Alcohol Dehydration Mechanisms

Overview

• Topic: Dehydration of alcohols to form alkenes via elimination reactions.
• Main distinction: mechanism depends on whether alcohol is primary, secondary, or tertiary.
• Two mechanisms: E2 (bimolecular) and E1 (unimolecular).

Classification Of Alcohols

• Primary: OH-bearing carbon connected to one other carbon.
• Secondary: OH-bearing carbon connected to two other carbons.
• Tertiary: OH-bearing carbon connected to three other carbons.
• Primary alcohols typically undergo E2; secondary and tertiary undergo E1.

E2 Mechanism (Bimolecular)

• Key feature: rate-determining step involves collision of two species.
• Typical conditions: acid-catalyzed (e.g., H2SO4) and a base or conjugate base present.
• Mechanistic steps:
  • Protonation: Lone pair on OH takes a proton from acid, converting OH to OH2+ (alkyl oxonium ion).
  • Concerted elimination (rate-determining): Base (conjugate base) removes a hydrogen from the β-carbon while C–O bond breaks and C=C double bond forms simultaneously.
  • Products: Alkene + H2O; acid is regenerated (catalyst).
• Notes:
  • Water is a good leaving group once protonated.
  • Electrons flow toward electronegative oxygen during bond breaking.
  • Overall termed E2 because two species are involved in the slow step.

E1 Mechanism (Unimolecular)

• Key feature: rate-determining step involves only one molecule (formation of carbocation).
• Typical for: secondary and tertiary alcohols (stable carbocations).
• Mechanistic steps:
  • Protonation: OH protonated by acid to form OH2+ (alkyl oxonium ion).
  • Unimolecular ionization (rate-determining): C–O bond breaks, leaving water and forming a carbocation intermediate.
  • Deprotonation: A base (e.g., H2O or conjugate base) removes a β-hydrogen; electrons form the C=C double bond.
  • Products: Alkene + H2O; carbocation stability directs mechanism and product distribution.
• Notes:
  • Primary carbocations are too unstable for E1 pathways.
  • Carbocation intermediate allows multiple β-hydrogens to be abstracted, giving different alkene isomers.
  • Trans alkene generally favored over cis as major product; cis formed as minor product.

Regioselectivity And Stereochemistry

• Choice of β-hydrogen influences double-bond location and stereochemistry.
• Steric hindrance and blocking groups affect which hydrogen is removed.
• For simple symmetrical cases, multiple alkenes (positional and stereoisomers) can form.
• Trans alkene usually preferred as the major stereoisomer in E1 eliminations.

Key Terms And Definitions

• Alkyl Oxonium Ion: Protonated alcohol (R–O–H2+) formed after protonation of OH.
• Leaving Group: Water (after protonation) is a good leaving group in acid catalysis.
• Carbocation: Positively charged carbon formed in E1 mechanism; stability increases with more alkyl substitution.
• E2: Bimolecular elimination; concerted removal of proton and leaving group.
• E1: Unimolecular elimination; rate-determining formation of carbocation followed by deprotonation.

Reaction Summary Table

Action Items / Study Tips

• Practice identifying primary, secondary, tertiary carbons on structures.
• Draw full mechanisms showing electron flow for both E2 and E1.
• Predict major alkene products using carbocation stability and Zaitsev’s rule.
• Consider stereochemical outcomes (cis/trans) and why trans is often favored.
• Remember acid catalysis: protonate OH first to make water a good leaving group.