SYNTHESIS OF 1-BROMO-3-METHYLBUT-2-ENE

CAUTION: The entire preparation must be carried out in an efficient fume cup-board since the product is highly lachrymatory.

• Weigh a 100-ml three-necked flask fitted with stoppers. Remove the stoppers and attach to the flask a mechanical stirrer, a calcium chloride guard-tube to the outlet of which is connected a tube leading to the fume cup-board drain, and a gas inlet tube terminating in a glass frit and attached to a dry hydrogen bromide gas generator. 

• Charge the flask with 34 g (0.5 mol) of redistilled 2-methylbuta-l,3-diene (isoprene) (1), cool in an ice-salt bath and pass dry hydrogen bromide gas slowly through the reaction mixture until an increase in weight of 40 g is obtained (2); this may require about 6 hours. 

• Now fit the flask with a Vigreux column in the central joint and with a stopper and a suitable capillary air leak in the side joints. 

• Fractionally distil the crude l-bromo-3-methylbut-2-ene under reduced pressure (ensure that suitable potassium hydroxide traps are sited between the apparatus and the oil immersion rotatory pump) and collect the pure product of b.p. 56-57 °C/25 mmHg; the yield is 58 g (78%).

Notes to keep in  mind:

1. Isoprene is purchased in sealed capsules. These are usually stored in a refrigerator and only removed just prior to opening and redistillation of the isoprene. A simple distillation unit may be employed using a double surface condenser, the rubber tubing water leads being immersed in a large container of ice to effect more efficient cooling. The distillation receiver should be cooled and the outlet protected with a calcium chloride guard-tube. Pure isoprene has a b.p. 33-34 °C.

2. The reaction flask should be removed and replaced by a similar flask to ensure that the stirrer and gas inlet tube do not become unnecessarily contaminated with moisture. The reaction flask should be stoppered with the same stoppers used in the original weighing.