SYNTHESIS OF 2-BROMO-3-METHYLPENTANOIC ACID

• Hydrolyse 108g (0.5 mol) of diethyl s-butylmalonate (1) with aqueous ethanolic potassium hydroxide solution following the procedure described for the hydrolysis of diethyl proplymalonate. 

• Transfer the dried ether extract of the product to a 1-litre three-necked flask fitted with a sealed stirrer unit, an efficient reflux condenser and a dropping funnel charged with 80g (26 ml, 0.5 mol) of bromine. 

• Stir the solution, add about 2-3 ml of bromine, and when this has reacted completely drop in the remainder of the bromine at such a rate that the ether refluxes gently. 

• When all the bromine has reacted, add 100 ml of water cautiously from the dropping funnel to decompose any acyl bromide. 

• Separate the ether layer, dry it over anhydrous sodium sulphate and remove the ether (rotary evaporator). 

• Decarboxylate the residue by heating it for 5 hours in an oil bath maintained at 130°C and then distil the resulting bromo acid under reduced pressure, collecting the fraction of b.p. 125-140 °C/20mmHg. The yield is 65g (67%).

Notes to keep in mind:

1. Diethyl s-butylmalonate, b.p. 110-120°C/20mmHg, is prepared by reacting 2-bromobutane with diethyl malonate.