Kinetic isotope effects and aliphatic diazo-compounds. Part II. Experiments at high buffer concentrations

Abstract

The general acid-catalysed decomposition of 3-diazobutan-2-one and ethyl diazopropionate exhibits a decrease in the expected rate as the buffer concentrations is increased. This decrease is attributed to the second step in the reaction, the loss of nitrogen, becoming partially rate-determining. The free-energy difference between the transition states has been determined for both compounds. For 3-diazobutan-2-one the solvent isotope effect in the second transition state confirms the analysis. An alternative explanation of the decrease in rate in terms of the dimerisation and association of the buffer species is considered but it is shown that all the data for the hydrolysis of cyanoketen dimethyl acetal can be fitted to the second transition state hypothesis with only one extra parameter. The solvent isotope effect indicates that in this case, the second transition state involves a proton transfer which is similar to the rate-determining step in the hydrolysis of ethyl orthocarbonate.