What determines whether a reaction is classified as sn1 or sn2?

Question

Zoe Alexander · Accepted Answer

See Explanation

It is good to know why they are called SN 1 and SN 2; in SN 2 reactions, the rate of the reaction is dependent on two entities (how much nucleophile AND the electrophile is around), and hence it is called SN2.
SN 1 reactions' rates are only dependent on on entity, the electrophile (loss of a leaving group is the first step of this reaction and it does not require a nucleophile at first to have the Leaving Group leave and form carbocation), therefore it is called SN1.

Now answering your question, here are how they differ and how you can determine which mechanism is happening:

1) The substrate (aka the electrophile): For SN2 reactions, a primary substrate is better, while a tertiary substrate is virtually unreactive. But for SN1 reactions, it is the opposite. Tertiary substrates are perfect for SN1 reactions and primary substrates are just not good!

Therefore, if you have primary or secondary substrates, then the reaction will proceed through SN2 mechanism. If you have Tertiary substrate, then it will proceed via SN1 mechanism.

Skylar Alexander · Answer

undefined The classification of a reaction as SN1 or SN2 depends on several factors, including the nature of the substrate, the nucleophile, the leaving group, and the reaction conditions. In SN1 reactions, the rate-determining step involves the formation of a carbocation intermediate, which means the reaction proceeds via a unimolecular pathway. In SN2 reactions, the rate-determining step involves the simultaneous interaction of the substrate and the nucleophile, leading to a bimolecular pathway. Substrate structure, steric hindrance, and nucleophile strength are key determinants in distinguishing between SN1 and SN2 reactions.