How can I determine SN2 or SN1?

1 Answer
Shulgin · Stefan V.
Apr 18, 2018

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:

![https://www.masterorganicchemistry.com](useruploads.socratic.org)
Here you can see that the leaving group (Br) is sitting on a tertiary carbon.

S_N2 reaction:

![https://www.masterorganicchemistry.com](useruploads.socratic.org)
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).

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