urbaf2

NITRO DERIVATIVES OF TOLUENE 333

This is a nucleophilic reaction, and its mechanism, according to the author of

the present book, may possibly be represented as follows:

The reaction was first mentioned by Laubenheimer [143] when examining

chloro-3,4-dinitrobenzene. However for a long time no notice was taken of the

possibility of putting it into practice. It was only during World War I that the

method was introduced in the U.S.A., and this happened quite accidentally. In

the search for methods of removing unsymmetrical isomers from crude TNT, the

reduction of trinitrotoluenes was studied. It was hoped that the nitro group in the

meta position, being chemically more active, would be more easily reduced, and

that the reduction product would be relatively soluble in water. Sodium polysulphide

was used for the reduction. However, it was found that the product of the reaction

was strongly contaminated with sulphur formed at the reaction. Among other re-

ducing agents used, sodium sulphite was shown to be a very efficient one in remov-

ing unsymmetrical isomers, its action consisting, however, not in the reduction

of the nitro group, but its replacement by a sulpho group.

As Muraour [103] found, the reaction of sodium sulphite was not confined to

unsymmetrical trinitro derivatives of toluene. α− Trinitrotoluene also reacted with

Na 2 SO 3 to form dinitrotoluene sulphonic acid, the difference lying in the fact

that the reaction proceeds much more slowly than with unsymmetrical isomers.

A 3% solution of Na 2 SO 3 , acting for 1 hr, dissolves at room temperature about

1% of α− trinitrotoluene.

According to the British literature of World War I, the optimum temperature

of sulphitation is 40-45°C. At higher temperatures α− trinitrotoluene reacts too

vigorously, which results in a certain loss of the product. On the other hand, the

action of sulphite on the unsymmetrical isomers at lower temperatures is too slow.

Since solutions of sodium sulphite are alkaline, which affects TNT unfavourably,

a number of workers have developed other methods consisting in lowering the pH

of the sodium sulphite solution by adding buffers.

Thus G. P. Davies [144] suggested the use of the Na 2 SO 3 solution with such

buffers as mono- and di-sodium phosphates or boric acid, which reduce the pH

to 7.0-8.3.

In industrial practice, losses of crude TNT in the sulphitation process amount

from 6 to 8%. Out of this 2%-3.5% is ascribed to the loss of α− trinitrotoluene and

3.5-4.5% to that of unsymmetrical isomers and other impurities, such as tetrani-

tromethane (p. 339) and trinitrobenzene which is easily soluble in sodium sulphite,

forming addition products.