Is the single transition-state model appropriate for the fundamental reactions of organic chemistry?*
Vernon D. Parker
Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322-0300, USA
Abstract: In recent years, we have reported that a number of organic reactions generally believed to follow simple second-order kinetics actually follow a more complex mechanism. This mechanism, the reversible consecutive second-order mechanism, involves the reversible formation of a kinetically significant reactant complex intermediate followed by irreversible product formation. The mechanism is illustrated for the general reaction between reactant and excess reagent under pseudo-first-order conditions in eq. i where kf' is the pseudo-first-order rate constant equal to kf[Excess Reagent].
Reactant + Excess reagent = Reactant complex = Products (i)
The mechanisms are determined for the various systems, and the kinetics of the complex mechanisms are resolved by our "non-steady-state kinetic data analysis". The basis for the non-steady-state kinetic method will be presented along with examples. The problems encountered in attempting to identify intermediates formed in low concentration will be discussed.
Keywords: Non-steady-state kinetics; mechanism analysis; extent of reaction dependence of KIE; mechanism probes; dynamic residual absorbance analysis.
*Paper based on a presentation at the 17th International Conference on Physical Organic Chemistry (ICPOC-17), Shanghai, China, 15-20 August 2004. Other presentations are published in this issue, pp. 1807-1921.