4.5 AlcoholsEdexcel IGCSE Chemistry: Revision notes

Alcohols are a homologous series containing the functional group -OH. The first four members required for this course are:

In the displayed formula, the -OH group is drawn on the last carbon of an unbranched chain, with an O-H bond shown explicitly.

Ethanol can be oxidised to ethanoic acid in three different ways:

1. Burning in air or oxygen — complete combustion produces carbon dioxide and water, releasing heat
2. Microbial oxidation — bacteria in the air oxidise ethanol using oxygen to form ethanoic acid (this is how wine turns to vinegar if left open)
3. Heating with potassium dichromate(VI) in dilute sulfuric acid — this oxidising mixture converts ethanol to ethanoic acid in the laboratory, and the orange dichromate(VI) solution turns green as it is reduced

Ethanol can be manufactured by two methods:

1. Reacting ethene with steam: ethene + steam ⇌ ethanol, using a phosphoric acid catalyst at about 300°C and 60-70 atm pressure. This is a continuous, fast industrial process but uses a non-renewable feedstock (ethene from crude oil) and requires high-energy conditions.
2. Fermentation of glucose: glucose → ethanol + carbon dioxide, using enzymes in yeast at around 30°C. This is a batch process, slower, but uses a renewable feedstock (sugar/glucose from plants) and needs only mild conditions.

Fermentation must be carried out without air (anaerobically) because if oxygen is present, the yeast enzymes and any airborne bacteria will oxidise the ethanol produced into ethanoic acid instead, spoiling the product.

Fermentation is carried out at an optimum temperature (around 30°C) because:

• too low a temperature makes the enzyme-catalysed reaction too slow
• too high a temperature denatures the enzymes in the yeast, stopping the reaction