Why is sn2 bimolecular?

Mar 14, 2016

Because that's how it's defined.

${\text{S}}_{N} 2$ means nucleophilic substitution bimolecular, as it should state in your textbook.

From this, it should be fairly easy to notice that $N$ stands for nucleophilic, $\text{S}$ stands for substitution, and $2$ stands for "bi"molecular.

Any bimolecular reaction will involve two molecules ("bi" is the prefix for $2$).

1. Nucleophiles, being lewis bases by definition, have to donate electrons to something. There cannot be electron donation into nothing.
2. Electrophiles, being lewis acids by definition, must accept the electrons.

Therefore, an ${\text{S}}_{N} 2$ reaction involves one nucleophile attacking one electrophile, donating electrons to the electrophile to make a bond.

An example is propyl bromide reacting with sodium cyanide dissolved in dimethylsulfoxide (DMSO).

CHALLENGE: Can you draw the ${S}_{N} 2$ mechanism for this? You don't have to draw the solvent.