Neighbouring Group Participation
Neighbouring Group Participation (NGP) is observed in nucleophilic substitution reactions, where a neighbouring group helps in the removal of the leaving group to form a reactive intermediate that leads to the formation of the product. Increase in the reaction rate and unexpected stereo chemical outcomes are associated in reactions involving NGP.
An atom having an unshared pair of electrons and also present at least beta to the leaving group can act as a neighbouring group. Also, NGP is mostly observed on solvolysis reactions where the solvent acts as the nucleophile.
A typical reaction involving NGP is shown below.
During NGP, the neighbouring group (G) attacks the electrophilic centre to eliminate the leaving group (L). This leads to the formation of a cyclic intermediate which is very reactive. This is called anchimeric assistance from the neighbouring group. The nucleophile (Nu-)then attacks this intermediate to form the product. If the attack happens of the carbon that was having the leaving group the configuration will be retained because the configuration at that carbon will be inverted twice.
Groups like halides, hydroxides, ethers, thio ethers, amino groups, carboxylates, phenyl group, pi-bonds etc. have been indentified to act as neighbouring groups in many reactions.
Some more examples of reaction involving NGP are shown below.