3.16 - 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:
  • ethanol → ethanoic acid

Ethanoic acid is the acid in vinegar.

Two ways it happens

1. With an oxidising agent - Warming an alcohol with a chemical oxidising agent converts it to the carboxylic acid quickly.
2. With the oxygen of the air (microbial oxidation) - Microorganisms use ethanol as an energy source, oxidising it slowly to ethanoic acid. This is why a bottle of wine left open goes sour.

What alcohols are used for

As solvents

• Alcohols dissolve most substances that water dissolves, as well as substances water won't dissolve (such as oils, fats and hydrocarbons).
• Methanol and ethanol are widely used as solvents in inks, paints and cleaning fluids.

As fuels

• Alcohols are highly flammable and burn cleanly with a blue flame, releasing a good deal of energy.
• Ethanol is used as a fuel in spirit burners and as a biofuel.

In alcoholic drinks

• Ethanol is the alcohol in alcoholic drinks.
• Health risks - Excessive consumption damages organs including the liver, heart and brain, and causes alcohol poisoning.
• Social and economic costs - Alcohol misuse contributes to violent crime, healthcare costs and policing expenses.