Alcohols

Ethanol connects several parts of the organic syllabus: homologous series, fuels, fermentation, industrial manufacture, oxidation and carboxylic acids. The high-value examination skill is not memorising disconnected facts. It is keeping each route, condition and product separate.

This page contains both Core and Supplement material. Section labels show the distinction.

Alcohols and the hydroxyl group (Core)

Alcohols contain the –OH functional group. The general formula for the homologous series is CnH2n+1OH.

The first four names are methanol, ethanol, propanol and butanol. A displayed formula must show the atoms and bonds clearly, including the O–H bond.

Two stated uses of ethanol are:

• as a fuel
• as a solvent

Making ethanol by fermentation (Core)

Fermentation uses:

• aqueous glucose or another suitable sugar solution
• yeast
• a temperature of about 25-35°C
• anaerobic conditions, meaning oxygen is excluded

The equation is:

C6H12O6 → 2C2H5OH + 2CO2

The temperature must be warm enough for a useful rate but not so high that the enzymes in the yeast are denatured. Fermentation gives a dilute mixture, so the ethanol must be separated and concentrated, for example by fractional distillation.

Making ethanol from ethene (Supplement)

Ethene reacts with steam in an addition reaction:

C2H4 + H2O ⇌ C2H5OH

Conditions:

• about 300°C
• about 60 atm
• phosphoric acid catalyst

The reversible symbol is appropriate because the industrial process establishes an equilibrium.

Comparing the two manufacturing routes (Supplement)

Factor	Fermentation	Hydration of ethene
Raw material	Sugars from renewable crops	Ethene obtained from petroleum
Rate	Slow	Fast
Operation	Batch	Continuous
Product	Dilute and requires separation	High-purity ethanol can be produced continuously
Conditions	Lower temperature and pressure	Higher temperature and pressure

A comparison must mention both routes. Do not write two isolated lists and expect every comparison mark.

Complete combustion of ethanol (Core)

Ethanol burns in oxygen to form carbon dioxide and water:

C2H5OH + 3O2 → 2CO2 + 3H2O

This is complete oxidation of the carbon and hydrogen in the fuel. It does not produce ethanoic acid.

The balancing check is important: the oxygen atom already present in ethanol must be included when counting oxygen atoms.

Oxidation of ethanol to ethanoic acid (Core and Supplement depth)

Ethanol can be oxidised to ethanoic acid in two syllabus-relevant ways:

1. Heating with acidified aqueous potassium manganate(VII). The purple oxidising agent is decolourised as ethanol is oxidised.
2. Bacterial oxidation in air. This slow process explains why ethanol-containing liquids can form ethanoic acid under suitable conditions.

A simple organic representation is:

ethanol + oxygen → ethanoic acid + water

or

C2H5OH + O2 → CH3COOH + H2O

Keep this separate from complete combustion, which produces carbon dioxide and water.

Worked examination question

Ethanol is manufactured by fermentation and by hydration of ethene.

(a) State three conditions needed for fermentation. [3]

• yeast [1]
• about 25-35°C [1]
• absence of oxygen / anaerobic conditions [1]

(b) Give one advantage and one disadvantage of fermentation compared with hydration. [2]

• advantage: fermentation uses a renewable raw material [1]
• disadvantage: fermentation is slow or produces dilute ethanol that requires separation [1]

(c) Name the organic product formed when ethanol is heated with acidified aqueous potassium manganate(VII). [1]

• ethanoic acid [1]

(d) State the products of complete combustion of ethanol. [2]

• carbon dioxide [1]
• water [1]

Parts (c) and (d) test the distinction that was previously misstated on this site: oxidation to ethanoic acid and complete combustion are different reactions with different products.

Common mistakes

1. Calling combustion a route to ethanoic acid. Complete combustion gives carbon dioxide and water.
2. Mixing the manufacturing conditions. Yeast and 25-35°C belong to fermentation; phosphoric acid, 300°C and 60 atm belong to hydration.
3. Forgetting anaerobic conditions. Fermentation must occur without oxygen.
4. Giving one-sided comparisons. Use “whereas” or another explicit comparative structure.
5. Ignoring the oxygen in C2H5OH when balancing combustion. Count every atom before choosing the coefficient for O2.
6. Writing ethanoate instead of ethanoic acid. The oxidation product asked for here is the acid.

Exam-ready wording

Weak wording	More precise wording
“Yeast makes alcohol”	“Enzymes in yeast convert aqueous glucose into ethanol and carbon dioxide at about 25-35°C without oxygen”
“The factory route is faster”	“Hydration of ethene is a fast continuous process, whereas fermentation is a slow batch process”
“Ethanol burns into acid”	“Complete combustion forms carbon dioxide and water; controlled oxidation forms ethanoic acid”
“The purple reagent changes”	“Acidified aqueous potassium manganate(VII) is decolourised as ethanol is oxidised”

Test yourself

Q1. Write the balanced equation for fermentation of glucose. [2]

Show answer

C6H12O6 → 2C2H5OH + 2CO2. One mark for correct formulae and one for balancing.

Q2. Write the balanced equation for complete combustion of ethanol. [2]

Show answer

C2H5OH + 3O2 → 2CO2 + 3H2O.

Q3. State two ways ethanol can be converted into ethanoic acid. [2]

Show answer

Heat with acidified aqueous potassium manganate(VII), or allow bacterial oxidation in air.

Continue with carboxylic acids and the organic Chemistry exam-technique guide.