Abstract We reported an efficient ligand-free Co (OAc) 2· 4H 2 O/NaOH/O 2/ethylene glycol reaction system that enables selective aerobic oxidation of a wide range of substrates covering 2, 6-di-EWG-, 2, 3, 6-tri-EWG-, 2-EWG-, and 2-EWG-6-EDG-substituted 4-cresols into the corresponding 4-hydroxybenzaldehydes. Based on the experimental investigations and well-defined p-benzoquinone methides, a plausible reaction mechanism was ...