what is anchimeric assistance or neighbouring group participation?

Neighbouring group participation is an important factor like polar effects(inductive and resonance) and polar effects which affects the rate and mechanisms of reaction.This factor also operates in various reactions.In this,we have a group attached to the carbon atom adjacent to the carbon atom where nucleophilic substitution occurs and during the course of reaction,becomes bonded or partially bonded to the reaction centre to form a non-classical or bridged ion as a result the rate or/stereochemistry of a reaction gets affected.Such reaction which is accelerated by neighbouring group participation is reffered to as anchimerically assisted.For anchimeric assistance to occur,the neighbouring group which behaves as nucleophilic reagent must be placed suitably placed stereochemically with respect to the group that is ejected and this is the trans-configuration.It may also involve a group further removed than the carbon atom adjacent to the reactive centre.

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