DECARBOXYLATION OF AZULENE-1-CARBOXYLIC ACID IN ACIDIC AQUEOUS SOLUTION

Azulene-1-carboxylic acid decarboxylates at a measurable rate at 25° in aqueous solution over a wide range of acidity. In dilute solutions of a strong acid (0.001-0.03 M) the reaction is acid catalyzed, but at slightly higher acidities (0.06-0.3 M) the reaction becomes independent of acid concentration. At still higher acid concentrations (0.5-6.0 M HClO4 or H2SO4), a gradual increase in rate is observed for which d log k/d(- H0) = 0.3, but this increase is the consequence of a positive salt effect superimposed on the pH-independent reaction. An increase in acidity beyond 6 M causes a rapid decrease in rate of decarboxylation due to the equilibrium formation of a stable cation by protonation of the ring at the 3-carbon. The rate data at low acidities are quantitatively consistent with a mechanism in which there is a slow protonation to give a 1-carbon protonated intermediate; the carboxyl group of this then ionizes to give a second intermediate, RH+CO2-, which ultimately decarboxylates. Deuterium solvent isotope effects support the proposed mechanism as does an analysis of the individual rate coefficients involved. Several other conceivable mechanisms can be ruled out as inconsistent with either the rate data or other properties of the system. © 1968, American Chemical Society. All rights reserved.