Undergraduate · 18 min

SN1 versus SN2: timing is everything

Step through the concerted SN2 path and the two-step SN1 path, then predict which mechanism each alkyl halide prefers.

Goal

Explain rate = k[R–X][Nu⁻] for SN2 vs rate = k[R–X] for SN1, inversion vs racemisation, and the role of carbocation stability.

Apparatus and reagents

The mechanism viewer below; a table ranking methyl, 1°, 2°, 3° substrates.

Procedure

Mode 0 (SN2): follow $\ce{HO-}$ into the backside of $\ce{CH3-Cl}$ ; note that bond making and breaking are drawn as one arrow.
Mode 1 (SN1): watch $\ce{Br-}$ leave first, creating the flat carbocation, before water attacks either face.
Mode 2: compare the two rates for a methyl vs a tertiary substrate and state which mechanism wins in each case.
Predict the stereochemical outcome for (R)-2-bromobutane under SN2 and under SN1 conditions.

What to observe

SN2 shows a single transition state with the carbon momentarily pentacoordinate; SN1 shows a real intermediate that can be captured by any nucleophile present.
Inversion (SN2) keeps one stereoisomer; the planar carbocation (SN1) is attacked on both faces and gives a racemic mixture.

Explanation

In SN2 the nucleophile must reach the σ* orbital from the side opposite the leaving group, so the rate depends on both concentrations and drops sharply with substitution (methyl > 1° > 2° >> 3°). In SN1 the leaving group departs first — the slow step is unimolecular — so the rate depends only on R–X and grows with carbocation stability (3° > 2° > 1°). Stereochemistry is the fingerprint: Walden inversion for SN2, racemisation for SN1.

Chemists behind it

Vladimir Prelog