Organic Syntheses Part 2

The quality of the benzyl chloride markedly affects the yield of pure benzyl cyanide. If a poor technical grade is used, the yields will not be more than 60-75 per cent of the theoretical, whereas consistent results of about 85 per cent or more were always obtained when a product was used that boiled over 10'0. The technical benzyl chloride at hand yielded on distillation about 8 per cent of high-boiling material; a technical grade from another source was of unusual purity and boiled over a 2'0 range for the most part.

It is advisable to distil off the last portion of alcohol and water _in vacuo_ and also to distil the benzyl cyanide _in vacuo_, since under ordinary pressures a white solid invariably separates during the distillation.

One method of purifying the benzyl cyanide is to steam distil it after the alcohol has been first distilled from the reaction mixture.

At ordinary pressures, this steam distillation is very slow and, with an ordinary condenser, requires eighteen to twenty hours in order to remove all of the volatile product from a run of 500 g.

of benzyl chloride. The distillate separates into two layers; the benzyl cyanide layer is removed and distilled. The product obtained in this way is very pure and contains no tarry material, and, after the excess of benzyl chloride has been removed, boils practically constant.

This steam distillation is hardly advisable in the laboratory.

The benzyl cyanide, prepared according to the procedure as outlined, is collected over a 5'0 range. It varies in appearance from a colorless to a straw-colored liquid and often develops appreciable color upon standing. For a product of special purity, it should be redistilled under diminished pressure and collected over a 1-2'0 range. For most purposes, such as the preparation of phenylacetic acid or ester, the fraction boiling 135-140'0/38 mm.

is perfectly satisfactory. 3. Other Methods of Preparation

Benzyl cyanide occurs naturally in certain oils.[1] The only feasible method of preparing it that has been described in the literature is the one in which alcoholic pota.s.sium cyanide and benzyl chloride[2]

are employed. The cheaper sodium cyanide is just as satisfactory as the pota.s.sium cyanide and therefore is the best material to use.

Gomberg has recently prepared benzyl cyanide from benzyl chloride and an aqueous solution of sodium cyanide.[3]

[1] Ber. 7, 519, 1293 (1874); 32, 2337 (1899)

[2] Ann. 96, 247 (1855); Ber. 3, 198 (1870); 14, 1645 (1881); 19, 1950 (1886).

[3] J. Am. Chem. Soc. 42, 2059 (1920).

IV

a, g-DICHLOROACETONE

CH2ClCHOHCH2Cl + O(Na2Cr2O7 + H2SO4)--> CH2ClCOCH2Cl + H2O

Prepared by J. B. CONANT and O. R. QUAYLE. Checked by A. W. DOX, L. YODER, and O. KAMM.

1. Procedure

IN a 2-l. flask are placed 375 g. of commercial sodium dichromate, 225 cc. of water, and 300 g. of dichlorohydrin (b. p.

68-75'0/14 mm.). The flask is set in a water bath and equipped with a thermometer and mechanical stirrer. The contents are vigorously stirred, and 450 g. of sulfuric acid, diluted with 115 g.

of water, are introduced during the course of seven to eight hours.

It is convenient to add the acid at ten-minute intervals.

The temperature is kept between 20'0 and 25'0 during the entire reaction; this is accomplished by adding a little ice to the water bath from time to time. The stirring is continued for sixteen to seventeen hours after all the acid has been added; as there is very little heat evolved during this part of the reaction, the water bath may be allowed to come to room temperature.

Sufficient water is now added to the mixture to dissolve the pasty chromium salts (300-800 cc.). The ma.s.s of crystals is then rapidly filtered on a Buchner funnel and sucked as dry as possible.

The crystals are then transferred to a small laboratory centrifuge and centrifuged for several minutes. The crystals are washed in the centrifuge with about 15-25 cc. of ice water, then with 10-15 cc.

of cold petroleum ether, and finally centrifuged till as dry as possible.

The crude dichloroacetone is dried in a vacuum desiccator over sulfuric acid overnight It weighs about 220 g.

The crude product is best purified by distillation from a 250-cc. distilling flask fitted with an air condenser.

A very small fraction (10-15 g.) of low-boiling material is obtained, and the dichloroacetone (170-175'0) is then collected. It solidifies in the receiver to a white crystalline ma.s.s which weighs 200-220 g.

(65-70 per cent of the theoretical amount). A few grams more may be obtained by chilling the low-boiling fraction and filtering off the water.

2. Notes

Great caution should be exercised in working with dichloroacetone, as it is extremely lachrymatory and blisters the skin.

In transferring the crystals from the reaction flask to the Buchner funnel it is necessary to use a certain amount of water to dissolve the pasty chromium salts which are otherwise quite impossible to filter. The amount necessary varies greatly in different runs, according to the manner in which the chromium salts separate.

The amount of this water is kept low in order to dissolve as little of the product as possible. Nevertheless, 10-15 g.

of dichloroacetone are thus dissolved; this material, together with a little unchanged dichlorohydrin, may be recovered by a long procedure involving extraction with ether and sodium bisulfite.

This is not profitable, however.

It is not necessary to wash the crystals in the centrifuge until they are white. A small amount of chromic salt will not interfere with the subsequent purification.

Commercial sodium dichromate is hygroscopic and contains varying amounts of water. The 375 g. required in these directions are equivalent to 319 g. of anhydrous material.

The total time required for the oxidation is twenty-four hours.

It is convenient to start the reaction in the morning.

In this way the last part of the reaction, which requires no attention, will be accomplished during the night.

The regulation of the temperature is necessary, as the reaction proceeds very slowly below 20'0; on the other hand, the dichloroacetone itself is oxidized at a somewhat higher temperature than 25'0. 3.

Other Methods of Preparation

The preparation of dichloroacetone by the following methods is described in the literature: the direct chlorination of acetone;[1] the oxidation of dichlorohydrin;[2] the action of silver chloride on diiodoacetone;[3] the action of dichloropropene (CH2Cl-CCl=CH2) and hypochlorous acid;[4] the action of hydrochloric acid on ethoxymonochloroacetoacetic ester;[5] and the hydrolytic cleavage of dichloroacetoacetic ester.[6]

[1] Jahresb. 1859, 345; 1871, 531; J. prakt. Chem. (2)4, 52 (1871); Ber. 7, 467 (1874); 8, 1330, 1438 (1875); 26, 598 (1893); 42, 3233 (1909); Ann. 279, 315 (1894)

[2] Ber. 6, 1210 (1873); 13, 1706 (1880); 42, 3233 (1909); Ann.

208, 355 (1881); 269, 46 (1892); Ann. chim. phys. (6) 9, 145 (1886); Bull. soc. chim. (2) 36, 19 (1881).

[3] Ann. 192, 93 (1878).

[4] Compt. rend. 94, 1428 (1882).

[5] Ann. 269, 18 (1892).

[6] Ber. 43, 3533 (1910).

V

_p_-DIMETHYLAMIn.o.bENZALDEHYDE

(CH3)2NC6H5 + HNO2--> (CH3)2NC6H4NO + H2O (CH3)2NC6H4NO + 2HCHO + 2C6H5N(CH3)2 --> (CH3)2NC6H4N = CHC6H4N(CH3)2 + 2H2) + (CH3)2NC6H4CHO (CH3)2NC6H4N = CHC6H4N(CH3)2 + HCHO-->( CH3)2NC6H4N = CH2 + (CH3)2NC6H4CHO

Prepared by ROGER ADAMS and G. H. COLEMAN. Checked by H. T. CLARKE and W. W. HARTMAN.

1. Procedure

IN a 3-l. round-bottom flask fitted with a mechanical stirrer 150 g.