4.3/4.4 Alcohols and Haloalkanes

• Alcohols


  Compounds with a hydroxyl (-OH) group
• Alcohols


  • Hydrogen bonding between the σ+ on H and lone pairs on O on other molecules
  • Alcohols have stronger hydrogen bonds than alkanes, requiring more energy to separate
• As the number of carbon atoms in the alcohol chain increases

  The boiling point increases due to more London forces that require more energy to overcome
• Solubility of alcohols


  Small alcohols are completely soluble in water, solubility decreases as the hydrocarbon chain length increases as they break hydrogen bonds with water molecules
• Types of alcohols


  • Primary
  • Secondary
  • Tertiary
• Complete combustion of alcohols

  Alcohol + Oxygen → CO2 + H2O
• Oxidation of alcohols


  1. Primary alcohol → Aldehyde → Carboxylic acid

  2. Secondary alcohol → Ketone

  3. Tertiary alcohol cannot be oxidised
• Oxidising agent for alcohols


  Acidified potassium dichromate (K2Cr2O7 / H2SO4), changes from orange to green
• Forming aldehydes and carboxylic acids from alcohols

  1. Alcohol + [O] → Aldehyde

  2. Alcohol + 2[O] → Carboxylic acid
• Forming ketones from alcohols

  Requires reflux
• Dehydration of alcohols


  1. Alcohol → Alkene + H2O

  2. With hot catalyst Al2O3

  3. Or excess, hot H2SO4 and H3PO4 catalyst
• Substitution reaction of alcohols

  1. Alcohol + MX → Haloalkane + H2O

  2. Reacts with MX by reacting NaX with H2SO4 or H3PO4
• Haloalkanes


  Much more reactive than alkanes due to the electronegative halogen
• Reactions of haloalkanes


  1. Nucleophilic substitution - halogen substituted for another group

  2. Elimination reaction - H eliminated, forms alkene
• Hydrolysis of haloalkanes


  1. Haloalkane + NaOH(aq) → Alcohol + NaX

  2. Rate depends on halogen, stronger = slower rate
• Measuring rate of hydrolysis


  Using AgNO3, faster rate forms yellow precipitate, then cream, then white
• Nucleophile


  Electron-rich species that donates a pair of electrons
• Hydrolysis mechanism of haloalkanes


  1. C-X + OH- → H-C-OH + X-
• 1. X bond enthalpy

  2. I bond has the lowest enthalpy, requires least energy to break, so is the weakest
• Fluoroalkanes, chloroalkanes, bromoalkanes, iodoalkanes are increasingly reactive
• CFCs are useful as they are inert, non-flammable and non-toxic
• CFCs are used as refrigerants and solvents for dry cleaning
• HFCs are an alternative to CFCs, chemically inert
• CFCs absorb UV radiation in the upper atmosphere, breaking down to form chlorine radicals which destroy ozone
• Nitrogen oxide radicals from combustion engines also cause ozone depletion
• Ozone depletion leads to increased exposure to UV, causing skin cancer, cataracts and weakened immune systems