The presence of a free carboxyl or ester function on the alpha-carbon of toluene induces the nitration of the phenyl ring in the ortho position at or above the statistical value (chaperon effect), when pure HNO3 is used in CH2Cl2 solution. This is at variance with the results of classical nitration in H2SO4, where p-nitration predominates by far and m-nitration occurs at a remarkable extent. The new finding is explained in terms of precomplex formation.
