Preparation of Acetophenones from Acetic Acid and Benzenes
An improved process of prepn. of acetophenone derivs. comprises the reaction of an aliphatic acid and an aromatic compd., having a substituent alkyl, cycloalkyl, or alkoxy group in the presence of P2O5 in disperse phase and a diatomaceous form of pure SiO2.
In a specific example to m-xylene (1.0 mol) 106g and glacial acetic acid (1.0 mol) 60g in a flask equipped with a stirrer and reflux condenser protected by a CaCl2 tube are added 3g. of diatomaceous, very pure SiO2, then 71g (0.5 mol) P2O5, the mixture stirred and refluxed for 2 hours, the fluid mass cooled, and the organic layer decanted from the semisolid inorganic matter which is extracted to increase the amount of desired product. Distillation of the final product under reduced pressure gives 52g 2,4-dimethylacetophenone, a colourless liquid, b(20) 120-5°C, and 10g dimethylacetylbenzene, a rapidly crystallizing oil, b(5) 100-50°C.
Propionic acid in place of AcOH with m-xylene gives 2,4-Me2C6H3COEt, while stearic acid with m-xylene in the presence of P2O5 as before gives 2,4-dimethylstearophenone.
Shulgin has a quite similar procedure in PIHKAL, using polyphosphoric acid. Not the best of yields, but super-easy.
Reference: British Patent 633,724 (1949)