Solvent Effects on Mechanisms and Stereochemistry of Organic Reactions

7 Solvent Effects on Mechanisms and Stereochemistry of Organic Reactions 

Elaborating on the preceding dicussion on the dual reactivity of ambident anions, this Section contains a selection of reactions of various types which have in common the fact that a change in solvent drastically changes the mechanism as well as the stereochemistry usually associated therewith. The examples are intended to demonstrate how, out of two or more alternatives, a single pathway for a reaction can be favoured through a 

An Sn2 (intermediate) mechanism has been proposed as a result of the consideration of the dependence of Sn1/Sn2 solvolysis rates on electrophilic (EPA) and nucleophilic (EPD) solvent assistance [664, 665]. 

SnI reactions of secondary and tertiary substrates R3C—X e.g. 2-adamantyl tosylate [665], t-butyl heptafluorobutyrate [666]) proceed via activated complexes with high carbenium ion character to give ion-pair intermediates. They exhibit electrophilic solvent assistance in protic solvents SOH (H-bonding to the leaving group), but they are practically insensitive to changes in solvent nucleophihcity cf. Eq. (5-135). 

It is difficult to objectively prove or disprove either the Sn2 (intermediate) or the ion-pair mechanism of Sn2 solvolysis reactions. According to Olah et al. [669], the Sn2 intermediate can indeed be best characterized as a trivalent carbocationic centre solvated from both sides by the negatively charged nucleophile and the leaving group . According to Tidwell et al. [667], this view deemphasizes the covalent character of the 

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