Kinetic Studies of Solvolysis. X. Transition from a van’t Hoff to an Arrhenius Intermediate Caused by Steric Hindrance at the Product-Formation Step of SN1 Reactions

Abstract
1. The SN1 reactions of benzhydryl halides and hindered nucleophiles, i. e., potassium 2,6-di-t-butyl-4-methylphenoxide (I), tri-n-butylammonium 2,6-di-t-butyl-4-methylphenoxide (III), tri-n-butylamine and tri-n-butylammonium phenoxide (II), proceed through an Arrhenius intermediate and obey good second-order kinetics in acetonitrile at 110–140°C, whereas in the same solvent the reaction with potassium phenoxide is a typical SN1 reaction with a van’t Hoff intermediate at 70–120°C. This transition of the characteristics of the intermediates from a van’t Hoff to an Arrhenius type has been explained as being due to steric hindrance at the product-formation step of the SN1 reactions.