Mechanism of decarboxylation of substituted salicylic acids. II. Kinetics and ¹³C-carboxyl kinetic isotope effects in nitrobenzene–quinoline mixtures

Abstract

The decarboxylation of 4-methylsalicylic acid in dilute solutions of quinoline in nitrobenzene at 200 OC is first order with respect to the salicylic acid and first order with respect to quinoline. The pscudo-firstorder rate constant reaches a maximum a t 0.25 M quinoline, then decreases as the quinoline concentration increases. The 13C-carboxyl kinetic isotope effect, 100(k,z/k,3 -l), is 2.2% in pure quinoline and 0.7% in 0.02 M quinoline. It is therefore concluded that the mechanism proposed in Part I of this series involves three steps: ionization of the salicylic acid to a quinolinium salicylate ion pair, reversible protonation by quinolinium ion of the salicylate ion at carbon 1 of the aromatic ring, and loss of carbon dioxide from the protonated salicylate ion. I n dilute quinoline solution the second step is slow, while in pure quinoline the third step becomes rate determining.Canadian Journal of Chemistry, 46, 3915 (1968) In Part I of this series (1) it was proposed that more negative than o (applicable to step l), so a number of 4-and 5-substituted salicylic acids that for these substituents cancellation is not decarboxylate in quinoline solution by the follow-complete. ing mechanismThe evidence did not permit a decision as to COOH COOwhether steps 2 and 3 are sequential, as shown, or. QH+ concerted. It was, therefore, the object of the OH present investigation to study the kinetics of decarboxvlation and the '3C-carboxvl kinetic H isotope effect in the "inert" solvent nitrobenzene. ExperimentalThe apparatus and method used to follow the kinetics of the reaction have been described previously (I), as OH OHThis mechanism was designed to account for 3 major experimental observations. (a) Although the decarboxylation in nitrobenzene solution is second order with respect to salicylic acid, in quinoline it is first order, so that the quinoline must be a reactant in the decarboxylation. (b) Most 4-and 5-substituents have little effect on the rate, so that their influence on one process (step 1) must be cancelled by that on another (step 2). (c) The rate is sharply increased by those substituents for which o+ (applicable to step 2) is distinctly Results and DiscussionThe decarboxylation of salicylic acid has been shown to be second order with respect to the acid in nitrobenzene solution (1). However, in nitrobenzene containing quinoline the decarboxylation of 4-methylsalicylic acid is now found to be first order with respect to the acid to more than 3 half lives, even when the concentration of the acid is in considerable excess over that of the quinoline. This, of course, is in agreement with the proposed mechanism since, according to it, Can. J. Chem. Downloaded from www.nrcresearchpress.com by 34.213.178.48 on 05/10/18For personal use only.