Org. Synth. 1939, 19, 24
DOI: 10.15227/orgsyn.019.0024
PHENACYL BROMIDE
[Acetophenone, α-bromo-]
Submitted by R. M. Cowper and L. H. Davidson.
Checked by Lee Irvin Smith and E. W. Kaiser.
1. Procedure
A solution of
50 g. (0.42 mole) of acetophenone (Org. Syn. Coll. Vol. I, 1941, 111) in
50 cc. of pure anhydrous ether (Note 1) is placed in a dry
three-necked flask fitted with a
separatory funnel,
mechanical stirrer, and
reflux condenser (Note 2). The solution is cooled in an
ice bath,
0.5 g. of anhydrous aluminum chloride is introduced
(Note 3), and
67 g. (21.5 cc., 0.42 mole) of bromine is added gradually from the separatory funnel, with stirring, at the rate of about 1 cc. per minute. The
bromine color disappears rapidly although very little
hydrogen bromide is evolved; towards the end of the reaction the solution becomes pink.
After the bromine has been added the ether and dissolved hydrogen bromide are removed at once (Note 4) under reduced pressure with a slight current of air. The phenacyl bromide remains as a solid mass of brownish yellow crystals (Note 5); the color is removed by shaking with a mixture of 10 cc. of water and 10 cc. of petroleum ether. The crystals are filtered with suction and washed several times with fresh portions of the solvent mixture, if necessary, until a white product is obtained (Note 6). The crude phenacyl bromide weighs 74–80 g. (88–96 per cent of the theoretical amount) and melts at 45–48°. This material is sufficiently pure for many purposes. If higher purity is desired the crude product may be recrystallized from 25–30 cc. of methanol, yielding 54–55 g. (64–66 per cent of the theoretical amount) of white crystals melting at 49–51° (Note 7).
2. Notes
1.
Dry
carbon tetrachloride may be used as a solvent but is less favorable than dry
ether.
2.
Quantities up to
200 g. of acetophenone may be brominated in a single operation with equally good yields but this is not generally advisable unless the product is to be used at once, since it becomes discolored on standing.
3.
Without
aluminum chloride the reaction is slow and incomplete.
4.
If the
ether and
hydrogen bromide are not removed immediately the solution blackens on standing and a lower yield of less pure product results.
5.
Phenacyl bromide is a lachrymator and should be manipulated carefully, to avoid contact with the skin and inhalation of the vapor.
6.
The water removes yellow color due to residual
hydrogen bromide, and the
petroleum ether removes unchanged
acetophenone or oily by-product. Since the product is quite insoluble in water and only slightly soluble in cold
petroleum ether, it may be washed several times with little loss.
7.
The checkers observed that all specimens of the product, even after recrystallization, although white at first, became dark and discolored on standing in a
vacuum desiccator over
calcium chloride.
3. Discussion
Phenacyl bromide has been prepared by the bromination of
acetophenone without a solvent,
1 in
carbon disulfide,
2,
3 in
acetic acid,
3,
4,
5 and in other organic solvents.
5 The quantitative aspects of the bromination in various solvents have been investigated by Kröhnke.
3 The use of
ether in the method described is based on the use of this solvent in the bromination of
desoxybenzoin.
6
Phenacyl bromide is a useful reagent for the identification of organic acids by conversion to crystalline phenacyl esters.
4,
7
This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
Desoxybenzoin
petroleum ether
calcium chloride (10043-52-4)
acetic acid (64-19-7)
methanol (67-56-1)
ether (60-29-7)
hydrogen bromide (10035-10-6)
bromine (7726-95-6)
carbon tetrachloride (56-23-5)
Acetophenone (98-86-2)
aluminum chloride (3495-54-3)
carbon disulfide (75-15-0)
Phenacyl bromide,
Acetophenone, α-bromo- (70-11-1)
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