[Snowy]

If you were given a mechanism with reactants only and you were told to show the products, how would you know if the reaction is Sn1, Sn2, E1, or E2?

Thank you.

[Honclbrif]

Ah, the bane of first semester sophmore organic: is it S_N1, S_N2, E1, or E2?

To determine if it is unimolecular or bimolecular (X₁ or X₂) look at both the structure of the substrate, and the solvent.

The first step in a unimolecular reaction is the formation of a carbocation, so you want to determine if cation formation is favorable or unfavorable. Usually, tertiary sites support the most stable cations, followed by allylic and benzyllic so reactions at these sites tend to be unimolecular. On the opposite end of the spectrum, methyl and primary sites form the least stable cations, so reactions at these sites tend to be bimolecular. Solvent also plays an important role in determining which path it will take. Polar protic solvents (those with an -NH or -OH) solvate carbocations, making them more stable. Reactions in these solvents tend to be unimolecular. Polar aprotic solventstend to make the base/nucleophile more reactive. Reactions in these solvents tend to be bimolecular.

Next, consider the nature of the base/nucleophile. Species which have less basic character tend to prefer substitution, especially if they do not have much steric bulk (halides, etc). Species which have a stronger basic character tend to prefer elimination, especially if they have a lot of steric bulk.