A process for selective oxidation of dimethyl-1,1′-biphenyl(s) to form methyl-1,1′-biphenyl mono-carboxylic acid(s), which can be esterified to form plasticizers, comprising contacting a solution of dimethyl-1,1′-biphenyl(s) in acetic acid in the presence of an oxidation catalyst and air under time and temperature conditions sufficient to oxidize the dimethyl-1,1′-biphenyl(s) into one or more methyl-1,1′-biphenyl mono-carboxylic acid(s) products, conducting at least one of (i) adding an antisolvent, or (ii) optimizing a total conversion of dimethyl-1,1′-biphenyl(s) by oxidation based upon a molar ratio of dimethyl-1,1′-biphenyl isomers, or (iii) precipitating the methyl-1,1′-biphenyl mono-carboxylic acid(s) products by lowering the temperature, or (iv) decreasing the oxidation reaction temperature to enhance conversion of aldehydes over methyl functional groups, so as to limit over-oxidation of the dimethyl-1,1′-biphenyl(s), wherein the oxidation reaction is conducted in the absence of bromide-containing catalysts.