Czech Translation by Karel, HTML by Rhodium
The preparation of phenylacetone from phenylacetic acid and acetic anhydride is described. A mixture of reactants with alkali acetate and copper sulphate is refluxed, extracted, decanted and distilled, giving the pure product in a minimum 50% yield. The advantages of this procedure consist in the higher efficiency of decarboxylation, a shorter reaction time, removal of acetic acid by decantation and easier extraction of the product without previous neutralisation with alkali.
Phenylacetone - benzylmethylketone, 1-phenyl-2-propanone, C9H10O, is important intermediate in chemical, pharmaceutical industry and cosmetics. It is prepared by various methods; one branch of them are they using Grignard reagents1,2, chloroacetone3 or by saponification of phenylacetonitrile4,5. The second branch is based on decarboxylation of phenylacetic acid (or its salts) and reaction with acetic anhydride. For those methods the most determining factor is velocity and completeness of its decarboxylation, which can be improved by various catalysts. Reaction can be conducted by dry distillation of calcium salts of phenylacetic and acetic acid6,7, boiling of mixture of these acids with alkali acetates8-10, pyridine or quinoline11, or methyllithium12. Advantageous are also the methods based on passing of phenylacetic-acetic acid mixture over various catalysts as ThO25,13 or MnCO314 at 350-400°C.
Crude product is usually purified by fractionation or bisulfite method. Pure phenylacetone has mp -15°C, bp 215°C, d. 1.0018, nD 1.5164; its semicarbazone8,11 has mp 188-198°C, thiosemicarbazone15 mp 155°C, phenylhydrazone9 mp 85°C, 4-nitrophenylhydrazone16 mp 143°C and 2,4-dinitrophenylhydrazone17 mp 156°C.
Disadvantageous is long reaction time 24-72 h for liquid-phase methods and workup of the reaction mixture by neutralization with hydroxide, extraction or fractionation of its large volume for all methods. Yields are 30-50% for liquid-phase methods and 40-65% for gas-phase methods. The improved method provides yields min. 50% with shorter reaction time, removing of the neutralization step, and replacement of extraction by easier decantation.
To a mixture of 136 g (1.0 mol) phenylacetic acid, 70 g sodium or potassium acetate, and 16 g (0.1 mol) anhydrous cupric sulphate is introduced 2000 ml anhydrous acetic anhydride* in 4000 ml flask. The mixture is refluxed 24 h. After cooling 500 ml of solvent (CCl4, CHCl3, CH2Cl2) is added and the mixture is poured to a flask containing 2000 ml ice-water. After separation of layers upper layer is removed and lower layer is three times decanted with water, separated, dried (Na2SO4, CaCl2), and distilled. The solvent is distilled off and fractionation column is placed on the top of the flask. Remaining acetic acid and acetic anhydride is then removed, pure product is collected at 100°C/15mmHg. Yield 70-90 g (52-67%).
* The technical product, which contains water, can be dried by adding 10 g thionyl chloride.