How can I determine SN2 or SN1?

Question

Nolan Allen · Accepted Answer

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

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 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).

Ruby Allen · Answer

undefined To determine whether a reaction follows an SN1 (substitution nucleophilic unimolecular) or SN2 (substitution nucleophilic bimolecular) mechanism, consider factors such as the nature of the substrate, the nucleophile, the leaving group, and the reaction conditions. SN1 reactions typically occur with tertiary substrates, polar protic solvents, and involve a carbocation intermediate. SN2 reactions are favored by primary or secondary substrates, polar aprotic solvents, and proceed with concerted backside attack by the nucleophile. Additionally, the rate of the reaction and the stereochemistry of the product can provide further clues to the mechanism.