Preparation of Alkenes
Synthesis from Alkynes, Alkyl Halides, Alcohols, and Electrolysis.
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond ($C=C$). They are generally prepared by Elimination reactions (removing atoms from adjacent carbons) or Partial Reduction of alkynes.
1. From Alkynes (Partial Reduction)
Stereoselective Hydrogenation
Alkynes react with dihydrogen to form alkenes. The stereochemistry depends on the reagent used.
Reagent: $H_2 + Pd/CaCO_3$ (poisoned with Sulfur/Quinoline).
Reagent: Sodium in liquid Ammonia ($Na / \text{liq. } NH_3$).
2. From Alkyl Halides (Dehydrohalogenation)
$\beta$-Elimination
When alkyl halides ($R-X$) are heated with Alcoholic KOH, they lose a hydrogen atom from the $\beta$-carbon and a halogen from the $\alpha$-carbon to form an alkene.
If more than one alkene can be formed, the more substituted alkene (the one with more alkyl groups attached to the double bond) is the major product due to greater stability (Hyperconjugation).
Exception: Hofmann product (Less substituted) is formed if the base is bulky (e.g., t-BuOK) or the leaving group is poor (e.g., F).
Reactivity Order: $3^\circ > 2^\circ > 1^\circ$ Alkyl Halide.
3. From Alcohols (Acidic Dehydration)
Dehydration
Alcohols lose a water molecule when heated with concentrated acids like conc. $H_2SO_4$ (at 443 K) or $H_3PO_4$.
- Protonation of Alcohol.
- Formation of Carbocation (Slow step/Rate determining).
- Removal of proton to form Alkene.
4. From Vicinal Dihalides (Dehalogenation)
Treatment of vicinal dihalides (two halogens on adjacent carbons) with Zinc dust causes the loss of the halogen molecule.
5. Kolbe's Electrolysis
Electrolysis of Salts
Electrolysis of a cold, concentrated aqueous solution of potassium or sodium salts of dicarboxylic acids (like Sodium Succinate) yields alkenes at the Anode.
At Anode: Ethene + $CO_2$ | At Cathode: $H_2$ gas
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