Why is sn2 bimolecular?

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Answer 1 · 2016-03-14T02:03:58

Because that's how it's defined.

#"S"_N2# 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, #"S"# stands for substitution, and #2# stands for "bi"molecular.

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

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

Therefore, an #"S"_N2# 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_N2# mechanism for this? You don't have to draw the solvent.