Preparation of alkene
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This is an organic chemistry question, not a medical one — the medical library skill doesn't apply here. Let me answer directly.
Preparation of Alkenes
Alkenes (compounds containing a C=C double bond) can be prepared by several methods:
1. Dehydration of Alcohols (Elimination of Water)
Alcohols undergo acid-catalyzed dehydration when heated with concentrated H₂SO₄ or H₃PO₄.
Mechanism: E1 (for 2° and 3°) or E2
CH₃CH₂OH →(conc. H₂SO₄, 170°C)→ CH₂=CH₂ + H₂O
(ethanol) (ethylene)
- Zaitsev's rule: The more substituted (more stable) alkene is the major product.
- Higher temperature favors elimination over substitution.
- Order of reactivity: 3° > 2° > 1° alcohols.
2. Dehydrohalogenation (Elimination of HX from Alkyl Halides)
Treatment of alkyl halides with a strong base (e.g., KOH/ethanol, NaOEt) causes elimination of HX.
CH₃CH₂Br →(KOH/EtOH, heat)→ CH₂=CH₂ + KBr + H₂O
- E2 mechanism (bimolecular): favored by strong base, high temperature.
- E1 mechanism: favored by weak base, polar protic solvent.
- Zaitsev's rule applies (more substituted alkene predominates), unless a bulky base (e.g., t-BuOK) is used → gives Hofmann (less substituted) product.
3. Dehalogenation (Vicinal Dihalides)
Vicinal dihalides (1,2-dihalides) react with zinc (Zn) in ethanol or sodium iodide (NaI) in acetone to give alkenes.
CH₂Br–CH₂Br →(Zn, EtOH)→ CH₂=CH₂ + ZnBr₂
This is an anti elimination.
4. Kolbe's Electrolysis (Industrial Method)
Not commonly used in labs, but electrolysis of sodium/potassium salts of dicarboxylic acids can yield alkenes.
5. Pyrolysis of Esters (Chugaev Reaction / Cope Elimination)
Esters undergo pyrolysis (thermal elimination, ~400–500°C) via a cyclic syn-elimination mechanism to give alkenes:
RCH₂CH₂OOCCH₃ →(heat, ~500°C)→ RCH=CH₂ + CH₃COOH
The Chugaev reaction uses xanthate esters and proceeds at lower temperatures (~100–200°C) — also syn elimination.
6. Reduction of Alkynes
Alkynes can be selectively reduced to alkenes:
| Reagent | Product Stereochemistry |
|---|---|
| H₂ / Lindlar's catalyst (Pd/CaCO₃ + Pb(OAc)₂) | cis (Z)-alkene |
| Na or Li in liquid NH₃ (Birch/dissolving metal) | trans (E)-alkene |
7. Wittig Reaction
A carbonyl compound (aldehyde or ketone) reacts with a phosphorus ylide to form an alkene:
R₂C=O + Ph₃P=CH₂ → R₂C=CH₂ + Ph₃P=O
- Highly useful for controlled synthesis; produces alkene at a specific location.
- Stabilized ylides → (E)-alkene; non-stabilized ylides → (Z)-alkene.
8. Decarboxylation / Cracking (Industrial)
- Steam cracking of petroleum fractions at high temperatures yields ethylene, propylene, and other alkenes (most important industrial method).
- Catalytic cracking uses zeolite catalysts at lower temperatures.
Summary Table
| Method | Starting Material | Reagent/Condition | Notes |
|---|---|---|---|
| Dehydration | Alcohol | conc. H₂SO₄ or H₃PO₄, heat | Zaitsev's rule |
| Dehydrohalogenation | Alkyl halide | Strong base, heat | E2 or E1 |
| Dehalogenation | Vicinal dihalide | Zn/EtOH or NaI/acetone | Anti elimination |
| Pyrolysis of ester | Ester | Heat ~500°C | Syn elimination |
| Chugaev | Xanthate ester | Heat ~150°C | Syn elimination |
| Alkyne reduction | Alkyne | Lindlar's catalyst | cis-alkene |
| Alkyne reduction | Alkyne | Na/liq. NH₃ | trans-alkene |
| Wittig | Carbonyl compound | Phosphorus ylide | Controlled regiochemistry |
| Industrial cracking | Petroleum | High temp / catalyst | Mixture of alkenes |
Key concept — Zaitsev vs. Hofmann rule:
- Zaitsev: Strong, small base (e.g., KOH) → more substituted alkene (thermodynamic)
- Hofmann: Bulky base (e.g., t-BuOK) → less substituted alkene (steric)
Let me know if you want a deeper dive into any specific method, mechanism, or stereochemistry.
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