Question

How can I determine SN2 or SN1?

Answer 1

There are many things you can look at to determine if a reaction is `S_N1` or `S_N2`.

Explanation:

The first thing you can look after is how large the substrate is. If we have a leaving group on a tertiary carbon atom the reaction would most likely be an `S_N1` reaction because of steric hindrance. If the leaving group sits on a methyl group it would most likely be an `S_N2` reaction.

Examples of these would be:

`S_N1` reaction:

Here you can see that the leaving group `(Br)` is sitting on a tertiary carbon.

`S_N2` reaction:

Here you can see that the leaving group `(Br)` is sitting on a methyl group.

Of course, this is not always the case (depending on solvent and nucleophile). The biggest indicator might be the nucleophile and the solvent.

In an `S_N1` reaction, the nucleophile is a neutral (`H_2O` or `R"−"OH`) with a polar protic solvent (e.g. alcohols). In an `S_N2` reaction, the nucleophile is negatively charged (`O^−`, `CN^−` ...) with a polar aprotic solvent (e.g. DMSO, acetone).