Submitted by R. Lespieau and M. Bourguel.
Checked by Roger Adams and M. M. Brubaker.
1. Procedure
A
3-l. three-necked round-bottomed flask is fitted with a
mechanical stirrer through a
mercury seal, a
reflux condenser and a
1-l. separatory funnel. A mixture of
200 g. (1 mole) of 2,3-dibromopropene (p. 209) and
200 cc. of dry ether is added. The flask is now cooled in an
ice bath, the stirrer is started, and one molecular equivalent of
cyclohexylmagnesium bromide, prepared from 31 g. (1.28 atoms) of magnesium, 204 g. (1.25 moles) of cyclohexyl bromide, and 400 cc. of dry ether (Note
1), is added at such a rate that the mixture refluxes gently (Note
2). The addition takes about one-half to three-quarters of an hour. Two layers are formed, and
magnesium bromide may separate.
The stirring should be continued and the ice bath replaced by a
hot-water bath, and the mixture refluxed gently for two hours. The flask is again cooled, and
30 cc. of concentrated hydrochloric acid in 350 cc. of water is added through the
separatory funnel. This should be added slowly as long as heat is evolved. The contents of the flask are then transferred to a separatory funnel, the
ether layer separated and dried over
calcium chloride.
The ether is distilled on a
steam bath (Note
3) and the residue fractionated under reduced pressure by means of a
20-cm. column (p. 130). After three fractionations, there is obtained
32–44 g. of a product boiling up to
100° /25 mm., which is mainly unchanged
dibromopropene (b.p.
42–43° /18 mm.), and
123–130 g. (
60.5–64 per cent of the theoretical amount, not taking into account recovered
dibromopropene) of
cyclohexylbromopropene (Note
4) boiling at
100–105° /25 mm. This material is satisfactory for most purposes. On redistillation it boils at
88–89° /14 mm.
2. Notes
2. The reaction is instantaneous and is not stopped by moderate cooling. The cooling may be dispensed with and the
cyclohexylmagnesium bromide added more slowly (two hours).
3. The ether solution may be added in several portions to a
500-cc. modified Claisen flask (p. 130) and the
ether distilled after each addition.
3. Discussion
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