Decarboxylation of substituted 4‐aminobenzoic acids in acidic aqueous solution

Rates of decarboxylation have been determined of the following substituted 4‐aminobenzoic acids 2‐H‐, 2‐CH3‐, 2‐Cl‐, 2‐Br‐, 2‐NO2‐, 2‐OH‐, 2‐OCH3‐, 3‐Br‐, 2,6‐(CH3)2‐, 2,6‐(C2H5)2‐, 2,6‐Cl2‐ and 2,6‐Br2‐ in aqueous hydrochloric acid solution (pH ranging from about 0 to about 2), at 50° and an ionic strength of 0.1 (except for the lowest pH values). Analysis of the data in terms of a second‐order mechanism involves predominantly the species neutral ampholyte (notzwitterion) and hydronium ion for the range 0 < pH < 2. A general rate expression has been derived with the following features: (a) both (neutral) ampholyte and anion decarboxylation and (b) hydronium‐ion inhibition at lower pH. Essential to the mechanism is that the σ‐complex formed by addition of a proton at the C(1)position of the neutral acid cannot decarboxylate unless it first ionizes into the corresponding anion (loss of the carboxyl proton). The formation of the σ‐complex is not necessarily rate determining. The reaction mechanism can explain not only all relevant facts concerning the acids investigated but also the data offered by the literature on substituted benzoic acids, in particular all available data on the13C‐carboxyl‐isotope effect and the (H2O, D2O)‐solvent‐isotope effect.