Unit Overview

Carbonyl compounds and carboxylic acids

CIE A-Level Chemistry· 5 min read 📊 n/a

1. Unit at a Glance

This unit follows a logical learning sequence: we first cover carbonyl compounds (aldehydes and ketones, which share the reactive C=O double bond core) before moving to the structurally related but chemically distinct carboxylic acid functional group.

A core unifying theme is understanding how changes to the groups bonded to the carbonyl carbon modify reactivity: aldehydes are more reactive than ketones towards nucleophiles, while the hydroxyl group in carboxylic acids completely changes the reaction profile.

This unit contains two core sub-topics:

Aldehydes and ketones

Covers nomenclature, structure, oxidation, reduction, nucleophilic addition and chemical identification tests for aldehydes and ketones.

★★★⏱ 20 min

Carboxylic acids

Explores acidity, structure, preparation, and key reactions of carboxylic acids and simple carboxylate salts.

★★★⏱ 18 min

2. Common Pitfalls

Wrong move:

Confusing the results of Fehling's and Tollens' tests for aldehydes vs ketones

Why:

Only aldehydes are easily oxidised under mild test conditions, leading to different outcomes

Correct move:

Remember: both tests give positive results (silver mirror for Tollens', red precipitate for Fehling's) only for aldehydes; ketones give no visible change

Wrong move:

Assume carboxylic acids react via nucleophilic addition to C=O like aldehydes/ketones

Why:

The adjacent hydroxyl group changes reactivity through resonance stabilisation of the carboxyl group

Correct move:

Most carboxylic acid reactions involve O-H bond cleavage (acid-base reactions) or substitution of the hydroxyl group, not addition across the C=O double bond

3. Quick Reference Cheatsheet

Concept / Test	Key Rule / Formula
Oxidation of aldehydes	$R C H O acidified K_{2} C r_{2} O_{7}, Δ R C O O H$
Nucleophilic addition to carbonyls	Mechanism: nucleophile attacks the partially positive carbonyl carbon
Tollens' test outcome	Aldehyde: silver mirror; Ketone: no change
Fehling's test outcome	Aldehyde: brick-red precipitate; Ketone: no change
Carboxylic acid acidity	$p K_{a} \approx 4 - 5$ , more acidic than alcohols/phenols due to resonance-stabilised carboxylate ion

What's Next

Begin this unit by learning about aldehydes and ketones, the first core sub-topic. After completing both sub-topics in this unit, you will progress to study acyl derivatives, the next class of oxygen-containing organic compounds in the CIE 9701 syllabus.