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

Abstract

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.

A guided inquiry experiment for the measurement of activation energies in the biophysical chemistry laboratory: Decarboxylation of pyrrole‐2‐carboxylate

“…Dunn and Lee measured rates of the decarboxylation of P2C at 50 °C in varying acid concentrations, along with the 13 C‐carboxyl kinetic isotope effects at high and low acidities, and proposed the reaction mechanism illustrated in Fig. 1 [10]. The results are consistent with a mechanism in which the relative rates of protonation and decarboxylation change with varying acid concentrations.…”

“…The decarboxylation of pyrrole‐2‐carboxylate (P2C) offers a convenient laboratory experiment in kinetics, yielding the measurement of rate constants of the decarboxylation reaction in a short time using a continuous ultraviolet (UV) spectrophotometric assay. The reaction is acid‐catalyzed, and a complex decarboxylation mechanism, changing slightly with increasing acid concentration, has been proposed with ample experimental support [10]. The nonenzymatic reaction proceeds readily in moderately high acid concentration, allowing measurement of the rate constants at multiple ambient temperatures for the analysis of temperature dependence.…”

Kinetics and Mechanism of Decarboxylation of some Pyridinecarboxylic Acids in Aqueous Solution

“…A rate maximum at the isoelectric p H can be explained either by a rate-controlling decarboxylation of the isoelectric species, as postulated by Hammick, or by a rate-controlling protonation of the monoanion, such as that found at low acidities for salicylic (ll), anthranilic (13), azulene-2-carboxylic (12), and pyrrole-2-carboxylic acids (16) and shown in eq. 3.…”

Five‐Membered Heterocycles: Pyrrole and Related Systems

“…Based on kinetic studies, the mechanism of the decarboxylation of pyrrole-2-carboxylic acid 198 in acidic media has been suggested to proceed through the intermediate 379, which eventually releases carbon dioxide (Scheme 4.117) [603]. A striking feature during saponification of pyrrolecarboxylates is the relatively high rate constant for pyrrole-2-carboxylates compared to that of the C3 substituted isomers.…”