AS · Practice questions · Oxidation of alcohols

Watch for green.

Six original Cambridge-style questions on the oxidation of alcohols.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Primary to aldehyde to acid; secondary to ketone.

K₂Cr₂O₇/H⁺: orange to green.
Distil for aldehyde; reflux for acid.
Tertiary: no reaction.
CHI₃ (yellow) for CH₃CH(OH)– and CH₃CO–.

Recall

[3 marks]

Give the organic product, if any, when each is heated with acidified K₂Cr₂O₇: (a) butan-2-ol, (b) 2-methylpropan-2-ol, (c) propan-1-ol under reflux.

(a) butanone. ✓
(b) no reaction (tertiary). ✓
(c) propanoic acid. ✓

Explain

[3 marks]

Explain how the apparatus is chosen to make ethanal rather than ethanoic acid from ethanol, and why it works.

Use distillation apparatus and add the oxidant slowly (ethanol in excess). ✓
Ethanal (b.p. 21 °C) boils off as soon as it forms. ✓
It is removed from the oxidising agent before it can be oxidised to ethanoic acid. ✓

Recall

[2 marks]

Write equations using [O] for the oxidation of ethanol to (a) ethanal and (b) ethanoic acid.

(a) CH₃CH₂OH + [O] → CH₃CHO + H₂O. ✓
(b) CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. ✓

Analysis

[3 marks]

Describe tests using acidified dichromate(VI) and alkaline iodine that would distinguish butan-1-ol, butan-2-ol and 2-methylpropan-2-ol.

Acidified dichromate: butan-1-ol and butan-2-ol turn it green; 2-methylpropan-2-ol stays orange. ✓
Alkaline iodine: only butan-2-ol gives a pale yellow precipitate (CHI₃). ✓
So butan-1-ol: green, no precipitate; butan-2-ol: green, precipitate; tertiary: no change in either. ✓

Explain

[2 marks]

Explain why tertiary alcohols are not oxidised by acidified dichromate(VI).

Oxidation removes the H from the O–H and an H from the carbon bearing the OH. ✓
In a tertiary alcohol that carbon has no hydrogen atom, so it cannot be oxidised without breaking C–C bonds. ✓

Analysis

[2 marks]

Which of these give a positive tri-iodomethane test: ethanol, methanol, pentan-3-ol, pentan-2-ol? Explain.

Ethanol and pentan-2-ol: both contain CH₃CH(OH)–. ✓
Methanol and pentan-3-ol lack a CH₃ group attached to the CH(OH) carbon, so are negative. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.