SIGMA VALUES FROM REACTIVITIES

Generalized reactivity parameters provide an important source of information on electronic distributions in organic molecules. The schematics for classification and treatment of reactivities of m- and p-substituted derivatives of benzene in terms of sigma rho correlations are presented. The limitations on precise Hammett linear free energy relationships which are imposed by reaction type and solvent conditions are emphasized and recently proposed modifications of the relationship are shown to be intelligible in terms of electronic distribution effects. The bi-section of tau values to contributing inductive and resonance effects adds insight into the electronic effects of substituent groups. The relationships observed support the classical notion that the resonating substituent localizes charge alternately on the ortho and pant positions of the ring. Deductions based upon reactivity relationships are shown to be powerfully confirmed by F-19 n.m.r. shielding parameters for m- and p-substituted fiuorobenzenes.