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

#S_N2# reaction:

https://www.masterorganicchemistry.com
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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