Preparation of Alkenes
Synthetic Methods & Mechanisms | Hydrocarbons
1. From Alkyl Halides (Dehydrohalogenation)
Reaction of alkyl halides with Alcoholic KOH leads to the elimination of a hydrogen halide ($HX$) molecule ($\beta$-elimination).
Saytzeff's Rule (Zaitsev's Rule)
- Major Product: More substituted alkene (Saytzeff product).
- Order of Reactivity: $3^\circ > 2^\circ > 1^\circ$ Alkyl Halides.
2. From Alcohols (Acidic Dehydration)
Alcohols lose a water molecule upon heating with concentrated acids like $H_2SO_4$ or $H_3PO_4$.
Mechanism Characteristics:
- It proceeds via a Carbocation Intermediate.
- Rearrangements (Hydride/Methyl shift) are possible to form a more stable carbocation.
- Reactivity Order: $3^\circ > 2^\circ > 1^\circ$ (Based on carbocation stability).
3. From Alkynes (Partial Reduction)
Alkynes can be partially reduced to alkenes. The stereochemistry depends on the reagent used.
A. Cis-Alkene (Lindlar's Catalyst)
Reagent: $H_2 + Pd/CaCO_3$ (poisoned with Quinoline/Sulphur).
$$ R-C \equiv C-R + H_2 \xrightarrow{Pd/C, \text{quinoline}} \text{cis-Alkene} $$The addition of hydrogen is Syn-addition.
B. Trans-Alkene (Birch Reduction)
Reagent: Sodium in liquid Ammonia ($Na / liq. NH_3$).
$$ R-C \equiv C-R + 2[H] \xrightarrow{Na / liq. NH_3} \text{trans-Alkene} $$The addition of hydrogen is Anti-addition.
4. From Vicinal Dihalides (Dehalogenation)
Treatment of vicinal dihalides (halogens on adjacent carbons) with Zinc dust eliminates the halogen molecule ($X_2$).
5. Kolbe's Electrolytic Method
Electrolysis of aqueous solution of sodium or potassium salt of a saturated dicarboxylic acid (e.g., Sodium Succinate) yields alkene at the Anode.
$$ \underset{\text{Sodium Succinate}}{(CH_2COONa)_2} + 2H_2O \xrightarrow{\text{Electrolysis}} \underset{\text{Ethene}}{CH_2=CH_2} + 2CO_2 + H_2 + 2NaOH $$Practice Quiz
Test your synthesis skills.
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