What are the structural differences between aldehydes and ketones in Carbonyl compounds?

Aldehydes and ketones both contain the carbonyl group (C=O), but they differ in their structure. In aldehydes, the carbonyl group is bonded to at least one hydrogen atom and is typically at the end of a carbon chain. In ketones, the carbonyl group is bonded to two carbon atoms and is located within the carbon chain. This structural difference affects their chemical properties and reactions, which is a key concept in AS-Level Organic Chemistry.

How can you distinguish between aldehydes and ketones using chemical tests?

A common method to distinguish between aldehydes and ketones is the use of Tollens' reagent and Fehling's solution. Aldehydes will react with Tollens' reagent to produce a silver mirror on the test tube, while ketones do not react. Similarly, aldehydes will reduce Fehling's solution, resulting in a red precipitate, whereas ketones will not. These tests are important for practical assessments in Cambridge International A Levels Chemistry.

What are the typical reactions involving aldehydes and ketones in AS-Level Organic Chemistry?

Aldehydes and ketones undergo several important reactions. Aldehydes can be oxidized to carboxylic acids, while ketones generally do not undergo oxidation under mild conditions. Both aldehydes and ketones can undergo nucleophilic addition reactions, such as the formation of alcohols when reacted with Grignard reagents. Understanding these reactions is crucial for mastering the Carbonyl compounds topic in Cambridge International A Levels Chemistry.

Why are aldehydes generally more reactive than ketones?

Aldehydes are generally more reactive than ketones due to steric and electronic factors. The carbonyl carbon in aldehydes is less hindered by surrounding groups, making it more accessible to nucleophiles. Additionally, aldehydes have only one electron-donating alkyl group compared to two in ketones, making the carbonyl carbon more electrophilic. This increased reactivity is a key point in understanding the behavior of carbonyl compounds in AS-Level Organic Chemistry.

What role do aldehydes and ketones play in biological systems?

Aldehydes and ketones play significant roles in biological systems. For example, glucose, an essential energy source, is an aldehyde, while acetone, a ketone, is involved in metabolic processes like ketosis. Understanding these compounds' roles in biological systems can provide insight into their importance beyond the laboratory, which is a valuable perspective in Cambridge International A Levels Chemistry.

Why is "Showing HCN (not CN⁻) attacking the carbonyl." a common mistake in Aldehydes and ketones?

Why it happens: Forgetting the KCN provides CN⁻. How to avoid it: CN⁻ is the nucleophile; H⁺ adds in the second step. Source: 9701 Examiner Reports 2022-2024

Why is "Using 2,4-DNPH to distinguish aldehyde from ketone." a common mistake in Aldehydes and ketones?

Why it happens: Both give a precipitate with it. How to avoid it: 2,4-DNPH only confirms a carbonyl; use Tollens'/Fehling's to distinguish.

Why is "Saying ketones give a positive Tollens'/Fehling's test." a common mistake in Aldehydes and ketones?

Why it happens: Treating both carbonyls the same. How to avoid it: Only aldehydes are oxidised; ketones give no change.

Why is "Saying all ketones give a positive iodoform test." a common mistake in Aldehydes and ketones?

Why it happens: Forgetting it must be a METHYL ketone. How to avoid it: Only CH₃CO– carbonyls react (e.g. propanone yes, pentan-3-one no).

Carbonyl compounds(AS-Level Organic Chemistry)Cambridge International A Level Chemistry 9701

Aldehydes and ketones

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Aldehydes and Ketones — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

How carbonyl compounds are made and reduced, the nucleophilic addition of HCN (with mechanism), and the tests that detect a carbonyl (2,4-DNPH) and distinguish aldehydes from ketones (Fehling's, Tollens', iodoform).

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

17.1.1 — Recall how aldehydes and ketones are produced: oxidation of primary alcohols (→ aldehydes) and secondary alcohols (→ ketones).
17.1.2 — Describe: (a) reduction with NaBH₄ or LiAlH₄ to alcohols; (b) reaction with HCN (KCN catalyst) to form hydroxynitriles.
17.1.3 — Describe the mechanism of the nucleophilic addition of HCN to aldehydes and ketones.
17.1.4 — Describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect carbonyl compounds.
17.1.5 — Deduce the nature (aldehyde or ketone) of an unknown carbonyl using Fehling's and Tollens' reagents.
17.1.6 — Deduce the presence of a CH₃CO– group using the iodoform reaction.

Making and reducing carbonyls

Oxidise alcohols to make them; reduce them back with NaBH₄/LiAlH₄.

Making carbonyls (oxidation with acidified K₂Cr₂O₇/KMnO₄):

Primary alcohol → aldehyde (distil off): $\text{CH}_3\text{CH}_2\text{OH} + [\text{O}] \rightarrow \text{CH}_3\text{CHO} + \text{H}_2\text{O}$ .
Secondary alcohol → ketone: $\text{CH}_3\text{CH(OH)CH}_3 + [\text{O}] \rightarrow \text{CH}_3\text{COCH}_3 + \text{H}_2\text{O}$ .

Reduction with NaBH₄ (or LiAlH₄) reverses this — it adds 2[H]:

Aldehyde → primary alcohol: $\text{CH}_3\text{CHO} + 2[\text{H}] \rightarrow \text{CH}_3\text{CH}_2\text{OH}$ .
Ketone → secondary alcohol: $\text{CH}_3\text{COCH}_3 + 2[\text{H}] \rightarrow \text{CH}_3\text{CH(OH)CH}_3$ .

1° alcohol → aldehyde; 2° alcohol → ketone (oxidation).
NaBH₄/LiAlH₄ reduces: aldehyde → 1° alcohol; ketone → 2° alcohol.
Use [O] / [H] shorthand in equations.

Nucleophilic addition of HCN

CN⁻ attacks the δ+ carbonyl carbon; the O⁻ then picks up H⁺ to give a hydroxynitrile.

The C=O bond is polar (O is more electronegative), so the carbonyl carbon is δ+ and is attacked by nucleophiles → nucleophilic addition.

With HCN and a trace of KCN catalyst (which provides CN⁻), the mechanism is:

The CN⁻ nucleophile attacks the δ+ carbonyl carbon (curly arrow from CN⁻ lone pair to C); the C=O π electrons move onto the oxygen (curly arrow to O), forming a negatively charged alkoxide intermediate.
The O⁻ then accepts a proton (H⁺) from HCN (or water), giving a hydroxynitrile and regenerating CN⁻.

$\text{CH}_3\text{CHO} + \text{HCN} \rightarrow \text{CH}_3\text{CH(OH)CN}$ (2-hydroxypropanenitrile)

CN⁻ adds to the δ+ carbon → alkoxide → protonated to a hydroxynitrile. This adds one carbon to the chain.

This reaction is useful in synthesis because it increases the carbon chain by one and introduces a new functional group.

C=O is polar → δ+ carbon attacked by nucleophiles.
CN⁻ attacks → alkoxide → protonated → hydroxynitrile.
Adds one carbon to the chain.

Tests: 2,4-DNPH, Fehling's, Tollens' and iodoform

2,4-DNPH detects any carbonyl; Fehling's/Tollens' single out aldehydes; iodoform detects CH₃CO–.

2,4-DNPH (Brady's reagent) — gives an orange/yellow precipitate with any aldehyde or ketone, confirming a C=O carbonyl group (the melting point of the purified precipitate can even identify which carbonyl).

Distinguishing aldehydes from ketones — aldehydes are easily oxidised (to carboxylic acids); ketones are not:

Fehling's solution (blue Cu²⁺): aldehyde → brick-red precipitate (Cu₂O); ketone → no change (stays blue).
Tollens' reagent (ammoniacal silver nitrate, [Ag(NH₃)₂]⁺): aldehyde → silver mirror (Ag deposited); ketone → no change.

Iodoform test — a CH₃CO– group (methyl ketone, or ethanal) gives a yellow precipitate of CHI₃ with alkaline aqueous iodine. e.g. propanone and ethanal are positive; propanal is negative.

2,4-DNPH: orange/yellow ppt → confirms a carbonyl.
Fehling's (brick-red) and Tollens' (silver mirror): aldehyde only.
Iodoform: CH₃CO– → yellow CHI₃.

How it’s examined

The nucleophilic addition mechanism and the carbonyl tests (2,4-DNPH, Tollens'/Fehling's, iodoform) are guaranteed Paper 2 marks and frequent in qualitative-analysis questions. Examiner reports flag showing HCN (not CN⁻) as the nucleophile, omitting the protonation step, and confusing which test is for the carbonyl vs which distinguishes aldehyde/ketone.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 17.1; 9701 Examiner Reports 2022-2024; 9701/22 & 9701/33 May/Jun 2024 question papers and mark schemes. Last reviewed 2026-05-20.

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Step-by-step worked examples — Aldehydes and ketones

Step-by-step solutions to past-paper-style questions on aldehydes and ketones, written exactly the way a tutor would explain them at the board.

1Confirming a carbonyl (2 marks)
Getting started• 2,4-DNPH
Question
What reagent confirms the presence of a carbonyl group, and what is observed?
Step-by-step solution
1. Step 1
  2,4-DNPH (Brady's reagent) with an aldehyde or ketone.
2. Step 2
  Orange/yellow precipitate forms.
Answer
2,4-DNPH → orange/yellow precipitate confirms a C=O group.
Examiner tip
2,4-DNPH confirms a carbonyl but does NOT distinguish aldehyde from ketone.
2Reduction of carbonyls (2 marks)
Getting started• reduction
Question
Give the product when (a) propanal and (b) propanone are reduced by NaBH₄.
Step-by-step solution
1. Step 1
  (a) Aldehyde → primary alcohol: propan-1-ol.
2. Step 2
  (b) Ketone → secondary alcohol: propan-2-ol.
Answer
(a) propan-1-ol; (b) propan-2-ol.
Examiner tip
NaBH₄ reduces C=O (aldehyde → 1° alcohol, ketone → 2° alcohol) but not C=C.
3Distinguishing aldehyde and ketone (3 marks)
Building confidence• tests
Question
Describe a chemical test to distinguish propanal from propanone, with observations.
Step-by-step solution
1. Step 1
  Add Tollens' reagent and warm (or Fehling's solution).
2. Step 2
  Propanal (aldehyde): silver mirror with Tollens' (or brick-red with Fehling's).
3. Step 3
  Propanone (ketone): no change (aldehydes oxidise, ketones don't).
Answer
Tollens'/Fehling's: aldehyde gives silver mirror/brick-red; ketone no change.
Examiner tip
Aldehydes are oxidised by these mild oxidants; ketones cannot be oxidised.
4Nucleophilic addition of HCN (4 marks)
Building confidence• mechanism
Question
Describe the mechanism for the reaction of propanone with HCN (KCN catalyst).
Step-by-step solution
1. Step 1
  CN⁻ attacks the δ+ carbonyl carbon (curly arrow from CN⁻ lone pair to C).
2. Step 2
  The C=O π electrons move onto the oxygen (curly arrow to O) → alkoxide intermediate.
3. Step 3
  The O⁻ accepts H⁺ (from HCN), regenerating CN⁻.
4. Step 4
  Product: 2-hydroxy-2-methylpropanenitrile, (CH₃)₂C(OH)CN.
Answer
CN⁻ adds to the δ+ carbon → alkoxide → protonated → hydroxynitrile.
Examiner tip
CN⁻ (not HCN) is the nucleophile; show the protonation step at the end.
5Iodoform test for carbonyls (3 marks)
Stretch• iodoform
Question
Which of ethanal, propanal and propanone give a positive iodoform test? Explain.
Step-by-step solution
1. Step 1
  Positive needs a CH₃CO– (methyl carbonyl) group.
2. Step 2
  Ethanal (CH₃CHO) and propanone (CH₃COCH₃) have it → positive (yellow CHI₃).
3. Step 3
  Propanal (CH₃CH₂CHO) does not → negative.
Answer
Ethanal and propanone give a positive iodoform test; propanal does not.
Examiner tip
Only methyl carbonyls (CH₃CO–) — and CH₃CH(OH)– alcohols — give the yellow CHI₃ precipitate.
6Identifying an unknown carbonyl (4 marks)
Stretch• deduction
Question
A compound gives an orange precipitate with 2,4-DNPH, no silver mirror with Tollens', and a yellow precipitate with alkaline iodine. Deduce the functional group and suggest a structure.
Step-by-step solution
1. Step 1
  2,4-DNPH positive → it contains a carbonyl (C=O).
2. Step 2
  Tollens' negative → it is a ketone, not an aldehyde.
3. Step 3
  Iodoform positive → it has a CH₃CO– group → a methyl ketone.
4. Step 4
  Simplest structure: propanone, CH₃COCH₃.
Answer
A methyl ketone (carbonyl, not aldehyde, with CH₃CO–) — e.g. propanone CH₃COCH₃.
Examiner tip
Combine the three tests: carbonyl present → ketone (not aldehyde) → methyl ketone.

Key Formulae — Aldehydes and ketones

The formulae you need to memorise for aldehydes and ketones on the Cambridge International A Level 9701 paper, with every variable defined in plain English and a note on when to use it.

General formula of aldehydes/ketones
$C_nH_{2n}O$
$n$
number of carbon atoms
When to use
Both aldehydes and ketones share CₙH₂ₙO; the position of the C=O distinguishes them.

Key Definitions and Keywords — Aldehydes and ketones

Definitions to memorise and the exact keywords mark schemes credit for aldehydes and ketones answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Carbonyl group
Examiner keyword
The C=O group; in aldehydes it is at the chain end (–CHO), in ketones within the chain (R–CO–R').
Nucleophilic addition
Examiner keyword
Addition to a C=O in which a nucleophile attacks the δ+ carbonyl carbon, forming an alkoxide that is then protonated.
Reduction (NaBH₄)
Examiner keyword
NaBH₄ reduces aldehydes to primary alcohols and ketones to secondary alcohols (it does not reduce C=C).
2,4-DNPH (Brady's reagent)
Examiner keyword
A reagent giving an orange/yellow precipitate with aldehydes and ketones, confirming a carbonyl group.
Tollens' / Fehling's
Examiner keyword
Mild oxidising tests for aldehydes: Tollens' gives a silver mirror; Fehling's gives a brick-red precipitate. Ketones do not react.
Iodoform test
Examiner keyword
Alkaline aqueous iodine gives yellow CHI₃ with methyl carbonyls (CH₃CO–) or CH₃CH(OH)– alcohols.

Common Mistakes and Misconceptions — Aldehydes and ketones

The traps other students keep falling into on aldehydes and ketones questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Showing HCN (not CN⁻) attacking the carbonyl.
9701 Examiner Reports 2022-2024
Why it happens
Forgetting the KCN provides CN⁻.
How to avoid it
CN⁻ is the nucleophile; H⁺ adds in the second step.
✕Using 2,4-DNPH to distinguish aldehyde from ketone.
Why it happens
Both give a precipitate with it.
How to avoid it
2,4-DNPH only confirms a carbonyl; use Tollens'/Fehling's to distinguish.
✕Saying ketones give a positive Tollens'/Fehling's test.
Why it happens
Treating both carbonyls the same.
How to avoid it
Only aldehydes are oxidised; ketones give no change.
✕Saying all ketones give a positive iodoform test.
Why it happens
Forgetting it must be a METHYL ketone.
How to avoid it
Only CH₃CO– carbonyls react (e.g. propanone yes, pentan-3-one no).

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Aldehydes and ketones — frequently asked questions

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Aldehydes and ketones

Short Study Notes in the form of Slides

Short Study Notes — Aldehydes and ketones

Detailed Notes

What you’ll learn

How it’s examined

1Confirming a carbonyl (2 marks)

2Reduction of carbonyls (2 marks)

3Distinguishing aldehyde and ketone (3 marks)

4Nucleophilic addition of HCN (4 marks)

5Iodoform test for carbonyls (3 marks)

6Identifying an unknown carbonyl (4 marks)

General formula of aldehydes/ketones

Carbonyl group

Nucleophilic addition

Reduction (NaBH₄)

2,4-DNPH (Brady's reagent)

Tollens' / Fehling's

Iodoform test

✕Showing HCN (not CN⁻) attacking the carbonyl.

✕Using 2,4-DNPH to distinguish aldehyde from ketone.

✕Saying ketones give a positive Tollens'/Fehling's test.

✕Saying all ketones give a positive iodoform test.

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Assess your understanding

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Aldehydes and ketones — frequently asked questions