A-Bromoheptaldehyde dimethyl acetal

B. a-Bromoheptaldehyde dimethyl acetal. A solution of 156 g. (177 ml., 1 mole) of the enol acetate and 200 ml. of carbon tetrachloride is placed in a l-l. flask and cooled in an ice-water bath. A mixture of 160 g. (51 ml., 1 mole) of bromine and 50 ml. of carbon tetrachloride is added slowly through a buret, the flask being constantly shaken and the rate of addition so controlled as not to allow the temperature of the brominated mixture to rise above 10° (Note 6). The addition of bromine takes from 20 minutes to 1 hour, and the end point is reached when the calculated amount is absorbed and the bromine is no longer decolorized. The brominated mixture is added to 600 ml. of anhydrous methanol (Note 7) and allowed to stand for 48 hours or longer. At the end of this period the mixture is diluted with 2 1. of water and the separated oil (lower layer) is washed with 1 1. of water and finally with 1 1. of 5% sodium carbonate (Note 8). The carbon tetrachloride and methyl acetate are removed by distillation at atmospheric pressure. The residual oil is then distilled under reduced pressure in the presence of a small amount of sodium carbonate. The fraction boiling at 117-119°/17 mm. is collected as pure a-bromoheptaldehyde dimethyl acetal, 1.4510-1.4520 df 1.180-1.195. The yield is 191-203 g. (80-85%) (Note 9). 

C. a-Bromoheptaldehyde. A mixture of 119.5 g. (100 nfl., 0.5 mole) of a-bromoheptaldehyde dimethyl acetal and 80 ml. of concentrated hydrochloric acid is boiled gently in a 250-ml. distilling flask, and the methanol liberated is removed by distillation, which is continued slowly until the vapor temperature reaches 90°, at which point the heating is stopped and the residue and distillate are combined and diluted with 200 ml. of water. The somewhat brownish oil which separates is distilled under reduced pressure from a 250-ml. Claisen flask. The yield of pure a-bromoheptaldehyde, boiling at 87-92°/17 mm., 1.4580-1.4600, df 1.210-1.230, is 87- 92.5 g. (90-95%). 

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See also in sourсe #XX -- [ ]

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