Organic Syntheses / An Annual Publication of Satisfactory Methods for the Preparation of Organic Chemicals

by Unknown

XIII

p-NITROBENZOIC ACID

(p)NO2C6H4CH>3s> + 3O(Na2Cr2O7 + H2SO4)—> (p)NO2C6H4CO2H + 3H2O

Prepared by O. KAMM and A. O. MATTHEWS. Checked by H. T. CLARKE and W. W. HARTMAN.

1. Procedure

IN a 5-l. round-bottom flask, fitted with a mechanical stirrer, are placed 680 g. of sodium dichromate, 1500 cc. of water, and 230 g. of p-nitrotoluene. Stirring is started, and 1700 g. of concentrated sulfuric acid are allowed to flow in during about thirty minutes. The heat of dilution of the sulfuric acid will cause the nitrotoluene to melt, and rapid oxidation will soon take place. The last half of the sulfuric acid must be added gradually, in order to prevent too violent a reaction. Since a small amount of nitrotoluene is volatilized, it is advisable to carry on this work under a hood.

After the sulfuric acid has been added and the spontaneous heating of the reaction mixture has subsided, the mixture is heated to gentle boiling for about half an hour. After the reaction mixture has cooled, 2 l. of water are added, the cooled solution is filtered through a cloth filter, and the product washed with about 1 l. of water. In order to remove the chromium salts as completely as possible, the crude nitrobenzoic acid is warmed on the water bath and agitated with 1 l. of dilute (5 per cent) sulfuric acid solution. After cooling, the product is again filtered. It is then dissolved in 5 per cent sodium hydroxide solution, filtered from any chromium hydroxide remaining, and also from unchanged nitrotoluene. The filtrate, which should be light yellow or greenish in color, is acidified with dilute sulfuric acid, with stirring. It is usually preferable to run the alkaline solution into the dilute sulfuric acid, rather than to use the reverse procedure, for the precipitation of the nitro acid. The precipitated product is filtered with suction, washed thoroughly, and dried. The product should possess only a light-lemon color. The yield should be 230-240 g. (80-85 per cent of the theoretical amount).

For a product of special purity, crystallization from benzene is advisable. For most purposes, however, the nitrobenzoic acid may be used without crystallization, since its melting point is found to be within 2'0 of the correct value of 238'0.

2. Notes

The above procedure differs from that recorded in the literature, mainly in the use of a fairly large excess of sulfuric acid. This shortens the reaction time from forty hours to about one hour, which is especially convenient in the preparation of the acid on a laboratory scale. Because of the use of this large excess of sulfuric acid, the reaction is apt to be rather violent if the directions given are not carefully followed. The oxidation should be carried out under a hood. Small amounts of nitrotoluene are lost by volatilization, but this loss is not serious, as can be seen from the yield of product obtained.

Ten or 20 g. of unchanged nitrotoluene can be recovered from the reaction mixture by steam distillation, but the value of the by-product would not pay for the time spent in recovery.

The washing of the crude reaction product with dilute sulfuric acid is advisable, if good material is to be obtained. If an efficient centrifuge is available for use at this stage of the operation, this separate washing may prove to be less essential.

When a sparingly soluble organic acid is precipitated from fairly concentrated solution, the precipitate is liable to carry down with it some of the salt of the organic acid. Addition of the salt solution to the mineral acid, with stirring, avoids this difficulty. 3. Other Methods of Preparation

The nitration of benzoic acid produces only very small yields of the p-nitro product.[1] The only practical method for the preparation consists in the oxidation of p-nitrotoluene, although for this purpose various oxidizing agents are used. In addition to nitrotoluene, p-nitrobenzyl alcohol, p-nitrocinnamic acid and similar compounds may be oxidized, but their cost is prohibitive in comparison with that of the cheaper nitro hydrocarbon.

p-Nitrotoluene may be oxidized by means of strong nitric acid,[2] chromic acid mixture,[3] or permanganates.[4] Electrolytic oxidation[5] has also been proposed. The procedure given above involves the use of chromic acid mixture, but, owing to a change in the concentration of sulfuric acid, the time of reaction is greatly shortened and the preparation is thus considerably improved.

[1] Ber. 8, 528, 536 (1875)

[2] Ann. 127, 137 (1863); 128, 257 (1863)

[3] Ann. 139, 335 (1866).

[4] J. Am. Chem. Soc. 41, 1575 (1919).

[5] R. P. 117, 129; Frdl. 6, 112.

XIV p-NITROBENZYL CYANIDE

C6H5CH2CN + HNO3—> (p)NO2C6H4CH2CN + H2O

Prepared by G. R. ROBERTSON. Checked by ROGER ADAMS and H. O. CALVERY.

1. Procedure

IN a 2-l. round-bottom flask, fitted with a stopper holding a dropping funnel and a mechanical stirrer, is placed a mixture of 275 cc. of concentrated nitric acid (sp. gr. 1.42) and 275 cc. of concentrated sulfuric acid (sp. gr. 1.84). This is cooled to 10'0 in a freezing mixture, and 100 g. of benzyl cyanide (free from alcohol and water) are run in slowly, at such a rate that the temperature remains at about 10'0 and does not exceed 20'0. After all the benzyl cyanide has been added (about one hour), the ice bath is removed, the mixture is stirred for an hour and then poured on to 1200 g. of crushed ice. A pasty mass slowly separates; more than half of this mass is p-nitrobenzyl cyanide, the other constituents being o-nitrobenzyl cyanide, and a variable amount of an oil which resists hydrolysis; apparently no dinitro compounds are formed. The mass is filtered on a porcelain funnel with suction, pressed well to remove as much oil as possible, and dissolved in 500 cc. of boiling alcohol (95 per cent). On cooling, p-nitrobenzyl cyanide crystallizes; the mother liquor, on distillation, gives an impure alcohol which can be used for the next run. Recrystallization from 550 cc. of 80 per cent alcohol (sp. gr. 0.86 to 0.87) yields 70 to 75 g. (50-54 per cent) of a product which melts at 115-116'0.

This product is satisfactory for most purposes, and incidentally for the preparation of p-nitrophenylacetic acid. Occasionally it must be free even from traces of the ortho compound, and in this case should be crystallized again from 80 per cent alcohol; it then melts at 116-117'0.

2. Notes

Fuming nitric acid may be used in nitrating benzyl cyanide, but the method here described is cheaper.

The yield of 70 g. is obtained from benzyl cyanide, which boils over a 5'0 range prepared as described in preparation III (p. 9). Very pure benzyl cyanide will give a slightly higher yield, while commercial grades may give only 50 g. of p-nitrobenzyl cyanide and much oil.

The reaction has been also carried out with 500 g. of benzyl cyanide. Under these conditions a 5-l. flask was used, and it required two and a half hours to add the benzyl cyanide. The yield of product was 325 to 370 g.

3. Other Methods of Preparation

Nitrobenzyl cyanide has hitherto been prepared by the action of fuming nitric acid[1] on benzyl cyanide.

[1] Ber. 17, 505 (1884); 33, 170 (1900); J. Biol. Chem. 39, 585 (1919); J. Am. Chem. Soc. 43, 180 (1921).