Organic Syntheses, Coll. Vol. 1, p.209 (1941); Vol. 5, p.49 (1925).
A 500-cc. round-bottomed flask
is connected by a wide, bent glass tube
to an efficient condenser
, which is provided with an adapter
leading into a 500-cc. receiving flask
immersed in an ice bath
. Two hundred grams (0.71 mole) of 1,2,3-tribromopropane (Note 1)
and 10 cc. of water are placed in the reaction flask, and 50 g. (1.25 moles) of sodium hydroxide (Note 2)
in small lumps (not pulverized) is added at once with shaking. The mixture warms up and the flask is immediately connected to the apparatus and heated directly by means of a Bunsen burner
held in the hand. It is advisable to use a slightly luminous flame about 15 cm. high and to heat with a rotary motion, directing the flame against the sides of the flask.
The mixture is shaken occasionally, and the alkaline layer soon becomes partly emulsified. Heat is applied until vigorous ebullition occurs, whereupon a spontaneous distillation of the reaction product takes place (Note 3)
. When the reaction slackens, the flask is heated as before. The mass becomes solid as the volatile products are removed. This solid mass, which is brownish in color, is heated until no more liquid distils (Note 4)
. The entire operation up to this point requires twenty to thirty minutes.
The distillate in the receiving flask separates into two layers: an upper layer of water; and a heavy layer of colorless oil, consisting of a mixture of 2,3-dibromopropene
and unchanged tribromopropane
. The distillate is transferred to a 500-cc. separatory funnel
and thoroughly agitated with an additional 150 cc. of water (Note 5)
. The lower layer is decanted and found to weigh 140–145 g.
It is subjected to a preliminary distillation under reduced pressure to separate most of the unchanged tribromopropane
. The material that passes over below 95° /75 mm.
consists of 2,3-dibromopropene
containing a small amount of water and tribromopropane
, and weighs 120–130 g.
The residue in the distilling flask consists of unchanged tribromopropane
) (Note 6)
The crude product is dried over calcium chloride
and fractionally distilled under reduced pressure (Note 7)
. The yield of pure, colorless, 2,3-dibromopropene
boiling at 73–76° /75 mm.
is 105–120 g.
per cent of the theoretical amount). A small quantity of tribromopropane
, usually about 5 per cent of the crude material, remains in the distilling flask (Note 6)
may be used in place of sodium hydroxide
, but it was found that the reaction is much more violent and irregular and the yields are somewhat lower. If potassium hydroxide
is used, it is advisable to provide the receiving flask with a reflux condenser
to avoid loss when the initial violent reaction occurs. Since the reaction is so rapid, it would be very dangerous to use more than 200 g. of tribromopropane
in a single run with potassium hydroxide
In the initial vigorous ebullition, about one-half of the material distils during a few minutes and it is necessary to wait for this reaction to slacken before continuing to heat the flask. If sodium hydroxide
is used, the reaction is vigorous but not violent, and it is possible to use 600 g. of tribromopropane
in a 1-l. flask
in a single run without danger if the reaction flask is connected with the condenser by means of a wide tube
. The yields in larger runs are as good as when smaller runs are made.
A small additional quantity of distillate may be obtained by partially evacuating the apparatus, after replacing the adapter and receiving flask by a 250-cc. distilling flask
. The amount of material obtained in this way amounts to only 1–2 g.
if the reaction flask has been properly heated. It is well to note that flasks which have been repeatedly heated with very strong alkalies become fragile, and care should be used not to reduce the pressure too greatly.
and the wash water have a strong lachrymatory effect and should be manipulated carefully.
When a sufficient quantity of these crude residues of tribromopropane
has been collected, it may be used for preparing 2,3-dibromopropene
. If allowance is made for this recovered material, the yield of 2,3-dibromopropene
per cent of the theoretical amount.
These substances may be distilled under atmospheric pressure with only slight decomposition, but the products become highly colored on standing.
This preparation is referenced from:
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