Aldehydes and Ketones

Chemistry· 20 min read

1. Structure and IUPAC Nomenclature★★☆☆☆⏱ 5 min

📘 Definition

Carbonyl group

C=O

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

Example:

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

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

Number the parent carbon chain starting from the end closest to the carbonyl group
For aldehydes, replace the -e suffix of the parent alkane with -al (the carbonyl is always position 1 for straight-chain aldehydes)
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 $C H_{3} C H (B r) C H_{2} C H O$

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

2. Oxidation Reactions★★★☆☆⏱ 7 min

📘 Definition

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
1. 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

3. Reduction Reactions★★★☆☆⏱ 4 min

Both aldehydes and ketones are reduced by common reducing agents like sodium borohydride ( $N a B H_{4}$ ) or lithium aluminium hydride ( $L i A l H_{4}$ ). 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 $N a B H_{4}$

1
1. Butanone is a ketone with carbonyl group at position 2 of a 4-carbon chain
2
1. The C=O double bond breaks, and a hydrogen and -OH group add across the double bond
3
1. 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

4. Qualitative Identification Tests★★★☆☆⏱ 6 min

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 $C u_{2} O$ precipitate	No visible change
Acidified $K_{2} C r_{2} O_{7}$	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
1. Tollens' reagent only gives a positive result for aldehydes
3
1. A negative Tollens' test rules out aldehydes
4
Conclusion: the unknown compound is a ketone

5. Common Pitfalls

Wrong move:

Claiming 2,4-DNP distinguishes between aldehydes and ketones

Why:

2,4-DNP reacts with all carbonyl groups, not just aldehydes

Correct move:

2,4-DNP only confirms a C=O group exists; Tollens' or Fehling's distinguish between aldehydes and ketones

Wrong move:

Stating ketones cannot be oxidized under any conditions

Why:

Ketones resist mild oxidation, but oxidize with strong reagents

Correct move:

State that ketones do not react with mild oxidizing agents used for distinguishing functional groups

Wrong move:

Predicting aldehydes reduce to secondary alcohols

Why:

Mixing up reduction products for aldehydes and ketones

Correct move:

Remember: Aldehyde → primary alcohol, Ketone → secondary alcohol

Wrong move:

Forgetting to add the position number for ketones in IUPAC naming

Why:

Unlike aldehydes, the carbonyl can be at any internal position, so the position is required

Correct move:

Always add the carbonyl position number before the -one suffix for ketones with 4+ carbons

6. Quick Reference 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

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
Carbonyl structure identification
2023 · 4
Reaction product prediction
2021 · 1
Test result interpretation

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.