Question
Predict the major product and mechanism (SN1 vs SN2) when:
(a) 2-bromobutane reacts with aqueous NaOH at room temperature. (b) Tertiary butyl bromide reacts with aqueous NaOH. (c) Methyl bromide reacts with NaCN in DMSO.
Justify your answers with reasoning about substrate, nucleophile, and solvent.
Solution — Step by Step
SN2 favoured by:
- Primary or methyl substrate
- Strong, charged nucleophile
- Polar aprotic solvent (DMSO, DMF, acetone)
SN1 favoured by:
- Tertiary substrate (stable carbocation)
- Weak or neutral nucleophile
- Polar protic solvent (water, alcohol)
Substrate is secondary (borderline). Nucleophile is strong (favours SN2). Solvent is water — polar protic (favours SN1).
For secondary haloalkanes with a strong nucleophile, SN2 typically wins narrowly. Product: butan-2-ol, with inversion of configuration at the chiral centre (Walden inversion).
Substrate is tertiary — SN2 is sterically blocked. Tertiary cation is stable. Polar protic solvent stabilises the cation.
Mechanism: SN1. Product: t-butyl alcohol (2-methylpropan-2-ol). The product is racemic if the carbon were chiral, since the planar carbocation can be attacked from either face.
Substrate is methyl — only SN2 is possible (no carbocation stability). Nucleophile is strong. Solvent DMSO is polar aprotic — does not solvate the nucleophile, keeping it “naked” and reactive.
Mechanism: SN2. Product: acetonitrile (). Reaction is fast.
Why This Works
Solvent choice is the most underappreciated factor. Polar protic solvents (water, methanol) stabilise carbocations and ions through H-bonding — this favours SN1. Polar aprotic solvents stabilise the cation but leave anions un-solvated, so the nucleophile is more reactive and SN2 races ahead.
The substrate hierarchy is rigid: methyl/primary do SN2, tertiary does SN1. Secondary is the battleground where solvent and nucleophile decide.
Alternative Method
Use the rate-law diagnostic. SN2 is second-order overall (depends on both substrate and nucleophile). SN1 is first-order (depends only on substrate). If a problem gives kinetic data, this is the cleanest tell.
Common Mistake
Students predict SN1 for primary substrates because the solvent is polar protic. Wrong — substrate trumps solvent. A primary cation is too unstable to form, so SN2 is the only option regardless of solvent.