Sn2 mechanism of nucleophile

Comparison of SnI and Sn2 Mechanisms of Nucleophilic Substitution in Alkyl Halides... 

The role of nitronium ion in the nitration of benzene was demonstrated by Sir Christopher Ingold—the same person who suggested the SN1 and Sn2 mechanisms of nucleophilic substitution and who collaborated with Cahn and Prelog on the ft and S notational system. 

Two mechanisms of isotopic exchange in alcohols can he considered, corresponding to the SN1 and SN2 mechanisms of nucleophilic substitution. Water is the nucleophile in this case, attacking a saturated carbon atom. In all the studies reported so far, exchange occurs only in acid solution and it has always been assumed that the conjugate acid of the alcohol is the reacting species. 

This process occurs as a concerted reaction, is stereospecific, and thus corresponds to the Sn2 mechanism of nucleophilic substitution at an sp -C-atom. For example, from ds-2,3-dimethy-l-oxirane, ( )-threo3-aminobutan-2-ol is obtained by reaction with NH3 ... 

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