The SN2 reaction is a bimolecular nucleophilic substitution reaction that involves a negative nucleophile generally paired with a cation. In the final product, the nucleophile sits in place of the leaving group and has the opposite stereochemical designation.
A Few Important Things About SN2 Reactions
The SN2 reaction works with zeroth (0º), primary (1º), and secondary (2º) leaving groups but NOT tertiary (3º).
It inverts stereochemistry, so watch out if you have a 2º leaving group because you could have a chiral center!
If the leaving group is on a chiral center, the stereochemical designation will be inverted—i.e. an R-designation will become an S-designation
Even if it’s not a chiral center, it’s still good practice to indicate inversion by switching the wedge/dash information
SN2 reactions tend to occur at room temperature or in the cold
Bulky bases do not work for SN2 reactions
And finally, SN2 reactions are not subject to rearrangement because there is no carbocation to rearrange
The SN2 Mechanism
In this example, the Z represents the leaving group, NU represents the nucleophile, and sodium is used as the cation but it could be potassium, lithium, or something else. The starting material used here has an S-designation.
The mechanism is a single-step reaction with two arrows. In what's called a backside attack, the nucleophile forms a bond to the carbon to which the 0º, 1º, or 2º leaving group is attached and kicks the leaving group off the carbon. In other words, the nucleophile forms a bond to the carbon holding the leaving group. Carbon can only have four bonds, so it breaks its bond to the leaving group upon the nucleophile’s attack.
In the transition state, the nucleophile is forming a bond to carbon and the leaving group's bond to carbon is breaking. Notice that at this point the leaving group and nucleophile both have partial negative charges as well as partial bonds, which are indicated by the dashed lines.
In our final product, the nucleophile is attached to carbon, stereochemistry at the reactive site has been inverted—in this case from S to R—and of course the leaving group is off to the side floating around in solution, probably paired with that cation that we had in the beginning.