title: Carbon kinetic isotope effects reveal variations in reactivity of intermediates in the formation of protonated carbonic acid.
abstract: Kinetic evidence suggests that acid-catalyzed decarboxylation reactions of aromatic carboxylic acids can occur by a hydrolytic process that generates protonated carbonic acid (PCA) as the precursor of CO2. Measurements of reaction rates and carbon kinetic isotope effects (CKIE) for decarboxylation of isomeric sets of heterocyclic carboxylic acids in acidic solutions reveal that C-C cleavage to form PCA is rate-determining with significant variation in the magnitude of the observed CKIE (1.018-1.043). Larger values are associated with the more reactive member in each isomeric pair. This variation is consistent with stepwise mechanisms in which C-C cleavage is competitive with C-O cleavage, leading to reversion to the protonated reactant to varying degrees with an invariant intrinsic CKIE for C-C cleavage. Thus, the relative barriers to reversion and formation of PCA control the magnitude of the observed CKIE in a predictable manner that correlates with reactivity. Application of the proposed overall mechanism reveals that carboxylation reactions in acidic solutions will proceed by way of initial formation of PCA.
authors.affiliation: Davenport Research Laboratories, Department of Chemistry, University of Toronto , 80 St. George Street, Toronto, Ontario, Canada M5S 3H6.
authors.forename: Adelle A
authors.initials: AA
authors.lastname: Vandersteen
authors.forename: Scott O C
authors.initials: SO
authors.lastname: Mundle
authors.forename: Georges
authors.initials: G
authors.lastname: Lacrampe-Couloume
authors.forename: Barbara
authors.initials: B
authors.lastname: Sherwood Lollar
authors.forename: Ronald
authors.initials: R
authors.lastname: Kluger