The Alcohol Functional Group

  • All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions.
  • As with other organic molecules, a large R represents the side chain which doesn´t affect the chemistry of the molecule.
  • Alcohols are a homologous series of compounds that have the general formula CnH2n+1OH
  • They differ by one -CH2 in the molecular formulae from one member to the next.

Alcohol- Functional Group, IGCSE & GCSE Chemistry revision notes

Diagram of the side chain and -OH group in ethanol which characterizes its chemistry

The First Four Alcohols

  • The names and structures of the first four alcohols are shown below.
  • In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘ol’.
  • Similar to propene and butene, the position of the -OH group can change and this can be reflected using numbers, where the number refers to which carbon atom the alcohol group is attached to.
  • Butan-2-ol therefore has 4 carbon atoms in a chain and the -OH group is on the second carbon atom from the left.

Alcohols - The First Four, IGCSE & GCSE Chemistry revision notes

Table showing the formulae and structures of the first four alcohols

Production of Ethanol by Fermentation

  • Ethanol (C2H5OH) is one of the most important alcohols.
  • It is the type of alcohol found in alcoholic drinks such as wine and beer.
  • It is also used as fuel for cars and as a solvent.
  • It can be produced by fermentation where sugar or starch is dissolved in water and yeast is added.
  • The mixture is then fermented between 15 and 35°C with the absence of oxygen for a few days.
  • Yeast contains enzymes that break down starch or sugar to glucose.
  • If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured.
  • The yeast respire anaerobically using the glucose to form ethanol and carbon dioxide:

C6H12O6 + Enzymes → 2CO2 + 2C2H5OH

  • The yeast are killed off once the concentration of alcohol reaches around 15%, hence the reaction vessel is emptied and the process is started again.
  • This is the reason that ethanol production by fermentation is a batch process.

Properties of Alcohols

  • Alcohols, like other homologous series, show a gradation in their physical properties:
    • Boiling points increase with increased carbon chain length.
    • Viscosity increases with increased carbon chain length.
    • Solubility in water decreases with increase carbon chain length.
  • Alcohols are colourless liquids that dissolve in water to form neutral solutions.
  • The first four alcohols are commonly used as fuels.
  • School laboratories use ethanol in spirit burners as it burns cleanly and without strong odours.
  • Methanol and ethanol are also used extensively as solvents.
  • This is because they can dissolve many substances that water cannot such as fats and oils, but can also dissolve most of the substances that water can.

Reactions of Alcohols

  • Alcohols undergo combustion to form carbon dioxide and water.
  • The complete combustion of ethanol produces 2 moles of CO2 and 3 moles of H2O:

CH3CH2OH + 3O2 → 2CO2 + 3H2O

  • Alcohols react with sodium metal to produce hydrogen gas and a metal salt.
  • The equation for the reaction of methanol with sodium is:

2Na + 2CH3OH → 2CH3ONa + H2

  • Alcohols undergo oxidation to produce carboxylic acids, an organic acid.
  • This is what happens to wine when it is left open as the microbial oxidation of ethanol will produce a weak solution of a carboxylic acid called ethanoic acid, the same acid used in vinegar.
  • Bacteria in the air (acetobacter) use atmospheric oxygen from air to oxidise the ethanol in the wine:

C2H5OH + O2 → CH3COOH + H2O

  • The acidic, vinegary taste of wine which has been left open for several days is due to the presence of ethanoic acid.

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.