Organic Syntheses, CV 3, 312
Submitted by C. F. H. Allen and Alan Bell.
Checked by R. L. Shriner and John C. Robinson, Jr..
1. Procedure
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).
2. Notes
1. The
column 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.
3. If too large a flame is used the column floods.
4.
2,3-Dimethylbutadiene 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]
Submitted by L. W. Newton and E. R. Coburn.
Checked by Nathan L. Drake and Richard Tollefson.
1. Procedure
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) or
pinacolone 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. (
79–86%) (Note
10). The diene is best stored over a small amount of
hydroquinone in a refrigerator.
2. Notes
1. The Pyrex tube should have a diameter of 3 cm. and a length of 70 cm.
2. 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).
3. 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).
4. The inlet tube should have a diameter of 10 mm.
5. This receiver serves to collect most of the water and polymer formed.
6. 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.
8. The contents of this receiver should be examined carefully for the presence of
2,3-dimethylbutadiene; the checkers usually found appreciable amounts present.
9. If the dehydration has been inefficient, some
pinacolone (b.p.
107°) may be present in this product. The
pinacolone may be distilled over the alumina again.
10. According to the submitters, the yield from
100 g. of pinacolone is
57–63 g. (
70–77%).
3. Discussion
The most convenient method for preparation of
2,3-dimethylbutadiene involves the dehydration of
pinacol. Many catalysts have been used, among them hydrobromic,
1 hydriodic,
1 and sulfuric acids;
2,3,4 sulfonic acids of the
benzene5 and
naphthalene6,7,8 series; acid
potassium sulfate;
9 alum;
10 aniline hydrobromide;
1 iodine;
11 hot
copper at 450–480°;
1 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;
15 by treatment of
tetramethylethylene dichloride with alcoholic potash;
4 from
pinacol hydrochloride and
sodium carbonate;
16 by the action of
sodium or
pyridine on
α,α,β-trimethyl-βγ-dibromobutyric acid;
17 by passing
1,2,4-trimethyl-4-isopropenylcyclohexene over hot
copper;
18 and by dehydration of
dimethylisopropenylcarbinol with hydrochloric
19 or
sulfuric acid.
17
This preparation is referenced from:
Appendix
Compounds Referenced (Chemical Abstracts Registry Number)
α,α,β-trimethyl-βγ-dibromobutyric acid
sulfonic acids
ethyl α-methacrylate
calcium chloride (10043-52-4)
sulfuric acid (7664-93-9)
Benzene (71-43-2)
formaldehyde (630-08-0)
potassium sulfate (37222-66-5)
hydroquinone (123-31-9)
HYDROBROMIC ACID (10035-10-6)
sodium carbonate (497-19-8)
copper (7440-50-8)
calcium sulfate (7778-18-9)
iodine (7553-56-2)
Pinacolone (75-97-8)
pyridine (110-86-1)
sodium (13966-32-0)
Naphthalene (91-20-3)
methylmagnesium iodide (917-64-6)
Pinacol hydrate (6091-58-3)
pinacol (76-09-5)
trimethylethylene (513-35-9)
2,3-DIMETHYL-1,3-BUTADIENE,
1,3-Butadiene, 2,3-dimethyl-,
2,3-Dimethylbutadiene (513-81-5)
dimethylbutadiene (926-56-7)
tetramethylethylene dichloride
pinacol hydrochloride
1,2,4-trimethyl-4-isopropenylcyclohexene
dimethylisopropenylcarbinol
aniline hydrobromide
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