6.8 - Alcohols

The alcohols

• An alcohol is a compound that carries an -OH group joined to a carbon atom.
• That -OH is the alcohols' functional group - the part that decides how the compound reacts, which is why all alcohols behave in much the same way.

Naming the alcohols

• The stem of the name shows how many carbon atoms are in the chain, and the ending -ol shows it's an alcohol (e.g. ethane becomes ethanol).
• The first four are methanol, ethanol, propanol and butanol, with one, two, three and four carbon atoms.

Formulae keep the -OH visible on purpose, which is why ethanol is written C₂H₅OH or CH₃CH₂OH, never C₂H₆O.

How alcohols burn

Alcohols are flammable, and burning them is their most useful reaction.

Burning in plenty of air

• With plenty of air an alcohol undergoes complete combustion.
• The carbon is oxidised to carbon dioxide and the hydrogen is oxidised to water, releasing a good deal of energy.
• ethanol + oxygen → carbon dioxide + water
• C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Balancing the equation

• The alcohol already contains an oxygen atom, which must be counted when balancing the equation.
• In the equation above, the right-hand side needs seven oxygen atoms (four in the CO₂, three in the H₂O). One comes from the ethanol itself, leaving six to come from oxygen gas - so three O₂ molecules.

Burning without enough air

• With a limited air supply, combustion is incomplete.
• Not all the carbon is oxidised to carbon dioxide - some is released as carbon in fine solid particles (soot) or as carbon monoxide, making the flame smoky.
• Less energy is released than in complete combustion.

Oxidising an alcohol

• An alcohol can be oxidised to form a carboxylic acid - an organic acid whose name ends in -oic acid.
• The acid has the same number of carbon atoms as the alcohol it came from:
  • methanol is oxidised to methanoic acid
  • ethanol is oxidised to ethanoic acid
  • propan-1-ol is oxidised to propanoic acid

For a carboxylic acid to form, the -OH must sit on a carbon at the end of the chain. An alcohol with its -OH part-way along the chain, such as propan-2-ol, is still oxidised and still turns acidified potassium dichromate(VI) green, but it gives a different kind of product.

With acidified potassium dichromate(VI)

• Warming an alcohol with potassium dichromate(VI) in dilute sulfuric acid oxidises it to a carboxylic acid.
• The orange solution turns green as the dichromate oxidises the alcohol.

With the oxygen of the air

• Microorganisms in the liquid slowly oxidise ethanol to ethanoic acid using oxygen from the air.
• It's why an open bottle of wine goes sour: the ethanol becomes ethanoic acid, the acid in vinegar.

Making ethanol

Ethanol is manufactured in two different ways.

Reacting ethene with steam

Ethene (from cracking crude oil fractions) reacts with steam in an addition reaction:

ethene + steam → ethanol

• Conditions:
  • temperature of about 300 °C
  • pressure of about 60 to 70 atmospheres
  • phosphoric acid catalyst
• It's a continuous process that produces high-purity ethanol.

Fermentation

Yeast converts glucose into ethanol and carbon dioxide:

glucose → ethanol + carbon dioxide

• Conditions:
  • optimum temperature of about 30 °C (higher denatures the enzymes)
  • no air (anaerobic) (prevents oxidation to ethanoic acid)
• Fermentation produces a dilute solution of ethanol that requires distillation to concentrate.