Alcohols can be made from alkenes or from sugar, and an alcohol can be oxidised to a carboxylic acid. Questions ask you to name the product, write the equation and choose a route. The skill belongs to organic reactions, and the syllabus wording decides how far you need to go.
How are ethanol and ethanoic acid connected?
Ethanol, C₂H₅OH, can be made in two ways. In fermentation, yeast converts glucose to ethanol and carbon dioxide in the absence of air. In hydration, ethene reacts with steam, as seen in the addition lesson.
Ethanol can then be oxidised. Exposed to a suitable oxidising agent, or left open to air with microbes, it changes to ethanoic acid, CH₃COOH. That is how wine turns to vinegar.
Ethanoic acid is a weak acid. It still shows the usual acid behaviour, for example releasing carbon dioxide from a carbonate.
Key equations
- Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
- Hydration: C₂H₄ + H₂O → C₂H₅OH
- Oxidation: C₂H₅OH + O₂ → CH₃COOH + H₂O
- Acid with a carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
Worked example
Use 180 g of glucose in fermentation. What masses of ethanol and carbon dioxide form, assuming complete conversion?
Step 1, equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Check the atoms: left 6 C, 12 H, 6 O. Right 4 + 2 = 6 C, 12 H, 2 + 4 = 6 O. Balanced.
Step 2, molar masses: glucose is 6(12) + 12(1) + 6(16) = 180. Ethanol is 2(12) + 6(1) + 16 = 46. Carbon dioxide is 44.
Step 3, amount of glucose: 180 ÷ 180 = 1.0 mol.
Step 4, use the ratio 1 : 2 : 2: 2.0 mol of ethanol and 2.0 mol of carbon dioxide.
Step 5, masses: ethanol 2.0 × 46 = 92 g. Carbon dioxide 2.0 × 44 = 88 g.
Check: 92 + 88 = 180 g, the same as the glucose. Mass is conserved.
Real fermentation gives less than this, because it is not fully complete. This is an ideal calculation.
The mistake to watch for
A common slip is to confuse the oxidation of ethanol with its combustion.
Mistaken answer: ethanol is oxidised to carbon dioxide and water.
That describes burning. In the controlled oxidation in this lesson, the product is ethanoic acid and water.
The correction is to read the question for the conditions. Complete burning gives carbon dioxide and water. Controlled oxidation of the alcohol group gives a carboxylic acid with the same number of carbon atoms.
Check yourself
1. Name the acid formed by oxidising propan-1-ol and write the equation with oxygen.
Show answer
Propanoic acid. C₃H₇OH + O₂ → C₂H₅COOH + H₂O.
Check: 3 C, 8 H, 3 O on each side.
2. Give one advantage each of fermentation and of hydration of ethene for making ethanol.
Show answer
Fermentation: it uses a renewable starting material (sugar). Hydration: it gives a purer product faster and in one continuous process. Either advantage with the reason earns the point.
3. A student adds ethanoic acid to sodium carbonate and sees bubbling. Which gas forms, and what does it show about the acid?
Show answer
Carbon dioxide forms. It shows that ethanoic acid behaves as an acid, reacting with a carbonate to give a salt, water and carbon dioxide.
Where this leads next
The alcohol and the acid join in the next lesson, interpreting an ester-forming relationship. The mole and equation-ratio tutor gives more practice with calculations like the fermentation one above.
If you can follow each step but lose the thread in an unfamiliar chain, our teachers can rebuild it with you in online one-to-one Chemistry tuition.