Organic Syntheses, Coll. Vol. 3, p.312 (1955); Vol. 22, p.39 (1942).
In a 2-l. round-bottomed flask
, surmounted by a packed fractionating column (Note 1)
, is placed a mixture of 354 g. (3 moles) of pinacol (Note 2)
and 10 ml. of commercial 48% hydrobromic acid
. A few boiling chips are added, and the flask is then heated slowly with a colorless flame about 3 in. high (Note 3)
. The distillate is collected until the thermometer
reads 95°; this requires about 2 hours when the rate of distillation is approximately 20–30 drops per minute. The upper, non-aqueous layer is washed twice with 100-ml. portions of water, 0.5 g. of hydroquinone
is added, and the liquid is dried overnight with 15 g. of anhydrous calcium chloride
. It is then fractionated as below, using the same column but a 1-l. flask (Note 4)
described in Organic Syntheses, 20, 96
, filled with glass helices, and wrapped with asbestos paper, is satisfactory. Carborundum may also be used as the filling, but a longer time is required for the distillation. Cold water is circulated through the cold finger.
If too large a flame is used the column floods.
can be kept, without appreciable change, for a limited time in a refrigerator
. If it is not to be used reasonably soon, a little hydroquinone
should be added as an inhibitor.
[II. ALUMINUM OXIDE METHOD]
A Claisen flask
, provided with the usual capillary inlet for air, is connected to a Pyrex tube (Note 1)
which is drawn out at one end and packed with 8-mesh alumina (Note 2)
. The tube is inserted in an electric furnace (Note 3)
capable of maintaining a temperature of 420–470°; the temperature is measured by a thermometer
placed alongside the tube in the furnace. The drawn-out end of the tube is connected by a rubber stopper
to an efficient Pyrex coil-condenser
, which is in turn connected by a rubber stopper
to the first of two receivers, arranged in series and connected by a short length of rubber tubing. Each receiver consists of a 500-ml. filter flask
, which carries an inlet tube (Note 4)
extending somewhat more than halfway to the bottom of the flask. The first receiver is immersed in an ice-salt mixture (Note 5)
; the second is immersed in a Dry Ice-methanol mixture contained in a Dewar flask
. The exit tube of the second receiver is connected to a manometer (Note 6)
and to a water pump
Pinacol (Note 7)
is placed in the Claisen flask
and is distilled, under the vacuum of a water pump, through the Pyrex tube, which is maintained at a temperature of 420–470°. About 100 g. of pinacol
is distilled in 15 minutes, and then the apparatus is swept out by maintaining the reduced pressure for 15 minutes longer. The water and resinous material collected in the ice-cooled receiver are discarded (Note 8)
. The second receiver is removed from the cooling bath, and the product is allowed to melt. Two layers are formed; these are allowed to separate, and then the receiver is replaced in the cooling bath until the lower water layer is frozen. The crude dimethylbutadiene
is then decanted. The yellow product is dried over anhydrous calcium sulfate
and is fractionated (Note 9)
through a Widmer or other column; the portion boiling at 67–70° is collected. The yield from 100 g. of pinacol
is 55–60 g.
) (Note 10)
. The diene is best stored over a small amount of hydroquinone
in a refrigerator.
The Pyrex tube should have a diameter of 3 cm. and a length of 70 cm.
The grade of alumina is important. With an ordinary grade of desiccator
alumina, the checkers were unable to obtain the yields stated. The alumina employed successfully was Alorco activated alumina, from the Aluminum Ore Company, East St. Louis, Illinois. The catalyst darkens in use, but it can be kept in a clean and active condition by the following treatment: after each run, or at least after two or three runs, the tube is heated to 420–470°, and a slow stream of air is drawn through it until the alumina is white (2–5 hours).
A very convenient furnace can be constructed by wrapping a 3-ft. length of 1-in. iron pipe with asbestos paper
and winding a suitable heating element over this paper (about 47.5 ft. of No. 18 B. & S. gauge Nichrome wire). The whole is enclosed in a length of steam-pipe insulation. The temperature is controlled by a properly chosen Variac (variable transformer).
The inlet tube should have a diameter of 10 mm.
This receiver serves to collect most of the water and polymer formed.
Another receiver, consisting of a 3 by 30 cm. side-arm test tube, may be used. It is immersed in a Dewar flask and cooled with Dry Ice-methanol. Only a small amount of material is collected in this receiver.
The contents of this receiver should be examined carefully for the presence of 2,3-dimethylbutadiene
; the checkers usually found appreciable amounts present.
If the dehydration has been inefficient, some pinacolone
) may be present in this product. The pinacolone
may be distilled over the alumina again.
According to the submitters, the yield from 100 g. of pinacolone
is 57–63 g.
The most convenient method for preparation of 2,3-dimethylbutadiene
involves the dehydration of pinacol
. Many catalysts have been used, among them hydrobromic,1
and sulfuric acids;2,3,4 sulfonic acids
of the benzene5
series; acid potassium sulfate
alum;10 aniline hydrobromide
and alumina at 400°.12,13,14
The diene has also been obtained by distillation of the product from the reaction between methylmagnesium iodide
and ethyl α-methacrylate
by treatment of tetramethylethylene dichloride
with alcoholic potash;4
from pinacol hydrochloride
and sodium carbonate
by the action of sodium
on α,α,β-trimethyl-βγ-dibromobutyric acid
by passing 1,2,4-trimethyl-4-isopropenylcyclohexene
over hot copper
and by dehydration of dimethylisopropenylcarbinol
or sulfuric acid
This preparation is referenced from:
Chemical Abstracts Nomenclature (Collective Index Number);
calcium chloride (10043-52-4)
sulfuric acid (7664-93-9)
potassium sulfate (37222-66-5)
HYDROBROMIC ACID (10035-10-6)
sodium carbonate (497-19-8)
calcium sulfate (7778-18-9)
methylmagnesium iodide (917-64-6)
Pinacol hydrate (6091-58-3)
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