Mechanism of decarboxylation of substituted salicylic acids. I. Kinetics in quinoline solution

Abstract

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...

Studies on the stability of preservatives under subcritical water conditions

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Development of a Manufacturing Process for Zatosetron Maleate

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Δ-Keto-acid/hydroxy-lactone isomerization in some lichen depsides, depsidones and diphenyl ethers

“…In this case, acid deprotonation, followed by a electrophilic substitution, has been proposed according to the literature. 13 Then, this decarboxylation absence with 5 and 6 is another strong argument in favor of the cyclic structure (Scheme 5 and S5 †). Mixtures of α (depsidones) and β isomers (diphenyl-ethers) of 5 and 6 are also signaled in acetonic extracts of lichens (for example, in Xanthoparmelia glabrans 14 ) .…”

“…3) and discussed below, could be proposed (see S4 †). On 13 C NMR spectra, ring inversion and tautomerism resulted in the disappearance of signals but in some cases C-2′′′ was observed near 206 ppm (ka) or 105 ppm (hl).…”

“…† X-ray data β-Collatone 9 CCDC 2241570. † 1 H, 2 H and 13 C NMR spectra Spectra were recorded at 300/75 MHz or 500/100 MHz (cryoprobe) respectively, using CDCl 3 or d 6 -acetone. Shifts (δ values) are given in parts per million ( ppm) and coupling constants ( J values) are given in Hertz (Hz).…”