Anchimeric Assistance (Neighboring Group Participation)
The participation of neighboring groups in an SN reaction is revealed by unique stereochemical results (retention in the substitution process) and also (usually) by obvious rate enhancements in comparison to a model in which neighboring group participation would be stereoelectronically impossible. Consider the case of trans-2-iodocyclohexyl brosylate (remember, brosylate is p-bromobenzenesulfonate, a very good leaving group, better even than tosylate (why?)).
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