Preparation of Haloalkanes from Alcohols

Preparation of Haloalkanes (Alkyl Halides) from Alcohols

• Alcohols react with Phosphorus Trichloride (PCl3) to produce alkyl chlorides and phosphorous acid.
• General Reaction: 3R-OH + PCl3 → 3R-Cl + H3PO3
  • R represents an alkyl group
  • 3R-OH represents an alcohol
  • 3R-Cl represents an alkyl chloride
  • H3PO3 is phosphorous acid
• Example: The reaction of 2-methylpropan-2-ol with PCl3 yields 2-chloro-2-methylpropane.

Chemical Reactions Involving Alcohols and Chlorides

• Alcohols react with phosphorus trichloride ($PCl_3$), phosphorus pentachloride ($PCl_5$), and thionyl chloride ($SOCl_2$) to form alkyl chlorides.
• Examples:
  • Methyl Alcohol (CH₃OH) + Phosphorus Trichloride ($PCl_3$) → Methyl Chloride ($CH_3Cl$)
  • Ethyl Alcohol ($C_2H_5OH$) + Phosphorus Pentachloride ($PCl_5$) → Ethyl Chloride ($C_2H_5Cl$)
• These reagents act as catalysts, replacing the hydroxyl group with a chlorine atom.

Chemical Reactions of Alcohols

• Reactions with Thionyl Chloride:
  • Ethyl Alcohol (C₂H₅OH) + thionyl chloride (SOCl₂) → ethyl chloride (C₂H₅Cl)
• Reactions with Lucas Reagent:
  • Primary alcohols don't react with Lucas reagent (concentrated HCl + Anhydrous ZnCl₂)
  • Secondary alcohols react slowly with Lucas reagent to form secondary alkyl halides.
  • Tertiary alcohols react rapidly with Lucas reagent to form tertiary alkyl halides.
• Reactions with Chlorine:
  • Methane (CH₄) + chlorine (Cl₂) in the presence of light → chloromethane (CH₃Cl) and hydrogen chloride (HCl)

Reactions of Alcohols and Alkanes

• Reaction of Ethyl Alcohol with Hydrogen Chloride
  • Ethyl alcohol (CH₃CH₂OH) + concentrated hydrogen chloride (HCl) → ethyl chloride (CH₃CH₂Cl) + water (H₂O).
• Lucas Reagent and Primary Alcohols
  • Primary alcohol + Lucas Reagent → primary alkyl chloride
• Chlorination of Methane
  • Methane (CH₄) + chlorine (Cl₂) in the presence of sunlight → various chloroalkanes
    • Chloromethane (CH₃Cl)
    • Dichloromethane (CH₂Cl₂)
    • Trichloroethane (CHCl₃)
    • Tetrachloromethane (CCl₄) (Carbon tetrachloride)

Organic Chemistry Notes

• Alkene Reactions
  • From Symmetrical Alkene:
    • Symmetrical alkene (like ethene) + HCl → chloroethane
  • From Unsymmetrical Alkene (Markovnikov's Rule):
    • Unsymmetrical alkene + hydrogen halide (HX) → negative part of HX attaches to the carbon with fewer hydrogens.
  • Anti-Markovnikov's Rule (Peroxide Effect/ Kharasch Effect):
    • Unsymmetrical alkene + hydrogen halide (HX) + peroxide → negative part of HX attaches to the carbon with more hydrogens.
• Other Reactions
  • Halogen Exchange Reaction: Alkyl chloride or bromide + sodium iodide in acetone → alkyl iodide.
    • R-Cl + NaI → R-I + NaCl
    • R-Br + NaI → R-I + NaBr
  • Finkelstein Reaction: Similar to halogen exchange but using sodium iodide.
  • Borodine Hunsdiecker Reaction: Silver salt of acids + bromine in the presence of silver → alkyl bromide.
    • R-COO-Ag + Br2 → R-Br + AgBr + CO2
• Reaction of Alkyl Halides with Metal Fluorides:
  • Alkyl chloride or bromide + metal fluorides (AgF, Hg₂F₂, etc.) → alkyl fluoride.
  • Example: CH₃-Cl + AgF → CH₃-F + AgCl (methyl chloride to methyl fluoride)

Halo Arenes

• Derivatives of aromatic hydrocarbons where hydrogen atoms are replaced with halogen groups.

Additional Notes

• Allylic Halide: The halogen atom is present at the allylic position.
  • Example: 3-bromocyclohexane
• Aralkyl Halides: Halogen atoms are attached to an aromatic ring that is part of a longer carbon chain.
  • Example: Benzyl halide