11.7 - 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

With an oxidising agent

Warming ethanol with acidified aqueous potassium manganate(VII) converts the ethanol to ethanoic acid.

For a carboxylic acid to form, the -OH must sit on a carbon at the end of the chain.

With the oxygen of the air

• Bacteria in the liquid use ethanol as an energy source and take oxygen from the air to oxidise it to ethanoic acid.
• It's why an open bottle of wine goes sour, and how vinegar is produced.

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.

Making ethanol

Ethanol is manufactured in two different ways.

Adding steam to ethene

Ethene (from cracking petroleum fractions) reacts with steam:

ethene + steam → ethanol

• Conditions:
  • temperature of about 300 °C
  • pressure of 6,000 kPa (about 60 atmospheres)
  • an acid catalyst
• It's a continuous process that produces pure ethanol.

Fermentation

Yeast cells convert glucose into ethanol and carbon dioxide:

glucose → ethanol + carbon dioxide

• Conditions:
  • temperature of 25 to 35 °C (higher denatures the enzymes)
  • no oxygen (anaerobic) (prevents oxidation to ethanoic acid)
• Fermentation produces a dilute solution of ethanol that is distilled afterwards to concentrate it.

Choosing between the two routes

Ethanol is manufactured either by fermentation or by the catalytic addition of steam to ethene.

Adding steam to ethene

• In its favour - The reaction is fast and runs continuously, producing close to pure ethanol.
• Against it - Ethene comes from non-renewable petroleum, and reaching the high temperature and pressure takes a great deal of energy.

Fermentation

• In its favour - The raw material is renewable sugar cane, and the conditions are mild (low energy cost and simple equipment).
• Against it - It's slow and runs as a batch process. It produces a dilute solution that must be distilled and purified.