Direct mononitration of p-xylene (1,4-dimethylbenzene) under conventional mixed-acid conditions produces exactly 1 ring-substituted isomer. The electrophile is the nitronium ion, NO2+, generated from nitric acid in the presence of sulfuric acid; the substitution proceeds through a cationic Wheland intermediate followed by deprotonation to restore aromaticity. In p-xylene, the two methyl groups occupy the para positions, and the four remaining ring positions C2, C3, C5, and C6 form one symmetry-equivalent set. The molecular framework possesses a C2 rotational axis through the midpoints of the C2–C3 and C5–C6 bonds, interchanging the methyl-bearing C1 and C4 positions and mapping C2 onto C3; a mirror plane through the C1–C4 axis maps C2 onto C6 and C3 onto C5. A second C2 axis and a second mirror plane complete the D2h symmetry description. Electrophilic attack at any unsubstituted carbon therefore yields the identical constitutional isomer after ring renumbering. The product is commonly designated 2-nitro-p-xylene; equivalent benzene-based names are 1,4-dimethyl-2-nitrobenzene and 2,5-dimethylnitrobenzene. The parent hydrocarbon exhibits a four-proton aromatic singlet at approximately δ 7.05 in CDCl3, confirming the magnetic equivalence of the four aromatic hydrogens. Because the mononitration product lacks the para-disubstituted symmetry of the parent, it shows three distinct aromatic proton environments rather than one.

Selective mononitration requires control of nitric acid stoichiometry and thermal severity. Excess nitric acid, prolonged residence time, or elevated temperature can initiate a second nitration step, producing dinitro compounds such as 1,4-dimethyl-2,5-dinitrobenzene; these are outside the mononitration isomer count. Industrial preparations of 2-nitro-p-xylene as an intermediate for 2,5-dimethylaniline are operated with controlled mixed-acid ratios and cooling to limit dinitration and oxidative by-products. Published quantitative selectivity data for specific plant-scale mixed-acid ratios are limited, although the symmetry equivalence of the four ring positions is sufficient to establish the mononitration isomer count without reliance on kinetic selectivity data.