SN1 or SN2 – what do the numbers mean?

Chances are, as a teacher or a student you will come across these terms (S_N1 or S_N2) in the teaching of IB Chemistry. They come up on a number of occasions in the organic chemistry part of the course (at Core, Higher or Option level).

As a student, I was happy with the S_N part – ‘S’ meant substitution – quite self explanatory, you swap one thing with another. ‘N’ was a little more difficult but I could get my head around it – it mean ‘nucleophilic’ which meant a nucleophile, a species that donate a pair of electrons. The ‘1’ or the ‘2’ were a little trickier. What did they mean?

Well, ‘1’ means ‘unimolecular’ and ‘2’ means ‘bimolecular’ – OK, so what, I still don’t understand! The ‘1’ and ‘2’ refer to the order of the reaction, the kinetics. Hm…, OK, I am still confused. At this point, my teachers explanation faltered and I was left unsure and not fully understanding.

It is actually quite simple:

In a unimolecular reaction, the rate determining step (slowest step in the mechanism) depends only on the concentration of the species being attacked and not on the concentation of the nucleophile.

In a bimolecular reaction, the rate determining step depends on the concentration of both the nucleophile and the species being attacked.

Simple! How I wish I had been told this! Hopefully, this will also help clarify your understanding of the terms. 😉