Kinetic isotope effects and aliphatic diazo-compounds. Part VI. The values of α and the Marcus theory

Abstract

The collected data for proton transfer to three diazo-compounds, namely diazoacetate anion, 3-diazobutan-2-one, and ethyl diazopropionate, show that although the rates of the reactions are different the details of the protontransfer part of the reaction are very similar. This qualitative conclusion is reinforced by quantitative analysis according to the Marcus theory. The Brønsted α changes if the catalyst is changed but not if the diazo-compound is changed. This difference is connected with the difference between C bases and O and N bases. A model in which the acid strengths of the C base varies continuously with the solvation co-ordinate is examined and rejected. An extended version of the Marcus expression is developed and it is shown that the diazo-systems and the protonation of nitro-compounds, for which α_s is negative, can be successfully described by the extended theory. The data for the nucleophilic attack in the second step of the decomposition of the three diazo-compounds are compared and the suggested differences in mechanism between diazoacetate anion and 3-diazobutan-2-one are shown to be reasonable.