Elimination Mechanism concept

Elimination Mechanisms (E1 & E2) – Concepts, Applications & Examples

In organic chemistry, elimination reactions occur when atoms or groups are removed from a molecule, usually leading to the formation of a double bond (alkene) or triple bond (alkyne). These reactions mainly follow two mechanisms: E1 (unimolecular) and E2 (bimolecular).

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🔹 E1 Mechanism (Unimolecular Elimination)

• Two-step mechanism.
• First step: Leaving group departs, forming a carbocation.
• Second step: Base abstracts a β-hydrogen → double bond forms.
• Favoured by weak bases and tertiary carbocations.

Example:
CH₃–C(OH)–CH₃ (tert-butanol)
→ (conc. H₂SO₄, heat) → (CH₃)₂C=CH₂ + H₂O

🔹 E2 Mechanism (Bimolecular Elimination)

• One-step concerted mechanism.
• Strong base abstracts β-hydrogen at the same time as leaving group departs.
• Favoured by strong bases and primary/secondary halides.

Example:
CH₃CH₂Br + alc. KOH → CH₂=CH₂ + HBr

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📘 Applications of Elimination Mechanism

1. Elimination in Alkyl Halides

When treated with alcoholic KOH, alkyl halides undergo elimination to give alkenes.

CH₃CH₂Br → (alc. KOH) → CH₂=CH₂

2. Elimination in Alcohols (Dehydration)

Alcohols, in the presence of concentrated H₂SO₄ and heat, lose water to form alkenes.

CH₃CH₂OH → (conc. H₂SO₄, Δ) → CH₂=CH₂ + H₂O

3. Vicinal Dihalides → Alkynes

Vicinal dihalides (two halogens on adjacent carbons) undergo double dehydrohalogenation to give alkynes.

CH₂Br–CH₂Br → (2NaNH₂, liq. NH₃) → HC≡CH

4. Haloarenes – Benzyne Mechanism

Although haloarenes are usually resistant, under strong base conditions elimination–addition occurs via benzyne intermediate.

C₆H₅Cl + NaNH₂ (liq. NH₃) → C₆H₅NH₂

5. Sulfonates (Tosylates, Mesylates, Triflates)

Good leaving groups like tosylates (–OTs) and mesylates (–OMs) also undergo elimination reactions similar to halides.

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✨ Key Rules of Elimination

• E1: Favoured by weak bases, polar protic solvents, and stable carbocations (tertiary).
• E2: Favoured by strong bases, polar aprotic solvents, and less stable carbocations.
• Zaitsev’s Rule: Major product is the more substituted (stable) alkene.
• β-hydrogen must be present next to leaving group.
• Leaving group should be good (Br, Cl, I, OTs).

Summary: Elimination reactions are applicable to alkyl halides, alcohols, vicinal dihalides, haloarenes (benzyne), and sulfonates. They proceed via either E1 (two-step) or E2 (one-step) mechanisms.