Preparation of Alkenes
Master the introduction of $\pi$ bonds. Decode stereoselective alkyne reduction and regioselective $\beta$-elimination to guarantee your hydrocarbon reaction marks.
Module Focus: Elimination & Partial Reduction
Alkenes are unsaturated hydrocarbons containing a $C=C$ double bond. Synthesizing them generally requires one of two strategies: Elimination (removing atoms from adjacent carbons of a saturated compound to form a $\pi$ bond) or Partial Reduction (adding exactly one mole of $H_2$ to an alkyne without going all the way to an alkane). In NEET, stereochemistry (cis/trans) and regiochemistry (Zaitsev/Hofmann) are heavily tested.
1. From Alkynes (Stereoselective Reduction)
Alkynes easily reduce to alkanes in the presence of standard catalysts ($Pt, Pd, Ni$). To stop the reaction at the alkene stage, we must use poisoned or specific catalysts. This allows us to control whether we get a cis or trans alkene.
- Reagent: $H_2$ gas with Palladium on Barium Sulfate ($Pd/BaSO_4$) or Calcium Carbonate ($Pd/CaCO_3$).
- Poison: Quinoline or Sulfur is added to "poison" (partially deactivate) the catalyst, stopping reduction at the alkene stage.
- Mechanism: Both H atoms attach from the surface of the metal $\rightarrow$ Syn-addition.
- Reagent: Sodium ($Na$) or Lithium ($Li$) in liquid Ammonia ($NH_3$).
- Mechanism: Involves a radical anion intermediate. Electron repulsion forces the intermediate into a *trans* geometry.
- Result: Hydrogen is added from opposite sides $\rightarrow$ Anti-addition.
2. From Alkyl Halides (Dehydrohalogenation)
Heating an alkyl halide with alcoholic KOH (potassium hydroxide dissolved in alcohol) causes the elimination of a hydrogen atom from a $\beta$-carbon and the halogen from the $\alpha$-carbon, forming a $\pi$ bond.
This reaction predominantly follows the E2 mechanism (bimolecular elimination). It is a concerted, single-step reaction requiring the $\beta$-Hydrogen and the Halogen to be anti-periplanar (180° apart) to each other.
If elimination can occur in two directions, the major product is the more highly substituted alkene (the one with more alkyl groups attached to the double bond carbons, making it more stable due to hyperconjugation).
2-Bromobutane $\xrightarrow{Alc. KOH}$ 2-Butene (Major, 80%) + 1-Butene (Minor, 20%)
If a very bulky base (like potassium *tert*-butoxide) is used, or if the leaving group is very poor (like Fluorine $-F$), the less substituted alkene becomes the major product (Hofmann product) due to steric hindrance.
3. From Vicinal Dihalides (Dehalogenation)
Vicinal dihalides (two halogens on adjacent carbon atoms) undergo dehalogenation when heated with Zinc dust in alcohol, forming an alkene.
4. From Alcohols (Acidic Dehydration)
Heating an alcohol with concentrated acids (like Conc. $H_2SO_4$ or $H_3PO_4$) at high temperatures results in the loss of a water molecule ($-H$ from $\beta$-carbon, $-OH$ from $\alpha$-carbon).
- Conc. $H_2SO_4$ at 443 K ($170^\circ\text{C}$).
- Temperature Trap: If heated at 413 K ($140^\circ\text{C}$) with excess alcohol, it forms an Ether instead of an alkene via $S_N2$.
- Alternative reagent: Heating with Alumina ($Al_2O_3$) at 623 K.
Proceeds via a Carbocation intermediate.
- Protonation of $-OH$ to form $-OH_2^+$ (good leaving group).
- Loss of water to form a Carbocation (Slow, Rate-determining step).
- Loss of $H^+$ from the $\beta$-carbon to form the $C=C$ bond.
Because dehydration proceeds via a carbocation, 1,2-hydride or 1,2-alkyl shifts will occur if they lead to a more stable carbocation before the final elimination step.
Dehydration of Neopentyl alcohol ($1^\circ$) completely rearranges to yield 2-Methyl-2-butene (Zaitsev major product from the $3^\circ$ cation).
NEET Grand Test: Alkene Preparation
15 High-Yield Questions testing stereoselectivity, Zaitsev's rule, and carbocation rearrangements.
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