Generally, in liquid phase air oxidation, the naphthenic hydrocarbons with side chain are more readily oxidized and yield of lower fatty acids are greater than those without side chain but some exceptions are recongnized. The authors have investigated about the liquid phase air oxidation of methylcyclohexane (I) and isopropylcyclohexane (II). (I) and (II) were picked out as the representation of the naphthenic hydrocarbons having superior and inferior producibility of lower fatty acids respectively.
When (I) was oxidized for 23hrs. after induction period at 160°C using Mn catalyst with max. partial press. of oxygen at 4.5atm. (calculated for 0°C), 66% yield of formic acid plus acetic acid were obtained based on reacted hydrocarbon. In this case, the oxidation of the starting hydrocarbon was completed in 12hrs., after which oxidation of intermediates mainly proceeded.
In the oxidation of (II) a large part of acetone and considerable part of cyclohexane were obtained in the primary products. This indicates that the oxygen begins its selective attack on the tertiary carbon of side chain, then breaks the C-C bond between the side chain and cyclohexane ring. The poor yields of formic and acetic acid from (II) was explainable from the inferior producibility of formic and acetic acid by oxidation of these intermediates (acetone, cyclohexane, and cyclohexyl radical). The phenomenon of selective oxygen attack on the tertiary carbon of the side chain was also observed in the oxidation of sec-butylcyclohexane.

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