Nucleophile
In the example below, the oxygen of the hydroxide ion donates an electron pair to bond with the carbon at the end of the bromopropane molecule.  The bond between the carbon and the bromine then undergoes heterolytic fission, with the bromine atom taking the donated electron and becoming the bromide ion (Br−). This is a SN2 reaction occurs by backside attack.  This means that the hydroxide ion attacks the carbon atom from the other side, exactly opposite the bromine ion.  Because of this backside attack, SN2 reactions result in a reversal of the configuration of the electrophile.  If the electrophile is chiral, it typically keeps its chirality, though the SN2 product's configuration is flipped as compared to that of the original electrophile (Walden inversion).