Reference Reaction Summary Tables Substitution and Elimination (SN1, SN2, E1, E2)
Substitution and Elimination (SN1, SN2, E1, E2)
Estimated reading time: 2 min
In this section
- Overview
- Substitution and Elimination (SN1, SN2, E1, E2)
- Addition to Alkenes and Alkynes
- Nucleophilic Addition to Aldehydes and Ketones
- Nucleophilic Acyl Substitution (Carboxylic Acid Derivatives)
- Enols, Enolates, and Carbon–Carbon Bond Formation
- Electrophilic Aromatic Substitution
- Addition vs. Substitution at Carbonyls — A Direct Comparison
- Cross-References
| Reaction | Typical Reagents/Conditions | Substrate | Mechanism | Product / Stereochemistry | Common Pitfall |
|---|---|---|---|---|---|
| SN2 | Strong nucleophile (e.g., NaOH, NaOR, NaCN, NaN₃, thiolate); polar aprotic solvent (DMSO, DMF, acetone) | Methyl, primary (secondary possible) | One step, concerted, backside attack | Inversion of configuration at the reactive carbon | Attempting SN2 on a tertiary substrate — sterics make backside attack impossible |
| SN1 | Weak nucleophile, often the solvent itself (H₂O, ROH); polar protic solvent | Tertiary (secondary possible) | Two steps via carbocation intermediate | Racemization (attack from either face of the planar carbocation) | Forgetting that carbocation rearrangements (hydride/alkyl shifts) can change the constitution of the product |
| E2 | Strong, often bulky base (e.g., NaOEt, KOtBu, DBU) + heat | Primary, secondary, tertiary | One step, concerted; requires anti-periplanar H and leaving group | Alkene; Zaitsev (more substituted) product usually favored unless the base is bulky | Ignoring the conformational requirement — if no anti-periplanar H is accessible, E2 cannot proceed from that conformation |
| E1 | Weak base, polar protic solvent, heat | Tertiary (secondary possible) | Two steps via carbocation intermediate | Alkene; Zaitsev product favored; rearrangements possible | Assuming E1 and SN1 don’t compete — they arise from the same carbocation and typically occur together |
Decision Framework
Reasoning through these four questions, in order, resolves most substitution/elimination problems:
- Is the substrate methyl or primary? → SN2 is likely (unless the base/nucleophile is bulky, which favors E2 even on primary substrates).
- Is the substrate tertiary? → SN1/E1 are likely if the nucleophile/base is weak; E2 is likely if the base is strong.
- Is the nucleophile/base strong and unhindered? → Favors the concerted pathways (SN2 on unhindered carbons, E2 otherwise).
- Is the solvent polar protic? → Favors the stepwise, carbocation pathways (SN1/E1).
See Chapter 10 for the underlying reasoning; this table is the condensed version for quick lookup.