Oxidation of Aromatic Side Chains
Oxidation of Aromatic Side Chains: Eperimental Procedure
In most cases, oxidations with KMnO4 are carried out in a two-phase system. Since KMnO4 is insoluble in non-polar hydrocarbons, solubility is achieved by adding a few moles of a transfer catalyst (crown ether or tetraalkylammonium salts) to the system.
To an aqueous solution of phase-transfer catalyst (0.5 ) and sodium carbonate is added the alkyl-substituted aromatic compound and the oxidizing reagent potassium permanganate in a ratio of 1:7. Proper cautionary steps should be taken because under certain conditions the reaction might be strongly exothermic The mixture is heated under reflux until disappearance of the permanganat color (ca. 45 ). After filtration of manganese dioxide, the carboxylic acid is recrystallized. Impurities caused by excess manganese dioxide can be removed by treatment with hydrogen sulfite or oxalic acid.
A possible, alternative methodology using nitric acid as oxidizing agent (see lower reaction) practically is of no importance.