Organic Syntheses, CV 2, 171
Submitted by H. R. Snyder and L. A. Brooks.
Checked by Lee Irvin Smith, R. T. Arnold, and John Ryan.
1. Procedure
In a
2-l. three-necked, round-bottomed flask, fitted with a
500-cc. separatory funnel, a
mechanical stirrer, and a
thermometer, is placed a solution of
123 g. (1.5 moles) of cyclohexene (Note
1) in a mixture of
300 cc. of carbon tetrachloride and
15 cc. of absolute alcohol. The flask is surrounded by an
ice-salt bath. The stirrer is started, and, when the temperature has reached −5°, a solution of
210 g. (67 cc., 1.3 moles) of bromine in
145 cc. of carbon tetrachloride is added from the separatory funnel at such a rate that the temperature of the reaction mixture does not exceed −1° (Note
2). The addition requires about three hours.
When the
bromine has been added the contents of the flask are transferred directly to a
1-l. modified Claisen flask and the
carbon tetrachloride and excess
cyclohexene are distilled from a
water bath (Note
3), (Note
4), and (Note
5). The water bath is replaced by an
oil bath and the product distilled under reduced pressure. There is a small low-boiling fraction, and then pure
dibromocyclohexane distils at
99–103°/16 mm. (
108–112°/25 mm.). The yield is
303 g. (
95 per cent of the theoretical amount) (Note
6) and (Note
7).
2. Notes
2. Unless the temperature is controlled carefully, the yield is poor because of substitution reactions. Even at this low temperature some substitution occurs unless the excess of
cyclohexene is used.
4. The dibromide decomposes on continued exposure to the air and becomes very dark. Hence the product should be distilled at once.
5. The low-boiling distillate contains a trace of the dibromide as shown by the fact that it darkens on exposure to the air.
6. The product is stored best in
sealed bottles with as little exposure to the air as possible.
7. A product which will not darken may be obtained by the following purification: The crude dibromide is shaken for five minutes with about one-third its volume of
20 per cent ethyl alcoholic potassium hydroxide. The mixture is diluted with its own volume of water and the organic layer is washed free of alkali, dried, and distilled. Material so treated will stay clear indefinitely. The loss in the purification is about 10 per cent. (Wm. Von Eggers Doering and Aldrich Durant, Jr., private communication.)
3. Discussion
This preparation is referenced from:
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