Intra- and inter-molecular catalysis and hydrogen isotope effects in the iodination of o- and p-carboxyacetophenone

Abstract

Measurements have been made of the rate of iodination of o- and p-carboxyacetophenone and their deuteriated analogues. These reactions are of zero order with respect to iodine, the measured rate representing the rate of ionization of the acetyl group. In self-buffered solutions of the ortho-compound the rate can be attributed solely to intramolecular proton transfer from the acetyl to the carboxylate group. In acetate and pyridine buffers there is also a contribution from intermolecular proton transfer to acetate ion or pyridine, and this is the only type of process detectable for the para-compound. The kinetic hydrogen isotope effect for the intramolecular process (k^H/k^D= 5·4) is considerably greater than that for any of the intermolecular processes (k^H/k^D= 4·0). A study was also made of the catalytic effect of the anions of the carboxyacetophenones in the decomposition of nitramide and the mutarotation of glucose. This leads to an estimate of the ‘true’ acid–base properties of the carboxy-group in o-carboxyacetophenone, and hence to a quantitative comparison between intra- and inter-molecular catalysis; it also shows that in aqueous solution 81% of the undissociated acid exists as the cyclic tautomer.