Kinetic isotope effects and aliphatic diazo-compounds. Part III. Solvent isotope effects for 3-diazobutan-2-one and ethyl diazopropionate

Abstract

Data are reported for the isotopic analysis of the products of the acid-catalysed decomposition of 3-diazobutan-2-one and ethyl diazopropionate and for the rate of reaction for both compounds in mixtures of H₂O and D₂O. The kinetic data are analysed to obtain the fractionation factors for the proton transferred to carbon and for the two secondary solvent protons. The fractionation factors from the product analysis agree with the factors for the proton transferred to carbon. A simple analytical correction for the breakdown of the rule of the geometric mean which will hold for most A–S_E2 transition states is presented and applied to the data.

The data for 3-diazobutan-2-one were obtained by the differential method and their higher precision allows the investigation of models other than the simple one for the A–S_E2 transition state and in particular the inclusion of transition-state solvation terms. The results are not completely conclusive but suggest that the simple model is the most probable one. The values of α deduced from the secondary solvent isotope effect do not agree with those from the Brønsted catalysis law.