Alcohols

Alcohols are a homologous series with the –OH group: methanol, ethanol, propanol and butanol. They burn to give carbon dioxide and water, react with sodium to give hydrogen, and are oxidised to carboxylic acids. Ethanol, the alcohol in drinks, is made by fermenting sugar with yeast, warm and without air.

Key facts

  • Alcohols contain –OH: methanol CH3OH, ethanol C2H5OH, propanol C3H7OH, butanol C4H9OH.
  • Alcohols burn to give carbon dioxide and water, react with sodium to give hydrogen, and are oxidised to carboxylic acids.
  • Fermentation: yeast turns sugar into ethanol and carbon dioxide, warm (about 30 °C) and without air: C6H12O6 → 2C2H5OH + 2CO2.

Worked examples

Example 1

What is the functional group of the alcohols?

  1. All alcohols contain the –OH group.
  2. It is what gives them their reactions.

Answer: –OH

Example 2

A small piece of sodium is added to ethanol. Which gas bubbles off?

  1. Sodium reacts with alcohols, more slowly than with water.
  2. Hydrogen gas is given off, and a salt (an alkoxide) forms.

Answer: hydrogen

Example 3

Ethanol is oxidised (for example by acidified potassium dichromate, or by microbes in the air). What is made?

  1. Ethanol has 2 carbon atoms.
  2. Oxidation turns the –CH₂OH end into –COOH.
  3. So ethanoic acid is made.

Answer: ethanoic acid

Example 4

In the complete combustion of one molecule of ethanol, chem C2H5OH, how many oxygen molecules are needed?

  1. Products: 2 CO₂ and 3 H₂O.
  2. Oxygen atoms needed: 4 + 3 = 7; the alcohol already has 1.
  3. 7 − 1 = 6 atoms, so 3 O₂ molecules.

Answer: 3

Example 5

What conditions are used to make ethanol by fermentation?

  1. Yeast ferments sugar into ethanol and carbon dioxide.
  2. It works best warm (about 30–35 °C), and without oxygen, so ethanoic acid does not form.
  3. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Answer: sugar solution with yeast, warm (about 30 °C), without air

Example 6

Write the word and symbol equations for making ethanol by fermentation, and give the conditions.

  1. Enzymes in yeast break glucose down into ethanol and carbon dioxide.
  2. C6H12O6 → 2C2H5OH + 2CO2
  3. Conditions: yeast, a warm temperature (about 35 °C) and no air.

Answer: glucose → ethanol + carbon dioxide; C6H12O6 → 2C2H5OH + 2CO2; yeast, about 35 °C, no air

Example 7

Why is fermentation carried out at about 35 °C, and not at 20 °C or 60 °C?

  1. Fermentation depends on enzymes in the yeast. Like most reactions, it is faster when warmer, so at 20 °C it is slow.
  2. But enzymes are proteins: too hot and their shape changes (they are denatured). At 60 °C fermentation stops.
  3. About 35 °C is a compromise: fast, but the enzymes still work.

Answer: at 20 °C the enzymes in the yeast work slowly; at 60 °C they are denatured and stop working; about 35 °C gives a good rate

Example 8

Why must air be kept out of a fermentation vessel?

  1. Fermentation is anaerobic: without oxygen, yeast makes ethanol.
  2. With oxygen, yeast makes carbon dioxide and water instead, and bacteria could turn the ethanol into ethanoic acid, which tastes of vinegar.

Answer: with oxygen the yeast respires aerobically, making carbon dioxide and water instead of ethanol; also, microbes in the air would oxidise the ethanol to ethanoic acid (vinegar)

Example 9

180 g of glucose is fermented: C6H12O6 → 2C2H5OH + 2CO2. What is the maximum mass of ethanol that can form? (Aᵣ: C = 12, H = 1, O = 16)

  1. Mᵣ of glucose. 6 × 12 + 12 × 1 + 6 × 16 = 180; 180 ÷ 180 = 1 mol
  2. 1 mol of glucose gives 2 mol of ethanol (Mᵣ = 46). 2 × 12 + 6 × 1 + 16 = 46
  3. Mass of ethanol. 2 × 46 = 92 g

Answer: 92 g