Halohydrin Formation and Stereochemistry

Overview

This lecture explains the formation of vicinal halohydrins from alkenes by electrophilic addition, emphasizing the reaction's mechanism, regioselectivity, and stereoselectivity.

Halohydrin Formation Reaction

• Halohydrin formation involves reacting an alkene with a halogen (Br₂ or Cl₂) and water.
• The product is a vicinal halohydrin: a molecule with a halogen and an alcohol group on adjacent carbons.
• The reaction follows Markovnikov’s rule for regioselectivity.
• The addition occurs with anti stereoselectivity, so the halogen and the alcohol group end up on opposite sides of the original double bond.

Example Reactions

• Example 1:
  Alkene + Br₂ + H₂O → Br and OH add to adjacent carbons (result: vicinal bromoalcohol).
  • The Br attaches to one double bond carbon, while the OH attaches to the other.
• Example 2 (unsymmetrical alkene):
  When Br₂ and H₂O are added to an unsymmetrical alkene, Br bonds to the less substituted carbon and OH to the more substituted carbon, in line with Markovnikov's rule.

Reaction Mechanism

• Step 1: The alkene’s π electrons react with the halogen molecule, leading to cleavage of the halogen-halogen bond and formation of a three-membered, bridged halonium ion intermediate (e.g., a bromonium ion).
• Step 2: Water acts as a nucleophile, attacking the more substituted carbon of the halonium ion and opening the ring.
  • For unsymmetrical alkenes, water attacks the carbon that can better stabilize the positive charge (more substituted).
• Step 3: The resulting oxonium ion is deprotonated by a halide ion (Br⁻ or Cl⁻), yielding the final vicinal halohydrin.

Regioselectivity and Stereoselectivity

• Regioselectivity: Water (the nucleophile) attacks the more substituted carbon of the halonium ion, following Markovnikov’s rule, because this carbon is better at stabilizing the positive charge.
• Stereoselectivity: The halogen and hydroxyl group add to opposite (trans) sides of the former double bond (anti addition).
• The final product is a pair of enantiomers (mirror-image isomers), since the O and halogen end up on opposite sides.

Key Terms & Definitions

• Halohydrin: A molecule with a halogen and an alcohol group on adjacent carbons.
• Vicinal: Groups attached to neighboring carbon atoms.
• Markovnikov's Rule: In addition reactions, the nucleophile adds to the more substituted carbon.
• Anti Addition: Two groups add to opposite sides of a double bond.
• Halonium Ion: A three-membered ring intermediate with a positive charge, formed during halogen addition to alkenes.
• Enantiomers: Non-superimposable mirror-image isomers.

Action Items / Next Steps

• Review and practice drawing the complete mechanism for halohydrin formation.
• Practice predicting products and stereochemistry, especially for unsymmetrical alkenes.
• Read about Markovnikov’s rule and how it applies in similar addition reactions.