Organic Syntheses, CV 5, 921
Submitted by Sidney I. Miller, Gene R. Ziegler, and R. Wieleseck
1.
Checked by William E. Parham and James N. Wemple.
1. Procedure
To a
2-l. bottle equipped with a
rubber stopper and immersed in a mixture of ice and water (slush) there is added a cold (about 0°) solution containing
300 g. (7.5 moles) of sodium hydroxide (Note
1) and 800 ml. of water. The mixture is swirled or stirred while
160 g. (2 moles) of bromine is added.
Phenylacetylene (84 g., 0.82 mole) (Note
2) is then added to the yellow solution, and the resulting mixture is stoppered and shaken. The rubber stopper is wired down, the bottle is covered with opaque cloth or paper, and the bottle is then placed in a mechanical shaker for 60 hours at room temperature (Note
3).
The crude oil is then separated from the aqueous phase, dried with
calcium chloride (Note
4). and fractionated (Note
5) at reduced pressure under
nitrogen (Caution! (Note 6)). The
distillation receiver should be cooled in an ice-salt or dry ice-acetone mixture. After a small fore-run of
phenylacetylene, there is obtained
109–124 g. (
73–83% yield) of water-white
phenylbromoethyne, b.p.
40–41° (0.1 mm.) n25D 1.6075 (Note
7).
2. Notes
2. Commercially available
phenylacetylene can be used. The checkers used material as obtained from Columbia Organic Chemicals co., Inc.
3. Vigorous shaking is essential. For this reaction rate =
k[C
6H
5 C

CH] [OBr
−] [OH
−] with
k = 7 M
−2 sec.
−1 at 25°.
2 Since the solubility of
phenylacetylene at 25° is 5.1 × 10
−3 M in water (2.0 × 10
−3 M in 2
M sodium chloride), efficient mixing of the reagents is of paramount importance.
2 An ordinary motor-drive stirrer proved to be inadequate.
Phenylbromoethyne gradually darkens when exposed to light or air. The product is best stored under
nitrogen in a refrigerator and should be distilled within a few days of its preparation.
4. The checkers observed that the
calcium chloride absorbs appreciable quantities of product. The crude oil was dissolved in
peroxide-free ether (about 300 ml.) prior to drying with
calcium chloride, or the
calcium chloride was extracted with several
50-ml. portions of dry ether after use. The ethereal extracts were concentrated under
nitrogen and added to the product before distillation.
5. The checkers distilled the product from a flask equipped with a Claisen head but no column.
6. No air should be allowed to come in contact with the hot pot liquid during the distillation, for an exothermic reaction may occur; at best this may fill the apparatus with tarry material and the room with noxious fumes; at worst, pressure built up may destroy all or part of the apparatus. As a precaution, this distillation should be carried out behind a safety shield.
7. The checkers observed that the refractive index of a sample stored for 5 days in the refrigerator in a
stoppered tube wrapped in
aluminum foil and cloth changed from
n25D 1.6074 to
n25D 1.6082.
3. Discussion
4. Merits of the Preparation
The hypohalite route to 1-chloro-, 1-bromo-, or 1-iodoalkynes is both general and convenient. The purity of the reagents does not appear to be critical.
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