Organic Syntheses Part 16

[6] Ber. 41, 3351 (1908); Ber. 42, 480 (1909).

[7] Ber. 41, 3318 (1908); J. Chem. Soc. 93, 754 (1908).

[8] Rec. trav. chim. (2) 30, 381 (1911).

[1b] Ber. 32, 1141 (1899); J. Chem. Soc. 95, 344 (1909).

[2b] Ber. 15, 130 (1882); 20, 2088 (1887); 41, 3351 (1908).

[3b] J. prakt. Chem. (2) 63, 170 (1901).

XXIV

1,3,5-TRINITROBENZENE

C6H2(NO2)3CO2H--> C6H3(NO2)3 + CO2

Prepared by H. T. CLARKE and W. W. HARTMAN. Checked by J. B. CONANT and J. J. TOOHY.

1. Procedure

THE crude trinitrobenzoic acid obtained by oxidation of 360 g.

of trinitrotoluene (prep. XXV, p. 95) is mixed with 2 l.

of water at 35'0 in a 5-l. flask provided with a stirrer.

Fifteen per cent sodium hydroxide solution is added, with continuous stirring, until a FAINT red color is just produced.

(See Notes.) The color is then immediately discharged by means of one or two drops of acetic acid, and the liquid is filtered from unchanged trinibrotoluene. The filtrate is transferred to a 5-l. flask, and 70 cc. of glacial acetic acid are added.

The mixture is then gently heated, with continuous stirring, when trinitrobenzene separates in crystalline condition, and floats on the surface of the liquid as a frothy layer.

After about one and a half hours the evolution of gas ceases; at this point the crystals begin to stir into the solution.

The heating and stirring is continued for three-quarters of an hour, when the mixture is allowed to cool, and the crystals filtered off.

A sample of the filtrate should be tested for undecomposed trinitrobenzoic acid: if a precipitate is produced by the addition of sulfuric acid the process must be continued.

After recrystallization from glacial acetic acid, the product melts at 121-122'0. The yield is 145-155 g. (43 to 46 per cent of the theoretical amount calculated from the trinitrotoluene). 2.

Notes

During the solution of the trinitrobenzoic acid, the temperature should not be below 35'0, owing to the slight solubility of trinitrobenzoic acid in cold water. The heat of neutralization raises the temperature to 45-55'0, but the latter temperature should not be exceeded, since any trinitrobenzene formed at this point would later be removed with the unreacted trinitrotoluene.

Care must be taken that no more alkali is added than is just sufficient to produce the faint red color. If an excess of alkali is added it produces a permanent color, which is not removed by acid and colors the final product.

When once the evolution of carbon dioxide sets in, the flame must be cut down so as to avoid the formation of a thick layer of froth which might foam over.

3. Other Methods of Preparation

1,3,5-Trinitrobenzene can be prepared by heating _m_-dinitrobenzene with nitric acid and sulfuric acid to 120'0;[1] by heating 2,4,6-trinitrotoluene with fuming nitric acid in a sealed tube at 180'0 for three hours;[2] by heating 2,4,6-trinitrobenzoic acid or its sodium salt with water, alcohol, dilute sodium carbonate or other suitable solvent.[3]

[1] Ber. 9, 402 (1876); Ann. 215, 344 (1882).

[2] Ber. 16, 1596 (1883).

[3] D. R. P. 77,353; Frdl. 4, 34 (1894).

XXV

2, 4, 6-TRINITROBENZOIC ACID

C6H2(NO2)3CH3 + 3O(Na2Cr2O7 + H2SO4)--> C6H2(NO2)3CO2H + H2O

Prepared by H. T. CLARKE and W. W. HARTMAN. Checked by J. B. CONANT and J. J. TOOHY.

1. Procedure

To 3600 g. of concentrated sulfuric acid, in a 5-l. flask placed in an empty water bath, are added 360 g. of technical trinitrotoluene, while the mixture is stirred mechanically. Sodium dichromate (Na2Cr2O7 2H2O) is now added in small quant.i.ties (PRECAUTION: see Notes), with constant stirring, until the temperature of the mixture reaches 40'0; the empty water bath is now filled with cold water and the addition of sodium dichromate continued at such a rate that the temperature remains at 45-55'0. In all 540 g. of sodium dichromate are added, the addition taking one to two hours. When all has been added, the mixture, which has now become very thick, is stirred for two hours at 45-55'0, and poured into a crock containing 4 kg.

of crushed ice. The insoluble trinitrobenzoic acid is filtered off, and carefully washed with cold water until free from chromium salts.

On drying it weighs 320-340 g.

The product is now mixed with 2 l. of distilled water at 35'0 in a 5-l. flask provided with a stirrer, and 15 per cent sodium hydroxide solution is dropped in with continuous stirring until a FAINT red color is just produced. Should this disappear, it is restored by the addition of a few drops more. When it has persisted for five minutes, the color is discharged by the addition of a few drops of acetic acid, and the insoluble unattacked trinitrotoluene filtered off and washed with a little water.

The trinitrobenzoic acid is precipitated from the filtrate by the addition of a slight excess of 50 per cent sulfuric acid.

The solution is chilled, and the acid filtered and washed free from salts with ice water. When dried in air it weighs 230-280 g.

(57 to 69 per cent of the theoretical amount).

2. Notes

The mother liquors and washings lose carbon dioxide on boiling, and the insoluble trinitrobenzene separates see preparation XXIV); after filtering, washing, and drying, it weighs 15-20 g.

(4 to 6 per cent of the theoretical amount).

It is essential that the stirring should be most efficient, so that when the mixture becomes thick the dichromate will be evenly distributed throughout the liquid, as rapidly as it is added.

If the stirring is not efficient, local reactions of extreme violence (in certain cases leading to conflagration) will occur.

An iron stirrer may be employed in the oxidation reaction, but not in the purification.

Technical sodium dichromate generally contains a certain amount of chlorides, and the chlorine liberated from these tends to cause a troublesome foam towards the end, of the reaction.

Only a very efficient stirrer, which draws down the surface of the liquid, is able to combat this difficulty. The amount of solid sodium dichromate given is for the dry crystalline compound containing two molecules of water of crystallization.

Great care should be taken in dissolving the crude acid in the alkali.

If an excess of alkali persists for any length of time, a permanent color is produced which will discolor the final product.

The acid is fairly soluble in cold water and should be washed with care.

3. Other Methods of Preparation

2,4,6-Trinitrobenzoic acid has been prepared by heating trinitrotoluene with fuming acid in a sealed tube to 100'0, for two weeks,[1a]

the oxidation being only partial. It can also be prepared by heating trinitrotoluene under a reflux condenser, with a mixture of 5 parts of concentrated nitric acid and 10 parts of concentrated sulfuric acid;[1] this method is, however, unsuitable in the laboratory owing to the difficulty of devising suitable apparatus.

Another method is to dissolve trinitrotoluene in nitric acid and, to this solution (at 95'0), to add pota.s.sium chlorate at such a rate that the temperature does not fall;[2] this method has been found to be difficult to control on a laboratory scale.