Reactions of Alcohols

Conversion to Alkyl Halides

• Primary Alcohols with HBr

  • Replace OH with Br atom, forming alkyl halide.
  • Mechanism: SN2 Reaction
    1. Protonation of OH group (turns into a good leaving group).
    2. Bromide ion attacks, expels H2O, forming alkyl halide.

• Primary Alcohols with HCl

  • Reacts slowly due to weak nucleophile Cl.
  • Enhanced by Lucas Reagent (ZnCl2) for faster reaction.
  • Mechanism:
    1. Oxygen attacks ZnCl2, expels Cl ion.
    2. Chloride ion attacks carbon, forming alkyl chloride.

Tertiary Alcohols

• With HI
  • SN1 mechanism due to sterics.
  • Steps:
    1. Protonation of OH group.
    2. Formation of tertiary carbocation intermediate.
    3. Iodide ion combines with carbocation.

Example Reactions

• 2-Methylcyclohexanol with HBr
  • Steps:
    1. Protonation of OH group.
    2. Formation of secondary carbocation.
    3. Hydride shift to form stable tertiary carbocation.
    4. Bromide ion attacks carbocation.

Use of Other Reagents

• PBr3

  • Converts OH to alkyl bromide via SN2 mechanism.
  • Mechanism involves nucleophilic attack by oxygen on phosphorus, forming intermediate, and expulsion of bromide.

• SOCl2 (Thionyl Chloride)

  • Converts OH to alkyl chloride via SN2.
  • Mechanism involves nucleophilic attack by oxygen on sulfur, followed by chloride attack.

Stereochemistry

• Secondary Alcohols with HBr

  • SN1 reaction leads to racemic mixture.
  • Both stereoisomers possible.

• Using PBr3 or SOCl2

  • SN2 reaction causes inversion of configuration at chiral center.

Tosylate Formation (TsCl)

• TsCl
  • Converts OH to a good leaving group (OTs).
  • Retains configuration.
  • Used for specific retention reactions and transformations.