5.5 - Alcohols

The alcohols

• An alcohol is a compound that carries an -OH group, the hydroxyl group, joined to a carbon atom.
• That group is the alcohols' functional group - the reactive part that decides how the compound behaves.
• The stem of the name shows how many carbon atoms are in the chain, and the ending -ol shows it's an alcohol.

The general formula

Every saturated, straight-chain alcohol fits the pattern:

C_nH_2n+1OH

n is the number of carbon atoms:

• n = 1: CH₃OH, methanol
• n = 2: C₂H₅OH, ethanol
• n = 3: C₃H₇OH, propanol
• n = 4: C₄H₉OH, butanol

The number in the name

• The position of the -OH group along the carbon chain can vary:
  • In propan-1-ol, the -OH is on the first carbon, at the end of the chain.
  • In propan-2-ol, it's on the middle carbon.
• The chain is numbered from the end that gives the lower number.

Methanol, ethanol, propan-1-ol, propan-2-ol and butanol are all liquids at room temperature and pressure.

Formulae keep the -OH visible on purpose, which is why ethanol is written C₂H₅OH, never C₂H₆O. The drawn form is the full structural formula; the condensed form along the chain is the shortened structural formula.

Trends along the series

• Mixing with water - Methanol, ethanol and propanol are miscible with water (they mix in any proportion). From butanol onwards, solubility falls off as the carbon chain gets longer.
• Melting and boiling points - Both rise as the molecules get bigger, because the forces between larger molecules are stronger, needing more energy to pull them apart.

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.

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.