7.7 - 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 react

The first four alcohols behave similarly because they share the same -OH functional group.

They burn

Alcohols are flammable. With plenty of air an alcohol burns completely: the carbon is oxidised to carbon dioxide and the hydrogen to water.

ethanol + oxygen → carbon dioxide + water

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

When balancing the equation, remember that the alcohol already contains an oxygen atom that must be counted on the left.

They dissolve in water

Methanol, ethanol, propanol and butanol all dissolve in water to form a neutral solution (pH 7).

They react with sodium

• Drop a small piece of sodium into an alcohol and it fizzes, releasing hydrogen gas.
• It's the -OH group that reacts - the hydrogen atom on the end of it is displaced.

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.

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.