Substitution and Elimination Map
Use this page when a question gives a substrate, nucleophile or base, solvent and leaving group, and asks for a likely mechanism or major product.
First-pass decision
Section titled “First-pass decision”| Signal | Likely pathway | Why it matters |
|---|---|---|
| Tertiary substrate + strong base | E2 | SN2 is blocked by sterics; strong base drives elimination. |
| Tertiary substrate + weak nucleophile in polar protic solvent | SN1/E1 mixture | Carbocation formation is plausible and solvent stabilises ions. |
| Primary substrate + strong non-bulky nucleophile | SN2 | Backside attack is accessible and nucleophile strength matters. |
| Primary substrate + bulky strong base | E2 | Bulky base removes a beta hydrogen instead of substituting. |
| Secondary substrate | Condition-dependent | Strong nucleophile, strong base, solvent and heat decide the route. |
Decision checklist
Section titled “Decision checklist”- Identify the substrate class: methyl, primary, secondary, tertiary, allylic or benzylic.
- Decide whether the reagent is mainly a nucleophile, a base, or both.
- Check steric bulk around the nucleophile or base.
- Check solvent: polar protic usually supports SN1/E1; polar aprotic often supports SN2.
- For elimination, check whether Zaitsev or Hofmann product is expected.
- For carbocation routes, check rearrangement.
- For E2, check anti-periplanar geometry.
Secondary substrate rule of thumb
Section titled “Secondary substrate rule of thumb”Secondary substrates are the most ambiguous.
- Strong non-bulky nucleophile in polar aprotic solvent: SN2 is plausible.
- Strong bulky base or heat: E2 becomes more likely.
- Weak nucleophile/base in polar protic solvent: SN1/E1 can compete.
Product traps
Section titled “Product traps”- SN1 can rearrange; SN2 cannot.
- E1 can rearrange; E2 cannot.
- SN2 inverts stereochemistry at the reacting carbon.
- E2 often needs anti-periplanar H and leaving group alignment.
- Bulky base can favour Hofmann elimination.