Organic Syntheses, Coll. Vol. 5, p.285 (1973); Vol. 47, p.31 (1967).
In a 3-l. three-necked, round-bottomed flask
fitted with a mechanical stirrer
and set up for a simple vacuum distillation are placed 500 ml. of triethylene glycol dimethyl ether (Note 1)
and 300 ml. of isopropyl alcohol
. Mechanical stirring is started, and 53.5 g. (2.23 moles) of sodium hydride
in a mineral oil suspension is added in small portions. After the addition is complete, the remaining neck of the flask is fitted with a Y-tube to which is connected a two-holed rubber stopper
containing a thermometer
which reaches into the flask below the liquid level and a piece of glass tubing which is connected to a nitrogen tank
. A pressure-equalizing dropping funnel
containing 242 g. (1.00 mole) of 1,2-dibromocyclohexane (Note 2)
is placed in the other arm of the Y-tube.
The temperature of the reaction flask is raised to 100–110°, and the receiving flask is cooled in a dry ice-isopropyl alcohol
bath as a rapid stream of nitrogen
is passed through the system. After most of the isopropyl alcohol
has been removed by distillation (Note 3)
, the receiver is changed, and the system is evacuated by a water aspirator (Note 4)
. Dropwise addition of 1,2-dibromocyclohexane
is begun, and the rate of addition is adjusted so that the temperature of the reaction mixture is maintained at 100–110° without external heating. The addition requires about 30 minutes; the reaction is terminated when distillation becomes very slow.
The distillate is washed four times with 200-ml. portions of water, and the organic layer is dried with anhydrous magnesium sulfate
. The yield of 1,3-cyclohexadiene
is 56 g.
) (Note 5)
. The diene can be separated from higher-boiling contaminants by a simple distillation at atmospheric pressure under nitrogen
; b.p. 78–80°
, yield 28–32 g.
) (Note 6)
was prepared by the method of Snyder and Brooks.2
If the cyclohexene
is cooled to ca.
−30° with a dry ice-isopropyl alcohol bath and the bromine
is not diluted, it is possible to run this preparation on a threefold scale in one-third of the recorded time. The product was always purified by the recommended procedure.
If the flow of nitrogen
is rapid, the distillation can be completed in about 1 hour; otherwise the distillation is very slow.
A dry ice-isopropyl alcohol trap
was inserted before the aspirator to catch any uncondensed product. The checkers also inserted a manometer between this trap and the aspirator, and maintained the pressure during the reaction at 130–170 mm. by careful adjustment of the regulator valve of the nitrogen cylinder.
If the temperature rises too high or the vacuum is not sufficient to flash out the diene as it forms, the product will be contaminated with small amounts of cyclohexene, benzene, and 1,4-cyclohexadiene
The checkers found that distillation without the use of a nitrogen
atmosphere gave 43–44 g.
) of product, b.p. 80–83°
, of excellent purity as shown by n.m.r. spectroscopy.
has been prepared by dehydration of cyclohexen-3-ol
by pyrolysis at 540° of the diacetate of cyclohexane-1,2-diol
by dehydrobromination with quinoline of 3-bromocyclohexene
by treating the ethyl ether of cyclohexen-3-ol
with potassium bisulfate
by heating cyclohexene oxide
with phthalic anhydride
by treating cyclohexane-1,2-diol
with concentrated sulfuric acid
by treatment of 1,2-dibromocyclohexane
with sodium hydroxide
in ethylene glycol
and with quinoline
and by treatment of 3,6-dibromo-cyclohexene
4. Merits of the Preparation
Because of its convenience and simplicity this procedure is the method of choice for laboratory preparation of 1,3-cyclohexadiene
. This olefin is an intermediate of some importance because it offers a route via the Diels-Alder reaction to a variety of bicyclic compounds.4,7,10
This preparation is referenced from:
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