Editing SN2 reaction (section)

===Substrate===
The substrate plays the most important part in determining the rate of the reaction. For S<sub>N</sub>2 reaction to occur more quickly, the nucleophile must easily access the sigma antibonding orbital between the central carbon and leaving group.

S<sub>N</sub>2 occurs more quickly with substrates that are more [[Steric effects|sterically accessible]] at the central carbon, i.e. those that do not have as much sterically hindering substituents nearby. Methyl and primary substrates react the fastest, followed by secondary substrates. Tertiary substrates do not react via the S<sub>N</sub>2 pathway, as the greater steric hindrance between the nucleophile and nearby groups of the substrate will leave the S<sub>N</sub>1 reaction to occur first.
[[File:Steric effects on SN2 reactivity.svg|center|750px]]

Substrates with adjacent pi C=C systems can favor both S<sub>N</sub>1 and S<sub>N</sub>2 reactions. In S<sub>N</sub>1, allylic and benzylic carbocations are stabilized by delocalizing the positive charge. In S<sub>N</sub>2, however, the [[Conjugated system|conjugation]] between the reaction centre and the adjacent pi system stabilizes the transition state. Because they destabilize the positive charge in the carbocation intermediate, electron-withdrawing groups favor the S<sub>N</sub>2 reaction. Electron-donating groups favor leaving-group displacement and are more likely to react via the S<sub>N</sub>1 pathway.<ref name="Clayden-2012" />
[[File:Benzylic chloride nucleophilic substitution.svg|center|650px]]