RENAL TRANSPLANTATION IN INDIA : A FRESH LOOK

Abstract

The lactonization of coumarinic acid and its anions and the hydrolysis of coumarin have been studied in the pH range of −0.3–11 in various buffer solutions between 10 and 25° by spectrophotometric methods. The significant influence of buffer concentration on the apparent first-order rate constant for lactonization was assigned to the acetic acid-catalyzed lactonization of the monoanion, HC−. The dependency of the apparent first-order rate constants extrapolated to zero buffer concentrations on the hydrogenion concentration was derived. It was consistent with hydrogenion-catalyzed lactonization and apparent spontaneous lactonization of undissociated coumarinic acid, H2C, to form coumarin, C, the spontaneous lactonization of the monoanion, HC−, and the nonreactivity of the dianion, C−2. An apparent discrepancy exists between the kinetic (3.0) and the spectral and potentiometric (4.0) pKa1′ which can only be rationalized by a more complex mechanism. This mechanism assumes the intramolecular formation of an orthoacid, H2C‡, which dehydrates spontaneously and by hydrogenion catalysis to give coumarin. The steady-state assumption for H2C‡ permits the fitting of the log k-pH profiles consistent with the analytical pKa1′ values and is consistent with this mechanism. Coumarin is hydrolyzed by specific hydroxyl-ion-catalyzed solvolysis and has been characterized as a function of pH. The hydrolyses of 3-chlorocoumarin, 3-bromocoumarin, and 3- and 4-methylcoumarin were studied in 30% dioxane for the acid-lactone equilibrium. The halogen substituents showed pronounced accelerating effects consistent with the electronegative group acceleration of hydroxyl-ion attack on the carbonyl carbon, whereas the methyl substituents did not significantly modify the reactivity of coumarin.