When aniline is treated with a mixture of nitric acid and sulfuric acid, the expected nitration product (para-nitroaniline) is obtained in poor yield. Instead, the major product from nitration is meta-nitroaniline. Apparently, the amino group is protonated under these acidic conditions, and the resulting ammonium group is a meta-director, rather than an ortho-para director. Propose a plausible method for converting aniline into para-nitroaniline.