Alcohols

• • Alcohols are a homologous series of organic compounds with the functional group -OH
  • suffix -ol
• Classifying alcohols:

  • Classification depends on the number of hydrogen atoms & alkyl groups attached to the carbon atom that contains the alcohol functional group
• Primary alcohols:

  • Methanol and ethanol are both primary alcohols
  • The -OH group is attached to a carbon atom that is attached to 2 hydrogen atoms & one alkyl group
• Secondary alcohols:

  • The -OH group is attached to a carbon atom that is attached to one hydrogen atom & 2 alkyl groups. E.g: Propan-2-ol & Propan-3-ol
• Tertiary alcohols:

  • The -OH group is attached to a carbon atom that is attached to no hydrogen atoms & 3 alkyl groups. E.g: 2-Methylpropan-2-ol
• Polarity:

  • Alcohols have a polar O-H bond because of the difference in electronegativity of the oxygen & hydrogen atoms
  • Alcohols are polar
  • Alcohols are soluble in water - due to the hydrogen bonding
  • The intermolecular forces will be weak London forces but there will also be much stronger hydrogen bonds between the polar O-H groups
• Alcohols have low volatility compared to alkanes:

  • Alcohols have strong H bonds
  • Alkanes have weaker London forces
  • Takes less energy to break
• Combustion of alcohols:

  • Alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water
  • The reaction is exothermic, releasing a large quantity of energy in the form of heat
  • As the number of carbon atoms in the alcohol chain increases the quantity of heat released per mole also increases
• Oxidising agent --> acidified dichromate
• • Primary alcohols are oxidised to aldehydes
  • Aldehydes can be oxidised further to produce carboxylic acids
  • To obtain the aldehyde from ethanol, you distill the mixture
  • To obtain the carboxylic acid from ethanol, you reflux the ethanol
• • Secondary alcohols oxidise to produce ketones
  • No further reaction as you can't oxidise a ketone
• • No reaction when oxidising a tertiary alcohol
• Dehydration of alcohols:

  • Dehydration is any reaction in which a water molecule is removed from the starting material
  • An alcohol is heated under reflux in the presence of an acid catalyst such as concentrated sulfuric acid, H2SO4
  • The product of the reaction is an alkene
  • Dehydration of an alcohol is an example of an elimination reaction
• elimination is the reverse of addition
• Elimination --> the removal of a molecule from a saturated molecule to make an unsaturated molecule
• A reagent for a elimination reaction is sulfuric or phosphoric acid
• Tertiary alcohols don’t undergo oxidation reactions, so the acidified dichromate (VI) remains orange when added to a tertiary alcohol
• Alcohols react with hydrogen halides to form haloalkanes. When preparing a haloalkane, the alcohol is heated under reflux with sulfuric acid & a sodium halide the hydrogen bromide is formed