# How can I determine SN2 or SN1?

Apr 18, 2018

There are many things you can look at to determine if a reaction is ${S}_{N} 1$ or ${S}_{N} 2$.

#### 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 ${S}_{N} 1$ reaction because of steric hindrance. If the leaving group sits on a methyl group it would most likely be an ${S}_{N} 2$ reaction.

Examples of these would be:

${S}_{N} 1$ reaction:

Here you can see that the leaving group $\left(B r\right)$ is sitting on a tertiary carbon.

${S}_{N} 2$ reaction:

Here you can see that the leaving group $\left(B r\right)$ 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 ${S}_{N} 1$ reaction, the nucleophile is a neutral (${H}_{2} O$ or $R \text{−} O H$) with a polar protic solvent (e.g. alcohols). In an ${S}_{N} 2$ reaction, the nucleophile is negatively charged (O^−, CN^− ...) with a polar aprotic solvent (e.g. DMSO, acetone).