Ion pairs in nucleophilic substitution

The concept of ion pairs in nucleophilic substitution is now generally accepted. Presumably, the barriers separating the intimate, solvent-separated, and dissociated ion pairs are quite small. The potential energy diagram in Fig. 5.4 depicts the three ion-pair species as being roughly equivalent in energy and separated by small barriers. 

See the discussion of the roles of ion pairs in nucleophilic substitution reactions beginning on page 480,... 

Detailed kinetic investigations, primarily concerned with salt effects on solvolysis reactions, led to the recognition of the importance of ion pairs in nucleophilic substitution. This work is most closely associated with Saul Winstein, who proposed the involvement of two distinct ion pair intermediates in solvolysis reactions. This concept has since been refined and elaborated on by others, and is the most generally accepted interpretation of nucleophilic substitution reactions. ... 

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See also in sourсe #XX -- [ , , , ]

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