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406 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

o- xylene f. p. -27.1°C

m- xylene f. p. -54.8°C

p- xylene f. p. +13.2°C

Such big differences enable the o- and p- isomers to be separated readily from

m- xylene.

A drawback of this method is that the mixture has to be cooled to a very low

temperature, which is costly. Further, if very pure m- isomer is to be prepared,

it should be crystallized out, which is a very expensive operation as it requires

cooling to a temperature below -55°C.

Chemical method of separating m- xylene. The method is based on the different

capacities of the isomers to undergo sulphonation, and on the different extent to

which the sulphonic acids are hydrolysed. It has been shown that o- and m- xylene

can be sulphonated with concentrated sulphuric acid under conditions which leave

p- xylene unaffected. m- Xylenesulphonic acid can be hydrolysed to m- xylene at

a temperature too low for the hydrolysis of o- xylenesulphonic acid.

In practice, the process is effected as follows. Crude m- xylene, containing 60-70%

of m- xylene, is mixed with sulphuric acid (sp. gr. 1.84) whereupon the temperature

rises to 45°C. Then the mixture is heated to 50°C and allowed to remain at this

temperature for 2 hr. Under these conditions sulphonation of the o- and m- isomers

takes place. The sulphonic acids may be separated from unconverted p- xylene

either by extraction with water or by expelling the p- xylene by steam distillation.

The temperature of the aqueous solution of sulphonic acids is brought to 130°C.

Hydrolysis of sulphonated m- xylene follows :

C 6 H 3 (CH 3 ) 2 SO 3 H + H 2 O -> C 6 H 4 (CH 3 ) 2 + H 2 SO 4 (3)

The sulpho derivative of o- xylene undergoes hydrolysis only at 160°C and it

remains unaffected during the reaction. This permits the separation of m- xylene

from an aqueous solution of sulphonic acid.

The m- xylene isolated in this way is of very high purity. When nitrated it yields

a product having a melting point only slightly lower than that of chemically pure

2,4,6-trinitro-m-xylene.

One-stage preparation of TNX

The nitration of xylene to trinitroxylene may be carried out in one, two or three

stages.

The one-stage nitration has the advantage of giving a higher yield of the nitro

compound as compared with other methods. On the other hand it has also a draw-

back, as it requires larger quantities of acids. The one-stage method of nitration

is recommended when the product must be obtained in a short time, and cost is

of minor importance.

The method was used in Russia during the 1914-1918 War. It was worked out

by Solonina [8] and applied in the Shterovka factory. As Kravchinskii [3] reported,

the operation was carried out in the following way.