# Carbonyl compounds and carboxylic acids

> CIE A-Level Chemistry · 9701
> Source: https://www.owlsprep.com/study/cie-9701-u15-overview/
> Weight: n/a

This unit explores the structure, reactivity and identification of aldehydes, ketones and carboxylic acids, core oxygen-containing functional groups central to organic synthesis and biological chemistry.

**Prerequisites:** [Basic organic functional group nomenclature](https://www.owlsprep.com/study/cie-9701-u11-intro-organic/); [Reactions of alcohol functional groups](https://www.owlsprep.com/study/cie-9701-u14-alcohols/)

## Learning objectives

- Distinguish between the structures and chemical properties of aldehydes, ketones and carboxylic acids
- Predict products of common reactions of carbonyl and carboxyl functional groups
- Interpret chemical test results to identify carbonyl-containing organic compounds

## 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](https://www.owlsprep.com/study/cie-9701-u15-aldehydes-and-ketones/) — Covers nomenclature, structure, oxidation, reduction, nucleophilic addition and chemical identification tests for aldehydes and ketones.
- [Carboxylic acids](https://www.owlsprep.com/study/cie-9701-u15-carboxylic-acids/) — Explores acidity, structure, preparation, and key reactions of carboxylic acids and simple carboxylate salts.

## Common pitfalls

- **Wrong:** Confusing the results of Fehling's and Tollens' tests for aldehydes vs ketones
  - Why it fails: Only aldehydes are easily oxidised under mild test conditions, leading to different outcomes
  - Correct: Remember: both tests give positive results (silver mirror for Tollens', red precipitate for Fehling's) only for aldehydes; ketones give no visible change
- **Wrong:** Assume carboxylic acids react via nucleophilic addition to C=O like aldehydes/ketones
  - Why it fails: The adjacent hydroxyl group changes reactivity through resonance stabilisation of the carboxyl group
  - Correct: 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

## Cheatsheet

| Concept / Test | Key Rule / Formula |
| --- | --- |
| Oxidation of aldehydes | $RCHO \xrightarrow{\text{acidified } K_2Cr_2O_7, \Delta} RCOOH$ |
| 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 | $pK_a \approx 4-5$, 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.

- [Aldehydes and ketones](https://www.owlsprep.com/study/cie-9701-u15-aldehydes-and-ketones/)
- [Carboxylic acids](https://www.owlsprep.com/study/cie-9701-u15-carboxylic-acids/)
- [Introduction to analytical chemistry](https://www.owlsprep.com/study/cie-9701-u16-overview/)

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