Kinetics and mechanism of the decarboxylation of salicylic acids

Willi, A. V.

doi:10.1039/tf9595500433

Cited by 25 publications

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“…Although the mechanism of decarboxylation of pyrrolecarboxylic acids has not been investigated, Corwin and Straughn have pointed out that the decarboxylation of pyrrolecarboxylic and hydroxybenzoic acids occur under similar conditions and are subject to similar substituent effects (2). Since the mechanism of decarboxylation of hydroxybenzoic acids (3,4) is very similar to that of the anthranilic acids previously studied in this laboratory (5-7), the present investigation of the decarboxylation of pyrrole-2-carboxylic acid uses the approach previously applied to anthranilic aicds.…”

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Kinetics and Mechanism of the Decarboxylation of Pyrrole-2-carboxylic Acid in Aqueous Solution

Dunn¹,

Lee²

1971

Can. J. Chem.

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The decarboxylation of pyrrole-2-carboxylic acid in aqueous buffers at 50" and ionic strength 1.0 has been found to be first order with respect to substrate at a fixed pH. As the p H is decreased, the rate constant increases slightly in the p H range 3-1, then rises rapidly from p H 1 to 10 M HCI. The '3C-carboxyl kinetic isotope effect is 2.8% in 4 M HCIO, and negligible at p H -3. These observations can be accounted for by a mechanism, previously proposed for the decarboxylation of anthranilic acid, in which the species undergoing decarboxylation is the carboxylate ion protonated at the 2-position of the pyrrole ring. This intermediate can be formed both by ring-protonation of the carboxylate anion and by ionization of the ring-protonated acid. At low acidities ring-protonation is rate determining, but at higher acidities the rate of protonation exceeds that of decarboxylation.I1 a t t t trouve, qu'a p H donnt dans des solutions tampons aqueuses a 50 "C et une force ionique de 1.0, la dtcarboxylation de I'acide pyrrole-2 carboxylique est du premier ordre en substrat. Quand le p H est diminut, la constante de vitesse augmente ltgerement entre p H 3 et 1 puis augmente rapidement du p H 1 a HCI 10 M . L'effet isotopique cinttique du carboxyle est de 2.8 % dans HCIO, 4 M e t ntgligeable a p H -3. I1 est possible de rationaliser ces observations a I'aide d'un mecanisme, propose prtctdemment pour la decarboxylation de l'acide anthranilique, dans lequel l'espece qui subit la dtcarboxylation est I'ion carboxylate protone a la position 2 du noyau du pyrrole. Cet intermediaire peut Ctre form6 soit par la protonation du noyau portant I'anion carboxylate soit par ionisation de l'acide proton6 sur le noyau. A basses aciditts, la protonation du cycle est I'etape determinante mais a des aciditts plus Clevees la vitesse de protonation est plus grande que la decarboxylation.

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confidence: 99%

Kinetics and Mechanism of the Decarboxylation of Pyrrole-2-carboxylic Acid in Aqueous Solution

Dunn¹,

Lee²

1971

Can. J. Chem.

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“…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.…”

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

Dunn¹,

Janzen²,

Rodewald³

1968

Can. J. Chem.

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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...

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“…Auf Grund der Beobachtung von allgemeiner Saurekatalyse [4] sowie der Messung von Substituenteneffekten [4], Losungsmittel-Isotopeneffekten [5] [6] und l3C-Isotopeneffekten [9]…”

Section: (3)unclassified

“…Saure nicht etwa in der Nahe des fruher gefundenen Wertes fur kHA (= 1,54s-l)[4], sondern steigt weiterhin an. Mit zunehmender Salzsaurekonzentration wird das Anwachsen schwacher.…”

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Der geschwindigkeitsbestimmende Schritt bei der Decarboxy‐lierung von 2,4‐Dihydroxybenzoesäure in mässig konzentrierter wässeriger Salzsäure

Willi

Cho

Won

1970

Helvetica Chimica Acta

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Summavy. The decarboxylation kinetics of 2,4-dihydroxybenzoic acid have been studied in 0 . 1 -8~ aqueous HCI at 50". At low HC1 concentrations, the observed first order rate constant, k, increases with increasing acidity of the solution. In solutions with 3 . 5 6~ HCI, k remains constant. It is concluded that an increase of the acidity of the solution causes a change of the rate determining step from slow proton transfer to rate limiting C-C bond cleavage.

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Kinetics and mechanism of the decarboxylation of salicylic acids

Cited by 25 publications

References 2 publications

Kinetics and Mechanism of the Decarboxylation of Pyrrole-2-carboxylic Acid in Aqueous Solution

Kinetics and Mechanism of the Decarboxylation of Pyrrole-2-carboxylic Acid in Aqueous Solution

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

Der geschwindigkeitsbestimmende Schritt bei der Decarboxy‐lierung von 2,4‐Dihydroxybenzoesäure in mässig konzentrierter wässeriger Salzsäure

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