Ortho Effect. II.

Abstract

In order to elucidate the nature of the ortho effect in the ionizations of substituted benzoic acids, microscopic dissociation constants of 2-substituted (OCH3, CH3, H, Cl, Br) 5-amino benzoic acids were determined in water at 25°C. Using those values, the substituent effects on the ortho effect in the ionization step were divided into those involved in the initial state and those in the final state of the equilibrium. Those substituent effects are very small compared with the ortho effect itself. Since the substituent effects on the steric inhibition of resonance are large (Part III), it is suggested that, in contrast with the views of the previous workers, the steric inhibition of resonance can not be the main cause for the ortho effect, and that the direct field effect would rather be the main factor responsible for this ortho effect.