Dehydration of Alcohols to Create Alkenes

Introduction

• Dehydration: Removal of water from alcohol to form alkenes.
• Process: Reversible reaction using heat and acid catalyst.
• Le Chatelier's Principle: To favor products, remove the alkene product to shift equilibrium.
• Distillation: Used to collect the alkene product.

Reaction Mechanism

• Resembles E1 Mechanism:
  • Poor leaving group in OH⁻ ion requires an acid catalyst.
  • Protonating OH makes it a good leaving group.
• Steps:
  1. Protonation: Hydroxyl group protonated, forming protonated alcohol.
  2. Leaving Group: Water leaves, forming a carbocation.
  3. Elimination: Carbocation eliminates to produce the alkene.

Characteristics

• Rate-Determining Step: Formation of the carbocation.
• Reactivity:
  • Tertiary alcohols > Secondary alcohols > Primary alcohols.
• Zaitsev's Rule: Major product is the more substituted alkene.

Example Reaction

• Alcohol Reaction with Acid and Heat:
  • Protonate hydroxyl using sulfuric acid.
  • Form carbocation (rate-determining step).
  • Deprotonate carbocation:
    • From left carbon: Forms less substituted (minor) alkene.
    • From right carbon: Forms more substituted (major) alkene.
    • Trans and cis isomers can form.

Products

• Possible Products:
  • Minor: Less substituted alkene.
  • Major: More substituted alkene (trans and cis isomers).
• Cis-Trans Formation: Both isomers form due to possible carbocation configurations.

Conclusion

• Dehydration of alcohols is a reversible process, favored by product removal and guided by principles like Zaitsev's rule and carbocation stability.