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Chapter 23 — Chapter 23 — Carbonyl Compounds and Carboxylic Acids
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Topic 1 of 9
📘Easy summary
The structural unit C=O is the carbonyl group; aldehydes (RCHO) and ketones (RCOR) contain it. Aldehydes occur in spices, fruits and natural fragrances (benzaldehyde gives the smell of bitter almond); ketones are used in pharmaceuticals, chemicals and paints; acetone (propanone) is in nail polish remover. Methanal is a gas; its 40% aqueous solution, formalin, preserves biological specimens. Aldehydes are prepared by oxidation of primary alcohols with acidified K₂Cr₂O₇ or KMnO₄: CH₃CH₂OH + [O] → CH₃CHO + H₂O. Dichromate is added dropwise to warm ethanol and ethanal is distilled off as soon as it forms, preventing oxidation to ethanoic acid; temperature is kept above the b.p. of ethanal but below that of ethanol. Ketones are prepared from secondary alcohols: propan-2-ol + [O] → propanone + H₂O; they are not oxidised further, so need not be distilled off immediately.
⭐Key points
- 1° alcohol → aldehyde (distil off at once).
- 2° alcohol → ketone.
- Oxidant: K₂Cr₂O₇/H₂SO₄.
- Formalin = 40% methanal.
🔬Diagram
🌿Real-life example
Ethanal (b.p. 21 °C) distils off before the ethanol (b.p. 78 °C) does.
💡Exam tip
- Explain WHY ethanal is distilled off: to stop further oxidation.
✅Quick check
1. Ketones are prepared by oxidation of:
2. Formalin is an aqueous solution of:
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