In the presence of an electron donating neighboring group, the reaction proceeds faster than expected. In addition, either inversion nor racemisation is observed in such cases.
In the following generalized representation, a neighboring group participation is illustrated. The lone-pair bearing atom/group such as Z would help in the removal of the leaving group by the mechanism shown below. (please note that the incoming nucleophile “Y” attacks the carbon atom of the three membered ring, not on the R group
