First-order rate constants for the decarboxylation of fourteen 4-and 5-substituted salicylic acids have been determined in quinoline solution in the temperature range 90-230 "C. Substituents have almost no effect on the rate constants, except those with large negative o-constants: p-arnino,p-hydroxy, p-ethoxy. The enthalpies and entropies of activation do not fit the isokinetic relationship, with the same three substituents deviating. It is suggested that the decarboxylation involves a preliminary ionization of the carboxyl group, followed by protonation of the aromatic ring of the anion so formed, and then loss of carbon dioxide. The isokinetic relationship fails because substituents affect all three steps differently, and the Hammett relationship fails because the substituent effect on the ionization is related to o while that on the other two steps follows of. The three substituents which deviate are those for which o and a+ differ widely.Canadian Journal of Chemistry, 46, 2905Chemistry, 46, (1968 The decarboxylation of substituted salicylic acids in aqueous solution has been thoroughly investigated. Schubert and Gardner (1) showed that the decarboxylation of 2,4,6-trihydroxybenzoic acid in aqueous perchloric acid is first order with respect to 2,4,6-trihydroxybenzoate ion and to hydrogen ion, so that the slow step could be either a unimolecular decomposition of the free acid or a bimolecular reaction between the anion and the proton. Willi (2) found that the decarboxylation of Csubstituted salicylic acids obeys the Hammett relationship using o+, and that the rate is increased by electron-releasing substituents. He therefore concluded that the slow step is protonation of the anion in the 1-position of the aromatic ring. Lynn and Bourns (3) demonstrated that protonation and decarboxylation are sequential rather than concerted processes by showing that the 13C-carboxyl kinetic isotope effect in the decarboxylation of 2,4-dihydroxybenzoic acid varies with buffer concentration. These results make it clear that the mechanism of decarboxylation of substituted salicylicacidsin aqueous solution is the following:In nonaqueous solution much less information is available. Brown, Hammick, and Scholefield (4) found that in resorcinol at 110-240 "C the first-order rates of decarboxylation increased as hydroxy groups were added to the 4-and 6-positions of salicylic acid, which suggested that here, too, protonation of the aromatic ring is an important part of the rate-determining process. Clark (5) studied the decarboxylation of 4-hydroxysalicylic acid in glycols and in quinoline and concluded that these solvents were acting as nucleophiles toward the carboxyl carbon.The present work reports the rate and activation parameters for decarboxylation of 14 substituted salicylic acids in quinoline solution. ExperimentalSynthetic quinoline distilled from and stored over barium oxide was used throughout. Salicylic acid and the 4-and 5-amino, 4-and 5-hydroxy, 4-and 5-nitro, 4-ethoxy, 5-methyl, 5-chloro, and 5-bromo substituted sali...

WIixt~rres of chlorobenzene and chloroben~ene-2-~H have been subjected to partial amination by sodamide in liquid ammonia and both the unreacted starting material and the product aniline have been analyzed for deuterium. Deuterium in the aniline is distributed approximately equally between the ortho and lneta positions. The results give strong support to the niecha~lisrn proposed by Roberts and co-workers in which the slow step is the formation of an intermediate, such as benzyne, which is symnietrical with respect to carbon atoms 1 and 2.The amination of aromatic halides by alkali metal amides in liquid ammonia was discovered in 1938 by Bergstrom and co-worlcers ( I ) , who were able to convert chloro-, bromo-, and iodo-benzene into aniline and a mixture of by-products containing diphenylamine, triphenylainine, and p-aminobiphenyl. Fluorobenzene did not react. In 1945 Gilman and Avalciail reported that a number of ortho-halogenated ethers on treatment by Bergstro~n's procedure gave ineta-aminoethers (2) as illustrated by the equation

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.