Kinetics of intramolecular catalysis.

Abstract

The ‘mixed’ acid dissociation constant values for a series of substituted aromatic ortho-acetyl carboxylic acids and one alicyclic ortho-acetylcarboxylic acid have been measured in water at 25°C. The ’true’ acid dissociation constant values have been found by first determining the catalytic constant for the decomposition of nitramide by the keto-acid anion in water at 25°C and then comparing these results with those already obtained for some simple carboxylic acids whose acid dissociation constants are known. The values obtained were checked in a few cases by determining the catalytic constants for the mutarotation of glucose in water at 18°C and comparing these with values already obtained for some simple carboxylic acids. Comparison of the ’mixed’ and ’true’ acid dissociation constants have been made and the percentage of ring-chain tautomerism for each keto-acid determined. Infra red data for each keto-acid are also provided. Rate constants for the intramolecular base catalysed enolization have been determined in water at 25°C and rate constants have also been obtained for intermolecular catalysis by acetate ions and pyridine. The reactions have been followed by recording spectrophotometrically the disappearance of tri-iodide at 353 nm. Some attempt has been made to relate the structure of the keto-acid to the rate of enolization. The rate for the bimolecular reaction involving two keto-acid anions has been estimated for each keto-acid and this value compared with that observed for the intramolecular base catalysed reaction. From these data the effectiveness of the intramolecular process compared to that of the intermolecular process has been estimated and is discussed in terms of effective molarity, c. The intermolecular processes involving pyridine and acetate ions have been compared in an attempt to estimate qualitatively steric effects associated with the acetyl group. Possible alternative mechanisms to explain apparent intramolecular catalysis are also suggested .