Intramolecular catalysis in some aliphatic keto-acids.

Abstract

Ring-chain tautomerism and the kinetics of halogenation have been investigated in the following keto-acids: CH3CO(CH2)nCO2H, where n = 2,3,4. PhCO(CH2)nC02H, where n = 2,3,4. 2-methyl levulinic acid. 2,2-dimethyl levulinic acid. 3-methyl levulinic acid. 3.3-dimethyl levulinic acid. 2,2,3-trimethyl levulinic acid. 5,5-dimethyl-4-oxo hexanoic acid. The percentage of ring-lactol present in dilute aqueous solution at 2 5.0°C has been estimated by the comparison of two measured dissociation constants: firstly, the ’mixed' dissociation constant i.e. the equilibrium mixture of both straight chain and ring-lactol tautomers, by the measurement of pH and secondly, the 'true' dissociation constant i.e. the straight chain tautomer alone, by the measurement of the kinetics of the general base-catalysed decomposition of nitramide. The results are discussed with reference to the structure of the keto-acids. Bredt's work on the tautomeric equilibrium in levulinic acid has been repeated and his conclusion concerning the structure of the acetyl derivative has been confirmed. Rates of halogenation of both self-buffered and acetate-buffered substrates have been measured spectrophotometrically at 25.0°C. Rate constants have been obtained for the intra- and inter-molecular processes contributing to the observed rate. For four of the keto-acids, in which two sites are available for reaction, the relative rates of deuterium exchange at the two sites have been estimated by N.M.R. spectroscopy. An attempt has been made to rationalise the observed correlations between rate of reaction, site of reaction and the structure of the keto-acids. Activation parameters have been measured for the halogenation of levulinic acid and 6-oxo heptanoic acid. The results have been rationalised in terms of the effect of substrate chain length on the transition states postulated for the intra-molecular and the inter-molecular processes considered. Acetyl levulinic acid has been used as a model to evaluate the role played by the ring acid tautomer in the kinetics.