# Aldehydes and Ketones

> Chemistry · CIE A-Level 9701
> Source: https://www.owlsprep.com/study/cie-9701-u15-aldehydes-and-ketones/

This module covers the structure, nomenclature, key chemical reactions, and qualitative identification of aldehydes and ketones, the core classes of carbonyl compounds for CIE A-Level. You will learn to distinguish these two functional groups for exam questions.

**Prerequisites:** [Organic functional group nomenclature](https://www.owlsprep.com/study/cie-9701-u2-functional-groups/); [Basic organic redox reactions](https://www.owlsprep.com/study/cie-9701-u11-organic-redox/)

## Learning objectives

- Distinguish between the structures of aldehydes and ketones
- Name aldehydes and ketones using IUPAC nomenclature rules
- Predict products of oxidation and reduction of carbonyl compounds
- Interpret results of qualitative tests for aldehydes and ketones

## Structure and IUPAC Nomenclature

**Carbonyl group** — A functional group consisting of a carbon atom double-bonded to an oxygen atom, with the carbon being electrophilic due to oxygen's higher electronegativity

*Notation:* C=O

*Example:* The carbonyl carbon in aldehydes bonds to at least one hydrogen atom

Aldehydes have the general formula $RCHO$, where the carbonyl group is always at the end (terminal position) of the carbon chain. Ketones have the general formula $RCOR'$, where the carbonyl carbon bonds to two alkyl/aryl groups, so it is always in an internal position on the chain.

1. Number the parent carbon chain starting from the end closest to the carbonyl group
2. For aldehydes, replace the -e suffix of the parent alkane with -al (the carbonyl is always position 1 for straight-chain aldehydes)
3. For ketones, replace the -e suffix with -one, and add the position number of the carbonyl group before the suffix

**Worked example:** Name the compound with structural formula $CH_3CH(Br)CH_2CHO$

1. 1. Count the longest carbon chain containing the carbonyl group:
2. $$\text{Total carbons} = 4 \rightarrow \text{Parent alkane} = \text{butane}$$
3. 2. The carbonyl is terminal, so suffix becomes -al, giving butanal
4. 3. The bromine substituent is located on carbon 3 (numbering from the carbonyl end)
5. Final IUPAC name: 3-bromobutanal

## Oxidation Reactions

**Oxidation of carbonyl compounds** — Aldehydes are easily oxidized by mild oxidizing agents, while ketones are resistant to mild oxidation due to the lack of a C-H bond on the carbonyl carbon

*Example:* Propanal oxidizes to propanoic acid, but propanone does not react under mild conditions

Common mild oxidizing agents in CIE exams are acidified potassium dichromate(VI), Tollens' reagent, and Fehling's solution. Ketones only oxidize under harsh conditions with strong oxidizing agents, which cleave C-C bonds to form shorter-chain carboxylic acids.

**Worked example:** Write the product of oxidation of pentanal by warm acidified potassium dichromate(VI)

1. 1. Pentanal is an aldehyde, so mild oxidation produces a carboxylic acid with the same number of carbon atoms
2. 2. The terminal -CHO group is converted to -COOH
3. $$CH_3CH_2CH_2CH_2CHO + [O] \rightarrow CH_3CH_2CH_2CH_2COOH$$
4. Product name: pentanoic acid

## Reduction Reactions

Both aldehydes and ketones are reduced by common reducing agents like sodium borohydride ($NaBH_4$) or lithium aluminium hydride ($LiAlH_4$). The reaction proceeds via nucleophilic addition of hydride ($H^-$) to the electrophilic carbonyl carbon.

- Aldehydes reduce to form **primary alcohols**
- Ketones reduce to form **secondary alcohols**

**Worked example:** Predict the product of reduction of butanone with excess $NaBH_4$

1. 1. Butanone is a ketone with carbonyl group at position 2 of a 4-carbon chain
2. 2. The C=O double bond breaks, and a hydrogen and -OH group add across the double bond
3. 3. The product is a secondary alcohol with -OH at position 2
4. $$CH_3COCH_2CH_3 + 2[H] \rightarrow CH_3CH(OH)CH_2CH_3$$
5. Product name: butan-2-ol

## Qualitative Identification Tests

Tests are used first to confirm the presence of a carbonyl group, then to distinguish between aldehydes and ketones. This is a very common short-answer exam topic.

| Test | Conditions | Aldehyde Result | Ketone Result |
| --- | --- | --- | --- |
| 2,4-DNP (Brady's) | Room temp | Yellow/orange precipitate | Yellow/orange precipitate |
| Tollens' reagent | Warm water bath | Silver mirror forms | No visible change |
| Fehling's solution | Warm water bath | Red $Cu_2O$ precipitate | No visible change |
| Acidified $K_2Cr_2O_7$ | Warm | Orange → green | No color change |

**Worked example:** An unknown compound gives a yellow precipitate with 2,4-DNP, no change with Tollens' reagent. What is the class of compound?

1. 1. A positive 2,4-DNP test confirms a carbonyl group is present
2. 2. Tollens' reagent only gives a positive result for aldehydes
3. 3. A negative Tollens' test rules out aldehydes
4. Conclusion: the unknown compound is a ketone

## Common pitfalls

- **Wrong:** Claiming 2,4-DNP distinguishes between aldehydes and ketones
  - Why it fails: 2,4-DNP reacts with all carbonyl groups, not just aldehydes
  - Correct: 2,4-DNP only confirms a C=O group exists; Tollens' or Fehling's distinguish between aldehydes and ketones
- **Wrong:** Stating ketones cannot be oxidized under any conditions
  - Why it fails: Ketones resist mild oxidation, but oxidize with strong reagents
  - Correct: State that ketones do not react with mild oxidizing agents used for distinguishing functional groups
- **Wrong:** Predicting aldehydes reduce to secondary alcohols
  - Why it fails: Mixing up reduction products for aldehydes and ketones
  - Correct: Remember: Aldehyde → primary alcohol, Ketone → secondary alcohol
- **Wrong:** Forgetting to add the position number for ketones in IUPAC naming
  - Why it fails: Unlike aldehydes, the carbonyl can be at any internal position, so the position is required
  - Correct: Always add the carbonyl position number before the -one suffix for ketones with 4+ carbons

## Cheatsheet

| Property | Aldehyde | Ketone |
| --- | --- | --- |
| General formula | RCHO | RCOR' |
| C=O position | Terminal | Internal |
| Mild oxidation product | Carboxylic acid | No reaction |
| Reduction product | Primary alcohol | Secondary alcohol |
| 2,4-DNP test | Positive precipitate | Positive precipitate |
| Tollens' test | Silver mirror | No reaction |
| IUPAC suffix | -al | -one |

## What's next

Understanding the structure and reactivity of aldehydes and ketones forms the foundation for studying nucleophilic addition reactions, a common mechanism topic in CIE A-Level Chemistry. This knowledge also supports learning about carboxylic acids and their derivatives, which share the electrophilic carbonyl group but have different reactivity patterns. Mastering identification tests for these functional groups will help you solve organic structure elucidation questions that often appear in Paper 2 and Paper 4.

- [Carboxylic Acids and Esters](https://www.owlsprep.com/study/cie-9701-u15-carboxylic-acids/)
- [Introduction to analytical chemistry](https://www.owlsprep.com/study/cie-9701-u16-overview/)
- [Mass spectrometry](https://www.owlsprep.com/study/cie-9701-u16-mass-spectrometry/)

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