Hammett Correlation in Competing Fragmentations of Ionized 2‐Substituted Benzanilides Generated by Electron Ionization Mass Spectrometry

Abstract

AbstractA good Hammett correlation has been found between the two principal fragmentations of four series of radical‐cations generated by electron ionization of 2‐substituted benzanilides of general structure XC6H4NHCOC6H4Y [X=CH3O, Cl, Br or I; Y=H, F, Cl, Br, I, CH3, OCH3, CF3or NO2in the 3 or 4 position]. When X is electron‐donating, with a negative substituent constant, σ, simple cleavage of these ionic species to form a substituted benzoyl cation, [YC6H4CO]+, is favoured. In contrast, when Y is electron withdrawing, with a positive σ constant, rearrangement leading to elimination of X⋅ by a proximity effect to form a protonated 2‐arylbenzoxazole competes more effectively. Reproducible trends are observed in the derived reaction constant, ρ, which declines on progressing from X=CH3O through Cl and Br to I. These trends establish that the mechanism of the proximity effect involves donation of electron density from the C6H4Y ring and is not materially altered by varying X.