Org. Synth. 1937, 17, 25
DOI: 10.15227/orgsyn.017.0025
1,3-BUTADIENE
Submitted by E. B. Hershberg and John R. Ruhoff.
Checked by W. H. Carothers and J. Harmon.
1. Procedure
Cyclohexene (
Org. Syn. Coll. Vol. I, 1941, 183, and
p. 152 below) is boiled in the
flask B, shown in
Fig. 3, and the vapor is passed over a cracking element consisting of an
expansible grid threaded with resistance ribbon L
(Note 1) and
(Note 2). The boiling flask B is supported over a
250-watt bowl heater provided with a rheostat, and the current in the cracking element is taken from a 115-volt a-c. or d-c. source and controlled by a second
rheostat of 10-ampere capacity (Note 3). In order to trap any
cyclohexene which passes the
coil condenser, the
gas-delivery tube D is connected to a tube leading close to the bottom of a
500-cc. distilling flask immersed in an
ice bath. The exit tube of the flask is connected with a short section of rubber tubing to a receiver for condensing the
butadiene, consisting of a
large test tube with the entrance tube leading halfway to the bottom and an exit tube at the top for conducting the
ethylene (saturated with
butadiene) to a
hood or outdoors. The receiver is cooled in a
Dewar flask containing solid
carbon dioxide and a eutectic mixture of equal parts by weight (or volume) of
chloroform and
carbon tetrachloride.
Fig. 3
The flask B is two-thirds filled with cyclohexene, and, with the cooling water flowing, this is heated to vigorous boiling. When the vapor has displaced the air from the apparatus completely (Note 4), the current is turned on in the cracking unit. By adjusting both the rate of boiling and the current the ribbon is maintained at a bright red heat over its entire length. Very rapid refluxing is necessary in order to prevent undue carbonization on the filament and tar formation on the glass walls (Note 5). The generator can be run intermittently or until the charge is exhausted.
The butadiene collected is purified by a bulb-to-bulb distillation, the receiver in the cooling mixture being replaced by a similar container to which it is connected by means of rubber tubing and into which the butadiene is allowed to distil. The product is quite satisfactory for most uses, as in the Diels-Alder reaction (Note 6). The cracking element uses about 500 watts at 8.7 amperes and produces 25–30 g. of butadiene per hour. The yield, based on the cyclohexene consumed (Note 7) and on redistilled product, is 65–75 per cent of the theoretical amount. High-boiling residues accumulating in the boiling flask should be removed after preparing 100–150 g. of butadiene (Note 8).
2. Notes
1.
Apparatus.—The unit shown in
Fig. 3 is constructed of Pyrex glass, and the following is a summary of satisfactory dimensions for the various tubings (outer diameters) and of other specifications: A, 32-mm. tubing; B,
500-cc. flask; C, 7-mm. tubing; D, 8-mm. tubing; E, No. 35 standard taper joint; F, coil of 3/16-in. copper tubing (see
(Note 2)); G,
cork stopper; H,
1/8-in. brass rod; I, rubber tubing; J, 3-mm. glass tubing; K,
brass electrical connectors; L, Chromel C resistance ribbon No. 37 B. and S. gauge, 1.9–2 ohms per foot, width 1/16 in., length 56 to 60 in. (see below); M, tungsten wire-loop supports sealed into the suspended glass rod; the center supports are of 0.01-in. (dia.) wire, those at the top and bottom of 0.015-in. (dia.) wire; N, copper wire No. 22 B. and S. gauge; O, 12-mm. tubing.
The ribbon L of the cracking element is threaded between the tungsten loops M, of which there are five each at the top and bottom and eight at the center. The unit is suspended with a copper wire from the glass cross-support as shown. The lower end is free to drop down as the resistance wire expands; this prevents short-circuiting of the element.
The composition of the filament is of importance in determining the yield and the performance of the apparatus. With nickel-chromium alloys excessive carbonization occurs and the yield is poor. Much better results are obtained with the nickel-iron-chromium alloys called Chromel C and Nichrome Alloy Wire.
2.
To provide for the proper functioning of the coil condenser even in warm weather it is advisable to increase the number of turns to 30–40, although the condenser as shown is satisfactory with tap water at 4–10°.
3.
A
slide-wire rheostat of 2-ampere capacity wound on a
hollow enameled iron tube will carry the required current if a stream of cooling water is passed through the tube.
4.
An explosion may occur if the filament is heated while an appreciable amount of air is still present. Furthermore, the heating element will burn out at once if an adequate supply of
cyclohexene vapor is not supplied, since the current passing through the wire is far above the normal rating for air.
5.
In case of excessive carbonization the rate of boiling should be increased or the filament temperature slightly lowered. In general the vapor velocity should be as high as possible without exceeding the capacity of the
copper condenser.
6.
The crude product contains appreciable amounts of C
2, C
3, and C
6 fractions. The actual
butadiene content lies between
82 and 88 per cent. If very pure material is desired the
butadiene is converted into the tetrabromide, which is crystallized and reconverted to the hydrocarbon by means of
zinc and
alcohol.
17.
The
cyclohexene collecting in the
ice trap ordinarily is returned to the boiling flask; in determining the percentage conversion this was combined with any material left in the boiler and the pure starting material present recovered by fractionation.
3. Discussion
The methods for the preparation of gaseous products containing more or less
butadiene are too numerous for profitable review here. Especially is this true since the development of commercial processes which are not particularly suitable for laboratory operation. The most satisfactory procedure for preparing
butadiene in the laboratory is the pyrolysis of
cyclohexene,
2 which has been shown to yield a product consisting essentially of
butadiene.
1,
3 For this pyrolysis, the apparatus described above and that described by Williams and Hurd
4 are available. Other laboratory preparations of
butadiene start from
butyl chloride,
5 2,3-dibromobutane,
6 crotyl chloride,
6 and
1,3-butylene glycol.
7This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
alcohol (64-17-5)
Cyclohexene (110-83-8)
carbon tetrachloride (56-23-5)
carbon dioxide (124-38-9)
acetone (67-64-1)
zinc (7440-66-6)
ethylene (9002-88-4)
Butyl chloride (109-69-3)
1,3-Butadiene,
butadiene (106-99-0)
2,3-dibromobutane (5408-86-6)
crotyl chloride (4894-61-5)
1,3-butylene glycol (107-88-0)
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