Solvolysis pseudo-first-order kinetics

For this expression, the quantity to the right of the first equal sign indicates that the order of the reaction with respect to either reactant is one, and the overall reaction order is the sum of the orders of the respective reactants. Hence, a Sn2 reaction is second-order overall. In certain cases, such as solvolysis, the rate expression takes on pseudo-first-order kinetic behavior. 

Solvolysis of the complex (64) in aqueous acetone follows pseudo first-order kinetics with the formation of the ring-expanded compound (65 equation 198).558 A carbonium ion pathway is supported by 13C labeling experiments. 

The points that we have emphasized in this brief overview of the S l and 8 2 mechanisms are kinetics and stereochemistry. These features of a reaction provide important evidence for ascertaining whether a particular nucleophilic substitution follows an ionization or a direct displacement pathway. There are limitations to the generalization that reactions exhibiting first-order kinetics react by the Sj l mechanism and those exhibiting second-order kinetics react by the 8 2 mechanism. Many nucleophilic substitutions are carried out under conditions in which the nucleophile is present in large excess. When this is the case, the concentration of the nucleophile is essentially constant during die reaction and the observed kinetics become pseudo-first-order. This is true, for example, when the solvent is the nucleophile (solvolysis). In this case, the kinetics of the reaction provide no evidence as to whether the 8 1 or 8 2 mechanism operates. 

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