Second order nucleophilic substitution is a concerted processes in which a nucleophilic Lewis base displaces a nucleofugal Lewis base from a Lewis acid centre. In organic chemistry, SN2 processes usually occur at a delta+ sp3 carbon centre. SN2 processes cause the tetrahedral carbon to invert like an umbrella via a five valent carbonium ion transition state. For more information look in the Chemogenesis
webbook sections on substitution
reactions and substitution,
transfer, abstraction, displacement (STAD).