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

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Abstract
The first-order rate constants for decarboxylation of picolinic and five substituted picolinic acids in buffered aqueous solution at 150° and ionic strength 1.0 increase as pH increases above 0, go through a maximum at pH near 1, then level off at about half the maximum. For quinolinic acid at 95° the rate maximum occurs at the isoelectric pH. It is therefore concluded that the anion decarboxylates about half as fast as the isoelectric species. Anion and isoelectric species show similar carboxyl-carbon kinetic isotope and substituent effects, so they probably decarboxylate by similar mechanisms. The methyl betaine of picolinic acid decarboxylates about 200 times faster than the anion. From these facts it is concluded that the isoelectric species which decarboxylates is probably zwitterion rather than neutral acid, and that anion and zwitterion decarboxylate by loss of carbon dioxide to form 2-pyridyl carbanion and ylid, respectively. The slower decarboxylation of isoelectric species compared to betaine suggests that only a small fraction of the isoelectric species is zwitterion. However, qualitative estimates from spectra of the fraction of zwitterion present in the isoelectric species at 150° suggest that this cannot be the whole explanation. The relative reactivities of the 2-and 4-positions are similar for decarboxylation and hydrogen exchange in pyridinium ions, but not in the unprotonated species.

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