SYNTHESIS OF GLUTARIC ACID (Pentanedioic acid)

Tetraethyl propane-1,1,3,3-tetracarboxylate 

• Cool a mixture of 320 g (302 ml, 2 mol) of redistilled diethyl malonate and 80 g ( 1 mol) of 40 per cent formaldehyde solution ('formalin') contained in a 1 -litre round-bottomed flask to 5 °C by immersion in ice, and add 5 g (7 ml) of diethylamine. 

• Keep the mixture at room temperature for 15 hours and then heat under a reflux condenser on a boiling water bath for 6 hours. 

• Separate the aqueous layer, dry the organic layer with anhydrous sodium sulphate and distil under reduced pressure. 

• Collect the tetracarboxylate ester at 200-215 °C/20mmHg. The yield is 250g (75%).

Glutaric acid

• Heat a mixture of 125g (0.376 mol) of the preceding ester and 250 ml of 1:1-hydrochloric acid under reflux with stirring in a 1-litre two-necked flask equipped with a mechanical stirrer and reflux condenser. 

• Continue the heating until the mixture becomes homogeneous (6-8 hours). 

• Evaporate the contents of the flask to dryness on a steam bath (rotary evaporator) and distil the residual glutaric acid under reduced pressure. 

• Collect the fraction boiling at 185-195°C/10mmHg; it crystallises on cooling. 

• Moisten with a little water (to convert any glutaric anhydride present into the acid), heat gently and dry at 30 °C. 

• Recrystallise from chloroform (or benzene): the resulting practically pure glutaric acid, m.p. 96-97 °C, weighs 40 g (81%).