Quick Recap — Mechanisms (SN1 vs SN2)

  • SN2: one step, backside attack, inversion of configuration; favoured by 1∘^\circ halides, strong nucleophile, polar aprotic solvent; rate ∝\propto [substrate][nucleophile].
  • SN1: two steps via a carbocation, racemisation; favoured by 3∘^\circ halides and polar protic solvent; rate ∝\propto [substrate].
  • Reactivity: SN1 3∘>2∘>1∘3^\circ>2^\circ>1^\circ; SN2 1∘>2∘>3∘1^\circ>2^\circ>3^\circ.
  • Elimination (E) competes with substitution, especially for 3∘^\circ halides and with strong bulky bases.