SYNTHESIS OF METHYL y-BROMOCROTON ATE (Methyl 4-bromobut-2-enoate)

Methyl crotonate:

• Purify commercial crotonic acid by distilling lOOg from a flask attached to an air condenser, use an air bath (Fig. 2.46(a)). 

• The pure acid passes over at 180-182 °C and crystallises out on cooling, m.p. 72-73 °C; the recovery is about 90 per cent. 

• Place 75 g (2.34 mol) of absolute methanol, 5 g (2.7 ml) of concentrated sulphuric acid and 50 g (0.58 mol) of pure crotonic acid in a 500-ml round-bottomed flask, and heat under reflux for 12 hours. 

• Add water, separate the precipitated ester and dissolve it in ether; wash with dilute sodium carbonate solution until effervescence ceases, dry with magnesium sulphate and remove the ether on a water bath. 

• Distil and collect the methyl crotonate at 1 18-120 °C; the yield is 40 g (69%).

Methyl y-bromocrotonate:

• Mix 36 g (0.2 mol) of JV-bromosuccinimide, 20 g (0.2 mol) of methyl crotonate and 60 ml of dry, redistilled carbon tetrachloride in a 500-ml round-bottomed flask. 

• Reflux on a water bath for 12 hours; by this time all the solid should have risen to the surface of the liquid.

• Filter off the succinimide at the pump and wash it with a little dry carbon tetrachloride. 

• Remove the solvent on a water bath and distil the residue under reduced pressure through a short fractionating column. Collect the methyl y-bromocrotonate at 77-78 °C/8mmHg; the yield is 31 g (86%).