EFFECTS OF WATER AND ONIUM ION PHASE-TRANSFER CATALYST ON THE RATE OF LIQUID-PHASE OXIDATION OF HYDROCARBONS

The liquid-phase oxidations of tetralin, cumene, cyclohexene and methylcyclohexane were investigated in the presence of water and different types of surfactants. It was found that water first increases, then decreases, the rate of oxidation. Aromatic compounds are oxidised more readily than non-aromatic ones. [H2O]max is mostly determined by the concentration of hydroperoxide present, and it does not depend on the nature of the hydrocarbon. H2O and D2O have similar catalytic effects; both inhibit the oxidation when applied in a larger excess. Anionic and non-ionic surfactants reduce the catalytic effect of water, because access to the hydroperoxide molecules by the water droplets is considerably hindered by the surrounding surfactant ions or molecules. In contrast, cationic surfactants catalyse the oxidation, their efficiencies largely depend on the nature of their anions. It is assumed that the water catalysis arises from the H-bonding interaction of the hydroperoxide with the discrete or less-associated water molecules. Water and onium ion have a joint catalytic effect. The onium-ion pair attacks at the more nucleophilic O-atom of the hydroperoxide, while the outer O-atom is activated by water, through H-bond formation.