Applications of non‐steady‐state kinetics in physical organic chemistry: guidelines for the resolution of the kinetics of complex reaction mechanisms

The resolution of the kinetics of the reversible consecutive second‐order reaction mechanism$$ \hbox{A} + \hbox{B} \lower8pt\hbox{${\buildrel{{\buildrel{k_{\rm f}} \over {\longrightarrow}}} \over {{\buildrel{\longleftarrow}\over {k_{\rm b$} \hbox{I} {\buildrel{k_{\rm p}} \over {\longrightarrow}} \hbox{P} $$involving the formation of a kinetically significant intermediate, which does not reach steady state before late in the first half‐life, followed by an irreversible product‐forming reaction is discussed. It is shown that an apparent second‐order rate constant k app and an extent of reaction–time profile are the only experimental data necessary for the evaluation of k f and k b (the forward and reverse rate constants) as well as k p (the microscopic rate constant for the product forming reaction). When the product‐forming step involves the cleavage of a CH bond, for which there is a deuterium kinetic isotope effect on k p , the resolution of the kinetics is enhanced. In this case, the experimental data include two apparent rate constants ( $k_{\rm app}^{\rm H}$ and $k_{\rm app}^{\rm D}$ ) and two extent of reaction–time profiles, one for normal reactants and the other for isotopically substituted reactants. Under these circumstances, a unique highly resolved experimental to theoretical data fit is found that results in the evaluation of all four microscopic rate constants: $k_{\rm f},\,k_{\rm b},\,k_{\rm p}^{\rm H}$ and $k_{\rm p}^{\rm D}$ . An alternative, when a kinetic isotope effect is not involved, is to fit the extent of reaction–time profiles for two or more concentrations of reactants concurrently. This procedure results in the resolution of the three microscopic rate constants for the reaction. Copyright © 2001 John Wiley & Sons, Ltd.