SYNTHESIS OF 2-PROPYGLUTARIC ACID

• Add 8.0g (10.0ml, 0.15 mol) of redistilled acrylonitrile to a stirred solution of diethyl propylmalonate (30.2 g, 0.15 mol) and of 30 per cent methanolic potassium hydroxide (4.0g) in t-butyl alcohol (100g). 

• Keep the reaction mixture at 30-35 °C during the addition and stir for a further 3 hours. 

• Neutralise the solution with 2 M-hydrochloric acid, dilute with water and extract with ether. 

• Dry the ethereal extract with anhydrous sodium sulphate and distil off the ether: the residue [diethyl (2-cyanoethyl)-propylmalonate; 1 1 g] solidifies on cooling in ice, and melts at 31-32 °C after recrystallisation from ice-cold ethanol. 

• Boil the cyanoethyl ester (10g) under reflux with 40 ml of 48 per cent hydrobromic acid solution for 8 hours, and evaporate the solution almost to dryness under reduced pressure. 

• Add sufficient water to dissolve the ammonium bromide, extract several times with ether, dry the ethereal extract and distil off the solvent. 

• The residual oil (4.5 g, 66%) soon solidifies: upon recrystallisation from water, pure 2-propylglutaric acid, m.p. 70 °C, is obtained.

Notes to keep in mind:

1.  Acrylonitrile forms an azeotropic mixture with water, b.p. 70.5 °C (12.5% water). The commercial product may contain the polymer; it should be redistilled before use and the fraction, b.p. 76.5-78 °C, collected separately as a colourless liquid.