Reaction C.—Oxidation of the Side Chain in Aromatic Compounds

Reaction C. Oxidation of the Side Chain in Aromatic Compounds. (A., 122,184 133, 41 137, 308 141,144 147, 292 B., 7,1057 19, 705 Z. Ch., 4, 119) (Fittig).—When aromatic compounds containing aliphatic side chains attached to the nucleus are treated with certain oxidising agents (potassium permanganate, dilute nitric acid, and chromic acid), the side chain is oxidised until only a carboxylic group attached to the nucleus remains the end methyl group, if there are several carbon atoms 

—Mesitylenic acid 50% theoretical (12 gms.). Uvitic acid 40% theoretical (12 gms.). Mesitylenic acid forms colourless monoclinic crystals difficultly soluble in hot water easily soluble in cold alcohol M.P. 166°. Uvitic acid forms colourless fine needles insoluble in cold and hot water readily soluble in alcohol and ether M.P. 287°—288°. (A., 122, 184 141, 144 147, 292 Z. Ch 4, 119.) 

The above illustrates the action of nitric acid in oxidising only one or two, and not all of the alkyl groups present, unless the heating is very prolonged. In the next preparation Method I. shows how chromic acid or alkali bichromate and sulphuric acid completely oxidises all the side chains present Method II. indicates how to oxidise completely the partially oxidised compound, potassium permanganate being sufficient. 

Method I. (from p-xylene).—20 gms. (1 mol.) of p-xylene are refluxed for 24 hours with 80 gms. (excess) of sodium or potassium bichromate and 250 gms. of 50% sulphuric acid. The unoxidised hydrocarbon is removed by distillation in steam, and the cooled solution filtered. The precipitate is purified by reprecipitation with acid from its dilute solution in sodium carbonate or caustic alkali. 

—90% theoretical (12 gins.). Colourless crystals insoluble in water and in alcohol sublimes without melting at 300°. (A., 137, 308.) 

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