substrate structure effect

Hand a chemist a flask and ask "will this go SN1 or SN2?" and the first thing they look at is the shape of the substrate itself — specifically, how many carbon groups crowd the carbon bearing the leaving group. That single feature, the substrate structure effect, is often the deciding factor, because the two mechanisms have opposite structural preferences.

Here is the crux. SN2 wants an open, uncrowded carbon so the nucleophile can attack the back; so it runs best on methyl and primary substrates, struggles on secondary, and is shut down on tertiary by steric hindrance. SN1, by contrast, depends not on access but on how stable the carbocation it forms will be — and carbocation stability rises with substitution (tertiary > secondary > primary > methyl), thanks to hyperconjugation and the electron-donating push of attached alkyl groups. So tertiary substrates, which would never tolerate SN2, are the perfect partners for SN1. The orders are mirror images: SN2 favors methyl > primary > secondary; SN1 favors tertiary > secondary.

Secondary substrates sit in the contested middle ground — they can go either way, and the outcome is then decided by the other three levers (nucleophile strength, leaving group, and solvent). The structure effect is also why methyl and primary substrates essentially never do SN1: their cations are far too unstable to form. And it is why two special cases (allylic and benzylic substrates), whose cations are resonance-stabilized, can do SN1 even at a primary center. Reading the substrate first, then bringing in the other levers, is the practical recipe for predicting a substitution's pathway and stereochemical outcome.