<P> The S 2 reaction is a type of reaction mechanism that is common in organic chemistry . In this mechanism, one bond is broken and one bond is formed synchronously, i.e., in one step . S 2 is a kind of nucleophilic substitution reaction mechanism . Since two reacting species are involved in the slow (rate - determining) step, this leads to the term substitution nucleophilic (bi-molecular) or S 2, the other major kind is S 1 . Many other more specialized mechanisms describe substitution reactions . </P> <P> The reaction type is so common that it has other names, e.g. "bimolecular nucleophilic substitution", or, among inorganic chemists, "associative substitution" or "interchange mechanism". </P> <P> The reaction most often occurs at an aliphatic sp carbon center with an electronegative, stable leaving group attached to it (often denoted X), which is frequently a halide atom . The breaking of the C--X bond and the formation of the new bond (often denoted C--Y or C--Nu) occur simultaneously through a transition state in which a carbon under nucleophilic attack is pentacoordinate, and approximately sp hybridised . The nucleophile attacks the carbon at 180 ° to the leaving group, since this provides the best overlap between the nucleophile's lone pair and the C--X σ * antibonding orbital . The leaving group is then pushed off the opposite side and the product is formed with inversion of the tetrahedral geometry at the central atom . </P> <P> If the substrate under nucleophilic attack is chiral, this often leads to inversion of configuration (stereochemistry), called a Walden inversion . </P>

Where does the nucleophile attack in an sn2 mechanism