Properties of Alcohols

• • The alcohol homologous series has the general formula CnH2n+1OH.

  
• Primary, secondary and tertiary alcohols

  • An alcohol can be primary, secondary or tertiary, depending on which carbon atom the hydroxyl group -OH is bonded to.
  • Primary alcohols are given the notation 1° and the -OH group is attached to a carbon with one alkyl group attached.
  • Secondary alcohols are given the notation 2° and the -OH group is attached to a carbon with two alkyl groups attached.
  • Tertiary alcohols are given the notation 3° and the -OH group is attached to a carbon with three alkyl groups attached.

  
• Examples of Primary, secondary and tertiary alcohols

  
• Physical properties of alcohols


  • Alcohols are polar molecules due to the electronegative hydroxyl group which pulls the electrons in the C-OH bond away from the carbon atom.
  • The hydroxyl group (-OH) is also polar, with a delta- charge on the oxygen atom and a delta+ charge on the hydrogen atom.
  • The partial positive charge on the hydrogen atom in the hydroxyl group can attract the lone pairs on an oxygen from a neighbouring molecule, forming hydrogen bonds.

  
• Physical properties of alcohols (2)

  • When you mix an alcohol with water, hydrogen bonds form between the -OH group and H2O.
  • If it's a small alcohol (e.g. methanol, ethanol or propan-1-ol), hydrogen bonding lets it mix freely with water- it's soluble in water.
  • In larger alcohols, most of the molecule is a non-polar carbon chain, so there's less attraction for the polar H2O molecules.
  • This means that as alcohols increase in size, their solubility in water decreases.

  
• Physical properties of alcohols (3)

  • Alcohols also form hydrogen bonds with each other.
  • Hydrogen bonding is the strongest kind of intermolecular force, so it gives alcohols a relatively low volatility (they don't evaporate easily into a gas) and relatively high boiling points compared to non-polar compounds of similar sizes, e.g. alkanes.