Organic Reactions & Mechanisms (CHEM 220) · Module 4 of 9
The SN1 Mechanism in Depth
The stepwise, carbocation-based substitution pathway: rate, stability, racemization, and rearrangement.
The Two-Step Mechanism & Rate-Determining Step
- SN1 (substitution nucleophilic unimolecular) occurs in two steps: first, the leaving group departs to form a carbocation, the slow, rate-determining step; second, the nucleophile attacks the carbocation.
- Because only the substrate is involved in the rate-determining step, the rate law is first order: rate = k[substrate].
Why does the SN1 rate law depend only on the substrate concentration, not the nucleophile's?
Solution. The rate-determining step, formation of the carbocation, involves only the substrate breaking apart; the nucleophile doesn't get involved until the second, faster step, so it doesn't appear in the rate law.
Carbocation Stability & Substrate Reactivity
- SN1 reactivity correlates directly with carbocation stability: tertiary > secondary > primary, which essentially doesn't undergo SN1, since more substituted carbocations are stabilized by hyperconjugation and induction from surrounding alkyl groups.
- Allylic and benzylic carbocations are especially stable due to resonance delocalization, even if only "secondary" by substitution.
Why do tertiary substrates readily undergo SN1, while primary substrates essentially do not?
Solution. Tertiary carbocations are stabilized by hyperconjugation and induction from three surrounding alkyl groups, making them stable enough to form; primary carbocations are far too unstable to form under normal conditions, so primary substrates cannot proceed by this pathway.
Racemization & Carbocation Rearrangements
- Because the carbocation intermediate is planar (sp²) and can be attacked by the nucleophile from either face, SN1 typically produces a racemic mixture — a mix of both R and S products — rather than a single stereochemical outcome.
- Carbocations can rearrange, via hydride or alkyl shifts, to a more stable carbocation before the nucleophile attacks, sometimes yielding unexpected "rearranged" products.
Why does SN1 typically produce a racemic (or nearly racemic) mixture of products, unlike SN2?
Solution. The planar carbocation intermediate can be attacked by the nucleophile equally from either face, producing roughly equal amounts of both stereochemical outcomes rather than a single inverted product.
Solvent Effects & Common Ion Effect
- Polar protic solvents, like water or alcohols, favor SN1 by stabilizing the developing carbocation and leaving group through solvation — ion-dipole interactions and hydrogen bonding.
Why do polar protic solvents favor SN1 reactions over SN2?
Solution. Polar protic solvents can hydrogen-bond with and stabilize both the developing carbocation and the departing leaving group through solvation, lowering the energy of the rate-determining ionization step.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What is the rate law for an SN1 reaction?
2. What is the reactivity order of substrates for SN1?