Organic Syntheses | Page 8

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 mass 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_-DIMETHYLAMINOBENZALDEHYDE
(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. of technical dimethylaniline are dissolved in 750 cc. of diluted hydrochloric acid (1 part concentrated acid to 1 part water). This solution is now cooled to 0'0 and a solution (previously cooled to 0'0) of 90 g. of technical sodium nitrite in 150 cc. of water is added through a separatory funnel. During the addition of the nitrite solution, mechanical stirring should be employed and the flask cooled well with ice and salt. The addition is made at such a rate (thirty to forty minutes for the entire addition) that the temperature does not rise above 5'0. The precipitate of nitroso dimethylaniline hydrochloride is filtered off with suction, then washed with about 300 cc. of diluted hydrochloric acid (1:1).
In a 2-l. beaker, 180 g. of technical dimethylaniline, 125 cc. of formaldehyde (technical 40 per cent), and 300 cc. of concentrated hydrochloric acid are mixed and heated for ten minutes on a steam bath. The mixture is now placed in a hood and the nitroso dimethylaniline added all at once, or as rapidly as possible. The beaker is then covered with a watch glass. A vigorous reaction soon occurs and is complete in about five minutes. The resulting solution is transferred to a 5-l. flask and diluted to 4 l.; stirring is started, and a 25 per cent solution of sodium hydroxide is added until the red color disappears (about 650 cc. are required). The yellow benzylidene compound separates, is filtered with suction and