Alcohols (AQA GCSE Chemistry)

The Alcohol Functional Group

• All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions

A molecule of ethanol

Diagram of the side chain and -OH group in ethanol which characterises its chemistry

The first four alcohols

• The names and structures of the first four alcohols in the homologous series are shown below

• In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘ol’

Table to show the name, formula and displayed formula of the first four alcohols

Name	Formula	Displayed formula
Methanol	CH3OH
Ethanol	C2H5OH
Propanol	C3H7OH
Butanol	C4H9OH

Table showing the formulae and structures of the first four alcohols

Production of ethanol by fermentation

Fermentation

• Ethanol (C₂H₅OH) is one of the most important alcohols

  • It is the type of alcohol found in alcoholic drinks such as wine and beer

  • It is also used as fuel for cars and as a solvent

• It can be produced by fermentation where sugar or starch is dissolved in water and yeast is added

• The mixture is then fermented between 15 and 35 °C with the absence of oxygen for a few days

• Yeast contains enzymes that break down sugar to glucose

• If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured

• The yeast respire anaerobically using the glucose to form ethanol and carbon dioxide:

C₆H₁₂O₆ + Enzymes → 2CO₂ + 2C₂H₅OH

• The yeast are killed off once the concentration of alcohol reaches around 15%, hence the reaction vessel is emptied and the process is started again

• This is the reason that ethanol production by fermentation is a batch process

Reactions of alcohols

Uses of alcohols

• Alcohols are colourless liquids that dissolve in water to form neutral solutions

• The first four alcohols are commonly used as fuels

• School laboratories use ethanol in spirit burners as it burns cleanly and without strong odours

• Methanol and ethanol are also used extensively as solvents

• This is because they can dissolve many substances that water cannot such as fats and oils, but can also dissolve most of the substances that water can

Combustion of alcohols

• Alcohols undergo combustion to form carbon dioxide and water

• The complete combustion of ethanol is as follows:

CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O

Reaction with sodium

• Alcohols react with sodium metal to produce hydrogen gas and a metal salt

• The word equation for the reaction of methanol with sodium is:

sodium + methanol → sodium methoxide + hydrogen

Oxidation of alcohols

• Alcohols undergo oxidation to produce carboxylic acids, an organic acid

• This is what happens to wine when it is left open as the microbial oxidation of ethanol will produce a weak solution of a carboxylic acid called ethanoic acid, the same acid used in vinegar

• Bacteria in the air (acetobacter) use atmospheric oxygen from air to oxidise the ethanol in the wine:

ethanol + oxygen → ethanoic acid + water

• The acidic, vinegary taste of wine which has been left open for several days is due to the presence of ethanoic acid