Organic Syntheses Part 9

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 ma.s.s slowly separates; more than half of this ma.s.s is _p_-nitrobenzyl cyanide, the other const.i.tuents being _o_-nitrobenzyl cyanide, and a variable amount of an oil which resists hydrolysis; apparently no dinitro compounds are formed.

The ma.s.s 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. .h.i.therto 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).

XV

_p_-NITROPHENYLACETIC ACID

(_p_)NO2C6H4CN + H2SO4 + 2H2O--> (_p_)NO2C6H4CH2CO2H + NH4HSO4

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

1. Procedure

IN a 1-l. round-bottom flask are placed 100 g. of _p_-nitrobenzyl cyanide. A solution of 300 cc. of concentrated sulfuric acid (sp. gr. 1.84) in 280 cc. of water is prepared, and two-thirds of this solution is poured on to the _p_-nitrobenzyl cyanide.

The mixture is shaken well, until the solid is all moistened with the acid. Any solid material sticking to the walls of the vessel is now washed down into the liquid with the remainder of the acid, the flask is attached to a reflux condenser, then set, without shaking, over a 10-cm. hole in a large sheet of asbestos board which rests on a tripod, and heated until the mixture boils.

The boiling is continued for fifteen minutes.

The reaction mixture, which becomes rather dark, is diluted with an equal volume of cold water and cooled to 0'0 or below.

The solution is filtered, the precipitate is washed several times with ice water and then dissolved in 1600 cc. of boiling water.

(A few grams of animal charcoal are added in dissolving the precipitate, if a technical _p_-nitrobenzyl cyanide has been used.) This solution is filtered as rapidly as possible through a large folded filter, preferably with a steam funnel. In spite of all precautions, however, some solid usually separates on the filter.

This must be redissolved in a minimum quant.i.ty of boiling water, and this solution then filtered into the main solution.

The _p_-nitrophenylacetic acid separates in long, pale-yellow needles, which melt at 151-152'0. The yield is 103 to 106 g.

(92- 3 per cent of the theoretical amount).

2. Notes

If the flask is not protected with an asbestos board or the equivalent, decomposition occurs where the substance is super-heated on the side walls of the flask. If crystals of the cyanide are allowed to remain on the upper walls of the flask, they are not easily washed down and so are not hydrolyzed.

The solubility curve of _p_-nitrophenylacetic acid is very steep at temperatures near 100'0, so that the filtering of the boiling solution should be rapid.