2 + ion decreases only slightly. Decreasing the concentration of sulphuric acid

below 92% H 2 SO 4 causes the NO 2 + concentration to fall more rapidly than the

HSO 4 - concentration increases, thus decreasing the reaction rate.

To check their theory, Bennett and his co-workers added KHSO 4 to the nitrating

mixture. As they expected, at concentrations of acid over 92% the reaction rate

increased. According to the theory, this was the result of increasing the concen-

tration of HSO 4 - ions. However, at concentrations below 92% of H 2 SO 4 , adding

KHSO 4 caused the reaction rate to fall, as increasing the concentration of the HSO 4 -

ions resulted in a decrease in the concentration of NO 2 + ions (Fig. 65).

% H 2 SO 4

FIG. 65. Influence of the concentration of sulphuric acid on the rate of nitration

of DNT in a heterogeneous medium (nitrating mixtures with a low content of HNO 3 ,

Bennett et al. [110])

As pointed out earlier (p. 36) the concentration of the proton acceptor (HSO 4 -

in our instance) is not the only factor determining the rate of nitration.

If the concentration of HSO 4 - appears to have a considerable influence on

the rate of reaction this may be due to the fact that in concentrated nitrating mix-

tures (over 92% H 2 SO 4 ) a high concentration of HSO 4 - is accompanied by

8 high concentration of NO 2

+

.

However, the equations of Bennett are useful for calculating the rate of nitra-

tion of dinitrotoluene to trinitrotoluene.

Orlova [110a] repeated the experiments of Bennett et al., studying the kinetics

of nitration of dinitrotoluene in nitrating mixtures rich in nitric acid that dissolve

dinitrotoluene, i.e. in homogeneous conditions. For 0.7 mole of dinitrotoluene,

3.8 mole of HNO 3 in 12 mole of H 2 SO 4 were used. The concentration of sulphuric

acid was changed from 87 to 100% H 2 SO 4. The temperature was 90°C. Contrary

to the results of Bennett, Orlova did not find any maximum of the rate of nitra-

tion (Fig. 66) which she attributes to the homogeneity of the reaction medium.