PURPOSE: To obtain the title substance in fast reaction rate and good yield by oxidizing 2,3,6-trimethylphenol with oxygen using copper chloride and Co compound as a catalyst in a solvent containing a polyethylene glycol alkyl ether.

CONSTITUTION: 2,3,6-Trimethylphenol is oxidized with oxygen in the presence of a catalyst consisting of copper chloride and Co compound in a solvent containing a compound (e.g. ethylene glycol dimethyl ether) expressed by the formula (n is 1-3; R is 1-4C alkyl; R' is H or 1-4C alkyl) to provide the title substance. The above-mentioned reaction is carried out in a concentration of the raw material compound of 5-50wt.% based on the reaction liquid at 60-100°C for 1-10 hr. As necessary, an auxiliary solvent can be added in an amount of about ≤20V% to the above-mentioned solvent. An amount of the catalyst used is preferably 0.1-0.5 times by mol based on the raw material compound in the case of copper (II) chloride and 0.05-5 times by mol in the case of Co compound. In the above-mentioned method, an expensive catalyst is made unnecessary.