SYNTHESIS OF 1-BROMO-3-METHYLBUTA-1,2-DIENE

STEP 1: In a 250-m1 two-necked, round-bottomed flask fitted with a stirrer and a dropping funnel, place a mixture of 14.3 g (0.1 mol) of powdered copper(r) bromide, 9.8 g (0.1 mol) of powdered ammonium bromide, 0.64 g (0.01 mol) of copper powder and 80 ml (c. 0.66 mol) of 46-48 per cent w/w concentrated hydrobromic acid (d 1.46-1.49). 

STEP 2: Stir the dark green mixture vigorously at room temperature and add 25.2 g (0.3 mol) of 2-me thylbut-3-yn-2-ol drop-wise during 5 minutes; maintain the vigorous stirring for 1.5 hours (1). 

STEP 3: Filter the reaction mixture through a sintered glass Buchner funnel and rinse the reaction vessel and funnel with 10 ml of light petroleum (b.p. 40-60 °C). 

STEP 4: Transfer the filtrate to a separatory funnel, discard the lower aqueous layer and remove any unchanged alcohol and copper salts from the almost colour­less organic layer by shaking with three 10 ml portions of concentrated hydrobromic acid. 

STEP 5: In the final washing the lower acid layer should show no trace of violet coloration. 

STEP 6: Dry the combined organic layers over a mixture of solid sodium hydrogen carbonate and magnesium sulphate, filter through a sintered glass Buchner funnel; rinse the flask and the inorganic solids with 5 ml of light petroleum (b.p. 40-60 °C). 

STEP 7: Transfer the filtrate to a 50-m1 distilla­tion flask fitted with a 9-cm fractionating column filled with glass helices and distil under reduced pressure, using a nitrogen bleed, from a bath held at 80­90 °C. 

STEP 8: Collect the 1-bromo-3-methylbuta-1,2-diene as a fraction, b.p. 52­54 °C/50 mmHg, in a flask cooled in ice-water (2). 

STEP 9: The yield is 28.1 g (64%). The purity of the product may be checked by g.l.c. analysis using a 1.5-m col­umn of 10 per cent Silicone oil on Chromosorb W held at 80 °C with a nitro­gen flow rate of 40 ml/minute; the material has t_R3.5 minutes. 

STEP 10: The infrared spectrum (liquid film) shows the characteristic strong allene (C=C—C) absorption band at 1950 cm ^-1 (see Fig. 3.20). On standing 1-bromo-3- methylbuta-1,2-diene tends to isomerise to 1-bromo-3-methylbuta-1,3-diene and polymerise.

Notes to keep in mind: 

1. The progress of the reaction may be monitored by occasionally halting the stirring and removing a portion of the clear upper organic layer for infrared examina­tion. The reaction is complete when a sample does not show hydroxyl and alkynyl hydrogen absorption at 3400 and 3300 cm -' respectively.

2. In common with the distillation of all unsaturated compounds in general, the resi­due in the distillation flask should be cooled to room temperature before admitting air into the system.