13.7 - 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.

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 by fermentation

Fermentation is how ethanol is produced for alcoholic drinks.

What is mixed, and what comes out

• A solution of sugar (from a crop such as sugar cane) is mixed with yeast in a sealed container and left somewhere warm.
• Enzymes in the yeast convert the sugar into ethanol and carbon dioxide:
  • sugar → ethanol + carbon dioxide

The conditions, and why each is needed

• Warm, about 30 to 40 °C - The enzymes work fastest in this range. Higher temperatures denature the enzymes and stop the reaction; lower temperatures slow it down.
• Slightly acidic - Provides the optimum pH for the enzymes.
• No air (anaerobic) - If air gets in, the ethanol is oxidised to ethanoic acid (vinegar).

Concentrating the product

• Fermentation stops at around 12 to 15% ethanol, because the alcohol kills the yeast.
• To produce higher concentrations, the mixture is separated by fractional distillation.