Replacement of hydrogen by carboxyl-oxygen

In some cases nitric acid is recommended as oxidant — provided that nitration does not occur. In the aromatic series it is often possible to oxidize one of several methyl groups while the others remain intact. Boiling o- or /7-xylene in a mixture of 1 volume of concentrated nitric acid (d 1.4) with 2-4 volumes of water gives the respective toluic acid 385 but a greater concentration of acid (2 volumes of concentrated nitric acid to 3 volumes of water) is required to oxidize m-xylene to w-toluic acid.386 Mellitic acid is formed in 35% yield on treatment of hexamethylbenzene with fuming nitric acid.387 

Fierz-David, L. Blangey, and W. von Krannichfeldt, Helv. Chim. Acta, 30, 247 (1947). 

OH ions are formed during oxidations by KMn04 in a neutral medium  

Most permanganate oxidations are carried out in a neutral medium. o-Xylene is converted by potassium permanganate into 0-phthalic acid under all conditions and 0-toluic acid cannot be isolated.388 The toluic acids are easily converted into the corresponding phthalic acids by alkaline permanganate.389 

Isonicotinic and picolinic acid are similarly obtained from 4- and 2-picoline, respectively, by oxidation with permanganate,391 yields being 50- 60% and 50% (as hydrochloride), respectively. 

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