SYNTHESIS OF 11-BROMOUNDECANOIC ACID

• Equip a 500-ml three-necked round-bottomed flask with a sealed stirrer unit, a wide-bore gas inlet tube reaching to the bottom of the flask and a two-necked multiple adapter fitted with a thermometer and a condenser protected with a calcium chloride guard-tube; arrange the apparatus so that occasional cooling can be effected with an ice-water bath. 

• In the flask place a solution of 27.6 g (0.15 mol) of undec-10-enoic acid in 220 ml of dry light petroleum (b.p. 40-60 °C) together with 1.5 g (0.006 mol) of benzoyl peroxide. 

• Pass a rapid stream of dry hydrogen bromide [from a cylinder or from 20 ml of tetralin and 17 ml of bromine] through the stirred mixture until it is saturated (about 0.75 hour) while maintaining the temperature between 10 and 20 °C by occasional cooling. 

• Should a white solid separate and tend to block the inlet tube towards the end of the reaction, either add a further small portion of dry light petroleum or maintain the temperature nearer to 20 °C when the material should dissolve.

• Decant the pale straw-coloured solution into a conical flask and rinse the reaction vessel with 40 ml of light petroleum (b.p. 40-60 °C) and combine with the main solution. 

• Cool the solution to — 10 °C and collect the solid which separates by filtration under suction and wash with about 40 ml of similarly cooled light petroleum. 

• A further quantity of the product may be obtained by concentrating the filtrate to about 40 ml and cooling. 

• Dissolve the crude product in approximately 200 ml of boiling light petroleum (b.p. 40-60 °C), add decolourising charcoal, filter, concentrate to about 1 50 ml and cool. 

• Collect the 11-bromoundecanoic acid which separates as microcrystalline platelike needles, m.p. 49-50 °C. 

• A further quantity of slightly less pure acid may be obtained by concentrating the mother-liquors to about 30 ml and cooling. The yield is 27.9 g (70%).