Orientation in Aromatic Nitration Reactions¹

Abstract

The isotope dilution method has been employed to determine isomer distributions in the nitration of chloro-, bromo- and iodobenzenes and the relative reactivity of iodobenzene with respect to benzene. For nitration at 25º, the following isomer distributions were found: chlorobenzene, ortho 29.6 ± 1.7%, meta 0.9 ± 0.0%, and para 69.5 ± 1.7%; bromobenzene, ortho 36.5 ± 1.7%, meta 1.2 ± 0.l%, and para 62.4 ± 2.8%; iodobenzene, ortho 38.3 ± 2.0%, meta 1.8 ± 0.3%, and para 59.7 ± 2.0%. The reactivity of iodobenzene relative to benzene at 25º was 0.13 in acetic anhydride and 0.22 in nitromethane. The degree of correlation of the nitration reactivities of benzene derivatives with Hammett σ-constants is discussed. Deviations from the Hammett relationship are interpreted in terms of important contributions of transition-state resonance.