SN1 vs SN2

by James

Let’s compare the two substitution reactions we’ve talked about so far. 

In the SN2 reaction, a nucleophile attacks a carbon (usually an alkyl halide) at the backside. The reaction is concerted (happens all at once), and the stereochemistry proceeds with inversion of configuration.

The big barrier to the SN2 is steric hindrance, because the nucleophile has to do a backside attack. This is why the rate is favored for primary > secondary >> tertiary alkyl halides, strong nucleophiles,  and polar aprotic solvents.

In the SN1 reaction, the leaving group leaves a carbon (usually an alkyl halide) to form a carbocation, which is then attacked by a nucleophile. The reaction is stepwise (happens in two steps) and the stereochemistry proceeds with a mixture of retention and inversion of configuration.

The big barrier to the SN1 is carbocation stability, which is why it is favored for tertiary > secondary >> primary alkyl halides and polar aprotic solvents. Also, since the carbocation is such a reactive electrophile, we only need a weak nucleophile.

Here’s a handy little chart.

Tomorrow: let’s go back and talk about curved arrows for a bit.

Thanks for reading! James

P.S. Further reading: trapped in SN1/SN2/E1/E2 hell?