A laboratory route to p-toluic acid involves oxidation of p-cymene with nitric acid.

[2] p-Toluic acid is an intermediate in the conversion of p-xylene to terephthalic acid, a commodity chemical used in the manufacture of polyethylene terephthalate.

It is generated both by the oxidation of p-xylene as well as the hydrogenolysis of 4-carboxybenzaldehyde.

In related processes it is converted to methyl p-toluate, which is oxidized to monomethyl terephthalate.

Material Safety Data Sheet