Kinetic hydrogen isotope effects in the ionization of some ketonic substances

Abstract

The kinetic hydrogen isotope effect has been measured for the water-catalysed bromination of acetylacetone, monobromoacetylacetone, ethyl acetoacetate, ethyl α-bromoacetoacetate, ethyl α-methylacetoacetate, ethyl malonate, ethyl monobromomalonate, and methyl methyl - malonate and for the bromination of ethyl a-methylacetoacetate catalysed by six basic anions. All these reactions are of zero order with respect to bromine, so that the process studied is the rate of transfer of a proton or deuteron from the organic molecule to the catalyst. For ethyl a-methylacetoacetate there is a correlation between the basic strength of the catalyst, the rate of proton transfer, and the magnitude of the isotope effect. Various possible explanations are considered for the variation in isotope effect, and it is concluded that current interpretations in terms of a three-centre model of the transition state are inadequate. The isotope effect also varies considerably with the nature of the organic molecule, but is not in general related to its reactivity. There is, however, a correlation between the magnitude of the isotope effect and the exponent β of the Brönsted relation between basic strength and catalytic power, suggesting that both quantities depend on the position of the proton in the transition state.