Elimination
A reaction in which a small molecule (e.g. H₂O or HX) is removed from the substrate, forming a C=C double bond.
E2 vs E1
| E2 | E1 | |
|---|---|---|
| Steps | One step (concerted) | Two steps (via carbocation) |
| Rate law | k[substrate][base] | k[substrate] |
| Base | Strong base needed | Weak base / no base |
| Substrate | Primary or secondary | Tertiary |
Zaitsev’s Rule
The major product of elimination is the more substituted alkene (more stable, more alkyl groups on the C=C). This is the Zaitsev product.
Substitution vs Elimination
| Factor | Favours Substitution | Favours Elimination |
|---|---|---|
| Temperature | Lower | Higher |
| Base strength | Weak / moderate | Strong base |
| Substrate | Primary (SN2) | Tertiary |
📋 Exam Tip
High temperature and strong base favour elimination. If the question mentions “hot concentrated NaOH” or “ethanolic KOH”, think elimination. “Dilute aqueous NaOH” favours substitution.