SYNTHESIS OF PROPYLMALONIC ACID

 

• Dissolve 156 g (2.78 mol) of potassium hydroxide in 156 ml of water in a 1.5-litre round-bottomed flask and add 500 ml of rectified spirit to produce a homogeneous solution. 

• Introduce 220g (1.09 mol) of diethyl propylmalonate slowly and with shaking. 

• Attach a double surface reflux condenser and reflux the mixture for 3 hours; hydrolysis is then complete, i.e. a test portion dissolves completely in excess of water. 

• Distil off as much ethanol as possible on a water bath, and dissolve the residue in a comparatively small volume of water. 

• Cool the solution in a large beaker surrounded by ice; add dilute sulphuric acid slowly from a suitably supported dropping funnel, whilst stirring vigorously with a mechanical stirrer, until the solution is acid to Congo red paper. 

• Extract the solution with three 150 ml portions of ether, dry the ethereal extract with anhydrous sodium sulphate and distil off the ether on a water bath. 

• Spread the syrupy residue in thin layers upon large clock glasses (1); after 2-3 days, filter off the crystals at the pump, using light petroleum, b.p. 40-60 °C, to facilitate the transfer from the clock glasses to the sintered glass filter funnel. 

• Spread the crystals on a porous tile to remove traces of oily impurities; the crude propylmalonic acid has m.p. 95-96 °C. 

• Spread the filtrate and washings on large clock glasses as before and filter off the solid which crystallises after 1 day. 

• Repeat the process until no further crystals are obtained. 

• Recrystallise all the crystals from hot toluene. The yield of pure propylmalonic acid, m.p. 96 °C, is HOg (69%).

Notes to keep in mind:

1.  An alternative procedure is to leave the syrupy residue in a vacuum desiccator over anhydrous calcium chloride and silica gel, and to filter off the successive crops of crystals as they separate. These are washed with light petroleum, b.p. 40-60 °C, spread on a porous tile and recrystallised.

Cognate preparations: Butylmalonic acid

• This acid may be similarly prepared from diethyl butylmalonate and melts at 102 °C after recrystallisation from benzene.

s-Butylmalonic acid

• From diethyl s-butylmalonate; the acid melts at 76 °C after recrystallisation from benzene.