SN2+reaction

The abbreviation SN2 stands for "substitution, nucleophilic, bimolecular". This means that a nucleophile will replace a leaving group in a reaction having second-order kinetics.

The SN2 reaction rate relies upon a number of important factors:
 * 1) The position of the leaving group
 * 2) Nucleophile strength
 * 3) Spatial interference (a.k.a. steric hinderance)

In terms of leaving group position, the SN2 reaction is essentially opposite the SN1 reaction. Recall that the SN1 reaction worked the best on tertiary carbons, or those adjacent to alkenes. These are typically sterically hindered sites where the SN2 reaction will not be favored. Look at the mechanism for SN2:



The nucleophile must approach the carbon atom opposite the leaving group, where X, Y, and Z will cause steric hinderance (get in the way). The smaller the atoms at X, Y, and Z (ideally, hydrogens), the better for SN2. This means that primary carbons (having two hydrogens) work best for SN2.

The SN2 reaction also differs from SN1 in the mechanism itself. Note that as the nucleophile is heterogenically bonding to the carbon, the leaving group is simultaneously heterolytically dissociating from the carbon. This occurs in the middle bracketed **transition state** shown above.

Here is a video showing the SN2 mechanism:

media type="youtube" key="h5xvaP6bIZI" height="315" width="560"

Below are a series of reaction mechanism animations to help see the transition state process:
 * from Bluffton: methyl bromide and the nitrile nucleophile;
 * from hkedcity: animated SN2 reactions.
 * Recent research news about the SN2 mechanism.

The only similarity to SN1 is in the fact that nucleophile strength still plays a role - the stronger the nucleophile, the better the reaction rate.

Here is another example of an SN2 reaction mechanism.



And another: