Bulletin of the Chemical Society of Japan
Online ISSN : 1348-0634
Print ISSN : 0009-2673
ISSN-L : 0009-2673
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
Kunio OkamotoYoshihisa MatsuiHaruo Shingu
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1965 Volume 38 Issue 11 Pages 1844-1852

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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.
2. The reactions of benzhydryl halides and some hindered nucleophiles, i. e., tri-n-butylamine, I and III, afford not products anticipated for the nucleophilic substitution in acetonitrile, but rather products derived from benzhydyl radicals, i. e., sym-tetraphenylethane, diphenylmethane and 1-(3′,5′-di-t-butyl-4′-hydroxyphenyl)-2,2-diphenylethane, whereas potassium phenoxide and II give rise to substitution products typical of the nucleophilic reactions, i. e., benzhydryl phenyl ether and p-benzhydrylphenol. A one-electron transfer mechanism between these hindered nucleophiles, such as tri-n-butylamine, I and III, and the SN1 intermediate has been suggested for the formation of benzhydryl radicals.

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