Reactions Involving Alcohols

Reaction with Hydrobromic Acid (HBr)

• Product: Alcohol is converted into an alkyl halide, specifically the OH group is replaced by a bromine atom.
• Mechanism:
  • Protonation: The OH group is protonated to become a better leaving group.
  • SN2 Reaction: A bromide ion attacks from the back, expelling water, and forming the alkyl halide.

Reaction with Hydrochloric Acid (HCl)

• Primary Alcohols: React slowly with HCl because chloride is a weaker nucleophile.
• Enhancement: Use Zinc Chloride (ZnCl2) as a Lewis acid (Lucas reagent) to increase reaction rate.
  • Mechanism:
    • Oxygen attacks Zinc Chloride, expelling a chloride ion.
    • Chloride ion attacks carbon, expelling the oxygen group and forming an alkyl chloride.

Reaction with Hydroiodic Acid (HI)

• Tertiary Alcohols:
  • Product: Replace OH with iodine atom.
  • Mechanism (SN1):
    • Protonation of OH group.
    • Leaving group leaves, forming a tertiary carbocation.
    • Iodide ion combines with carbocation to form the alkyl iodide.

Reaction with 2-Methylcyclohexanol and HBr

• Mechanism:
  • Protonation of OH group.
  • Leaving group leaves, forming a secondary carbocation.
  • Hydride Shift: Secondary carbocation shifts to more stable tertiary carbocation.
  • Bromide ion attacks the carbocation, forming a tertiary alkyl halide.

Other Methods for Converting Alcohols to Alkyl Halides

• Phosphorus Tribromide (PBr3):
  • Works via SN2 mechanism.
  • Converts OH to an alkyl bromide.
• Thionyl Chloride (SOCl2):
  • Works via SN2 mechanism.
  • Converts OH to an alkyl chloride.

Mechanism for PBr3

• Interaction: Oxygen attacks the phosphorus, expelling a bromide ion.
• Intermediate: An acid-base reaction with pyridine removes a hydrogen proton.
• Final Step: Bromide ion attacks carbon, expelling a group, and forming the alkyl bromide.

Mechanism for SOCl2

• Interaction: Oxygen attacks the sulfur, breaking a pi bond.
• Intermediate: Oxygen reforms pi bond, expelling a chloride.
• Final Step: Chloride ion attacks carbon, expelling the group, forming alkyl chloride, and producing SO2.

Stereochemistry Considerations

• Secondary Alcohol with HBr (SN1): Results in racemic mixture.
• Secondary Alcohol with PBr3 (SN2): Results in inversion of configuration.
• Thionyl Chloride (SOCl2) Reaction: Also results in inversion of configuration.

Para-Toluene Sulfonyl Chloride (TsCl)

• Reaction with Alcohol: Converts OH into a tosylate (OTs), a good leaving group.
• Mechanism:
  • Removal of HCl and linkage of R group to OTs, retaining stereochemistry.
• Product: R-OTs with retention of configuration.