Photoreactions of Heteroaromatic Compounds. IV. Solvent and Acidity Dependence of the Photoreactions of Nicotinic Acid and Its Derivatives

Abstract
Nicotinic acid and its derivatives show manifold photoreactivity depending on the solvent and the acidity of the solution. The cationic form of these compounds in general undergoes photo-hydroxylation in aqueous solutions, and photo-ethylation in ethanol solutions. The anionic form of nicotinic acid is resistant to UV-irradiation in strongly alkaline aqueous solution, but undergoes photo-decarboxylation in alkaline ethanol solution. Nicotinic acid yielded 2-(3-pyridyl)-5-pyridinecarboxylic acid in aqueous solutions at pH=4–6. Photo-substitution by 1-hydroxyethyl group occurs in the neutral forms of 3-cyanopyridine and ethyl nicotinate in ethanol solution. The photoreactivity of nicotinic acid is considered on the basis of CNDO/2 calculation.

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