Org. Synth. 1922, 2, 67
DOI: 10.15227/orgsyn.002.0067
PHENYLACETYLENE
[Benzene, ethynyl-]
Submitted by John C. Hessler
Checked by J. B. Conant and G. R. Barrett.
1. Procedure
In a 500-cc. Pyrex distilling flask (Note 1) is placed 150 g. (2.7 moles) of potassium hydroxide. The mouth of the flask is provided with a one-holed stopper holding a dropping funnel; the side tube of the flask is connected with a condenser set for downward distillation.
The distilling flask is gradually heated in an oil bath until the temperature of the bath is 200°, and 100 g. (0.55 mole) of β-bromostyrene is then dropped in upon the molten potassium hydroxide (Note 2), at the rate of somewhat less than a drop a second. Since the boiling point of phenylacetylene is 142–143°, and that of bromostyrene is 218–220°, the phenylacetylene distils away from the unchanged bromostyrene.
While the bromostyrene is being dropped in (Note 3), the temperature of the oil bath is raised very gradually to 215–220° and is kept there until all the bromostyrene has been added. Finally the temperature is raised to 230° and is held there until no more distillate comes over. The distillate is colorless; it consists of two layers, the lower one being water. The upper layer is separated and dried with solid potassium hydroxide. It is then distilled. The yield of the distilled phenylacetylene, boiling at 142–144°, is 37 g. (67 per cent of the theoretical amount) (Note 4).
2. Notes
1.
A single Pyrex flask can be used for only three or four runs. The flask should be emptied while still very hot. The use of
steel or copper vessels in place of a glass flask seems to diminish the yield slightly.
2.
The
potassium hydroxide usually available contains sufficient moisture so that it will liquefy at 200°. If pure dry
potassium hydroxide is used, it is necessary to add a little water so that the fusion point will be lowered to the temperature indicated.
It is suggested that equally good yields can be obtained if the eutectic of
sodium hydroxide and
potassium hydroxide is used, under which conditions the alkali is molten at 200° and the reaction takes place very readily.
13.
Toward the end of the reaction, a crust of
potassium bromide may tend to cover the melted
potassium hydroxide. The crust can be broken by shaking the distilling flask gently, or by using a
glass rod inserted through a second hole in the stopper holding the dropping funnel.
It is convenient to have such a rod or stirrer passing through a mercury seal in the stopper of the flask. An occasional turn of this stirrer breaks the crust and facilitates the operation. Mechanical stirring should not be employed, as it reduces the yield tremendously. Apparently this is because it facilitates the solution of bromostyrene in the tarry by-products and thus causes it to polymerize instead of reacting with the potassium hydroxide.
4.
The yield of material can be somewhat increased by working with small lots (
25 g. of β-bromostyrene).
3. Discussion
Two methods are available for preparing
phenylacetylene: the elimination of
carbon dioxide from
phenylpropiolic acid, and the elimination of halogen acid from either
styrene dibromide or an
α- or β-halostyrene.
Phenylpropiolic acid has been converted to
phenylacetylene by means of
phenol2 or
aniline3 or by heating with
barium hydroxide.
4 Styrene dibromide has been converted to
phenylacetylene by treatment with
sodium in
liquid ammonia,
5 or
sodamide,
6 7 or
potassium hydroxide in
alcohol.
8 α-Chlorostyrene and
α-bromostyrene have been converted to
phenylacetylene by means of
sodamide.
7,9 β-Chlorostyrene and
β-bromostyrene have been converted to
phenylacetylene by treatment with
sodium ethoxide or
potassium hydroxide in
alcohol10 with
sodium in liquid
ammonia,
5 with
sodamide,
7 with molten
potassium hydroxide,
11 and with the
sodium derivative of aniline.
12This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
α- or β-halostyrene
sodium derivative of aniline
alcohol (64-17-5)
ammonia (7664-41-7)
aniline (62-53-3)
sodium hydroxide (1310-73-2)
phenol (108-95-2)
carbon dioxide (124-38-9)
potassium hydroxide (1310-58-3)
sodium (13966-32-0)
sodium ethoxide (141-52-6)
potassium bromide (7758-02-3)
barium hydroxide (17194-00-2)
Phenylacetylene,
Benzene, ethynyl- (536-74-3)
β-bromostyrene,
bromostyrene (103-64-0)
Phenylpropiolic acid (637-44-5)
styrene dibromide (93-52-7)
α-Chlorostyrene
α-bromostyrene (98-81-7)
β-Chlorostyrene (622-25-3)
sodamide (7782-92-5)
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