Neighbouring group participation or anchimeric assistance

Two consecutive SN2 reactions will lead to a retention of configuration (i.e. two inversions = retention). This can occur when a neighbouring group acts as a nucleophile in the first of two SN2 reactions. 

The presence of neighbouring groups, including R2N and RS, can increase the rate of substitution reactions. 

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This influence of the nitrogen ion is known as neighbouring group participation or anchimeric assistance. 

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