Isotope effects in kinetics

A distinction was made between two proposed mechanisms for the fragmentation of an oxaziridine, using a combination of the primary kinetic isotope effect and stereochemistry [13], Amines can behave as bases and attack at hydrogen, or they can be nucleophiles and attack at carbon or nitrogen E2 and 8, 2 reactions are examples. In Equation 4.36, a tertiary amine (brucine) is shown in the role of base in a step resembling an E2 reaction. In Equation 4.37, the amine is shown attacking the nitrogen 

In order to disprove one of these alternatives, the dideutero analog 4.1 was prepared and the rates were measured for the fragmentation of 

The change in enthalpy and entropy from starting materials to products at 1M concentration (standard state) in a reaction may be calculated if an equilibrium constant can be measured for the reaction at several temperatures (Equations 4.38 and 4.39). The equilibrium constant gives the change 

If we could set up a simple equilibrium between starting materials and the transition state for a reaction and measure the amounts of each present at more than one temperature, we could calculate AG°, AH°, and AS° for that change. However, a transition state is too short-lived, and we cannot measure the molar concentration of it to calculate the equilibrium constant for its formahon. 

Consider the reachon in Equation 4.40 wherein starting materials A and B combine to give a short-lived transition state AB.  

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