Aldehydes and ketones

Q: What is the structural difference between an aldehyde and a ketone?

An aldehyde has its carbonyl group (\(C=O\)) at the end of the carbon chain (\(RCHO\)), whereas a ketone has its carbonyl group in the middle between two carbon atoms (\(RCOR'\)).

Q: Why is the carbonyl group polar and susceptible to nucleophilic attack?

Oxygen is significantly more electronegative than carbon, drawing electron density towards itself and creating a permanent dipole (\(C^{\delta+} = O^{\delta-}\)) where the \(C^{\delta+}\) atom attracts electron-rich nucleophiles.

Q: How do the boiling points of aldehydes and ketones compare to alkanes and alcohols of similar molecular mass?

Aldehydes and ketones have higher boiling points than alkanes due to permanent dipole–dipole attractions, but lower boiling points than alcohols because they cannot form intermolecular hydrogen bonds with themselves.

Q: Why are short-chain aldehydes and ketones soluble in water?

They can form hydrogen bonds with water molecules using the lone pairs of electrons on the carbonyl oxygen atom (\(O^{\delta-}\)) and the partially positive hydrogen atoms (\(H^{\delta+}\)) of water.

Q: How can aldehydes and ketones be distinguished using Tollens' reagent and Fehling's solution?

Aldehydes are easily oxidized, forming a silver mirror with Tollens' reagent and a brick-red precipitate with Fehling's solution, whereas ketones do not react with either.

Q: What are the reduction products when aldehydes and ketones react with \(NaBH_4\)?

Aldehydes are reduced to primary (\(1^\circ\)) alcohols, while ketones are reduced to secondary (\(2^\circ\)) alcohols.

Q: What type of reaction occurs when hydrogen cyanide (\(HCN\)) reacts with an aldehyde or ketone, and what is formed?

It is a nucleophilic addition reaction that produces a hydroxynitrile (cyanohydrin) and increases the carbon chain length by one carbon atom.

Q: Why does the nucleophilic addition of \(HCN\) to an unsymmetrical carbonyl compound produce an optically inactive racemic mixture?

The planar carbonyl group (\(C=O\)) can be attacked by the \(:CN^-\) nucleophile with equal probability from above or below the plane, producing an equimolar (50:50) mixture of enantiomers.

Q: How is Brady's reagent (2,4-DNP) used to detect and identify specific aldehydes and ketones?

2,4-DNP forms a bright orange/yellow precipitate with any aldehyde or ketone; purifying the derivative and determining its melting point allows precise identification by comparison with data tables.

Q: What structural feature is detected by the triiodomethane (iodoform) reaction, and what is the positive observation?

It detects a methyl carbonyl group (\(CH_3C=O\)), producing a pale yellow precipitate of triiodomethane (\(CHI_3\)) with an antiseptic smell.

Chemistry 1110 · CCEA A Level · 9 min read · Updated: 31 Aug 2026

Introduction to Aldehydes and Ketones

Welcome to one of the most exciting and essential areas of organic chemistry! In this chapter, we explore carbonyl compounds—specifically aldehydes and ketones. You encounter these molecules every day: the scent of vanilla (vanillin) and cinnamon (cinnamaldehyde) come from aldehydes, while acetone (propanone), the main ingredient in nail polish remover, is a ketone.

Don't worry if organic chemistry has felt tricky in the past. We will break down every mechanism, test, and concept step-by-step so you can master this topic with confidence for your CCEA A2 examinations.

1. Structure, Bonding, and Nomenclature

The Carbonyl Functional Group

Both aldehydes and ketones contain the carbonyl group, which is a carbon atom joined to an oxygen atom by a double bond, written as \(C=O\).

• Aldehydes have the carbonyl group at the end of a carbon chain. The carbonyl carbon is bonded to at least one hydrogen atom. Their general formula is written as \(RCHO\).
• Ketones have the carbonyl group in the middle of a carbon chain. The carbonyl carbon is bonded to two other carbon atoms. Their general formula is written as \(RCOR'\).

Memory Tip: Think of Aldehydes as being At the end, and Ketones as being in the Kentre (centre)!

Polarity in the Carbonyl Group

Oxygen is much more electronegative than carbon. This pulls the electron density in the double bond toward the oxygen atom, creating a permanent dipole:

\(C^{\delta+} = O^{\delta-}\)

This dipole is the secret to all the chemical reactions of carbonyls: the slightly positive carbon atom (\(C^{\delta+}\)) is vulnerable to attack by electron-rich species called nucleophiles.

Naming Aldehydes and Ketones

• Aldehydes end with the suffix -al. We always count the carbonyl carbon as carbon-1.
Examples: \(HCHO\) is methanal, \(CH_3CHO\) is ethanal, \(CH_3CH_2CHO\) is propanal.
• Ketones end with the suffix -one. For chains of five or more carbons (and sometimes four), we must specify the position of the \(C=O\) group.
Examples: \(CH_3COCH_3\) is propanone, \(CH_3COCH_2CH_3\) is butanone, \(CH_3COCH_2CH_2CH_3\) is pentan-2-one.

Common Mistake to Avoid: When writing aldehyde formulas, write \(CHO\) and never \(COH\). Writing \(COH\) looks like an alcohol!

Key Takeaway: Aldehydes have the \(C=O\) on an end carbon (\(RCHO\)), whereas ketones have the \(C=O\) between two carbon atoms (\(RCOR'\)). The \(C=O\) bond is polar (\(C^{\delta+} = O^{\delta-}\)), making the carbon atom electrophilic.

2. Physical Properties

Boiling Points

Let's compare carbonyl compounds to alkanes and alcohols of similar molecular mass:

1. Alkanes: Non-polar molecules held together only by weak London dispersion forces (van der Waals forces). They have the lowest boiling points.
2. Aldehydes and Ketones: Polar molecules that experience permanent dipole–dipole attractions alongside London forces. They have higher boiling points than alkanes.
3. Alcohols: Contain \(-OH\) groups and can form strong intermolecular hydrogen bonds. They have significantly higher boiling points than aldehydes and ketones.

Boiling Point Trend:
\(\text{Alkanes} < \text{Aldehydes / Ketones} < \text{Alcohols}\)

Solubility in Water

Small aldehydes and ketones (up to about \(4\) carbons, such as methanal, ethanal, and propanone) are completely miscible in water.

Why? Although aldehydes and ketones cannot hydrogen-bond to each other, they can form hydrogen bonds with water molecules. The lone pairs of electrons on the carbonyl oxygen atom (\(O^{\delta-}\)) form hydrogen bonds with the partially positive hydrogen atoms (\(H^{\delta+}\)) of water molecules.

As the hydrocarbon chain length increases, solubility decreases rapidly because the non-polar hydrocarbon tail disrupts the hydrogen bonding network of water.

Key Takeaway: Carbonyls have higher boiling points than alkanes (due to permanent dipole-dipole forces) but lower than alcohols (no intermolecular hydrogen bonding between carbonyl molecules). Small carbonyls dissolve well in water because they form hydrogen bonds with water molecules.

3. Formation and Oxidation Reactions

Formation from Alcohols

Recall how we make aldehydes and ketones using the oxidizing agent acidified potassium dichromate(VI), \(K_2Cr_2O_7 / H_2SO_4\):

• Primary alcohol \(\to\) Aldehyde (by distillation to prevent further oxidation) \(\to\) Carboxylic acid (under reflux with excess oxidant).
• Secondary alcohol \(\to\) Ketone (under reflux; cannot be oxidized further under mild conditions).

Oxidation Differences: Distinguishing Aldehydes and Ketones

Because aldehydes have a hydrogen atom attached to the carbonyl carbon, they can be easily oxidized to carboxylic acids. Ketones do not have this hydrogen, so they resist oxidation. We use this fundamental difference to distinguish between them.

Test 1: Acidified Potassium Dichromate(VI) (\(K_2Cr_2O_7 / H^+\))

• Aldehyde: Orange solution turns green (as \(Cr_2O_7^{2-}\) is reduced to \(Cr^{3+}\)).
• Ketone: Solution remains orange (no reaction).

Test 2: Tollens' Reagent (Ammoniacal Silver Nitrate)

Tollens' reagent contains the diamminesilver(I) complex ion, \([Ag(NH_3)_2]^+\). It acts as a mild oxidizing agent.

• Aldehyde: The aldehyde is oxidized to a carboxylate ion, and \(Ag^+\) ions are reduced to metallic silver. Observation: A silver mirror forms on the inner walls of the test tube.
Equation: \(RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \to RCOO^- + 2Ag(s) + 4NH_3 + 2H_2O\)
• Ketone: No reaction (solution remains colorless).

Test 3: Fehling's Solution

Fehling's solution contains copper(II) complex ions (\(Cu^{2+}\)) in an alkaline solution (deep blue color).

• Aldehyde: The blue \(Cu^{2+}\) is reduced to copper(I) oxide (\(Cu_2O\)). Observation: The blue solution forms a brick-red precipitate.
Equation: \(RCHO + 2Cu^{2+} + 5OH^- \to RCOO^- + Cu_2O(s) + 3H_2O\)
• Ketone: No reaction (solution remains blue).

Key Takeaway: Aldehydes are easily oxidized to carboxylic acids, giving positive results with Tollens' (silver mirror), Fehling's (brick-red precipitate), and acidified dichromate (orange to green). Ketones do not react.

4. Reduction Reactions

Reduction is the reverse of oxidation. We can reduce aldehydes and ketones back to alcohols using sodium borohydride, \(NaBH_4\), in aqueous or alcoholic solution.

The Products of Reduction

• Aldehyde reduced with \(NaBH_4\) \(\to\) Primary (\(1^\circ\)) Alcohol
\(RCHO + 2[H] \to RCH_2OH\)
Example: \(CH_3CHO + 2[H] \to CH_3CH_2OH\) (Ethanal \(\to\) Ethanol)

• Ketone reduced with \(NaBH_4\) \(\to\) Secondary (\(2^\circ\)) Alcohol
\(RCOR' + 2[H] \to RCH(OH)R'\)
Example: \(CH_3COCH_3 + 2[H] \to CH_3CH(OH)CH_3\) (Propanone \(\to\) Propan-2-ol)

Note: In balanced chemical equations, the reducing agent is represented simply as \([H]\).

Key Takeaway: \(NaBH_4\) reduces aldehydes to primary alcohols and ketones to secondary alcohols.

5. Nucleophilic Addition Reactions

Mechanism: Why Nucleophilic Addition Occurs

Because the carbonyl carbon is electron-deficient (\(C^{\delta+}\)), it attracts nucleophiles (species that donate an electron pair). The double bond breaks, adding the attacking group to the carbon and converting the \(C=O\) into a single \(C-O^-\) bond, which then picks up a proton (\(H^+\)).

Addition of Hydrogen Cyanide (\(HCN\))

The reaction of carbonyl compounds with hydrogen cyanide produces hydroxynitriles (also called cyanohydrins).

• Reagents: Potassium cyanide (\(KCN\)) and dilute sulfuric acid (\(H_2SO_4\)) or acidified \(NaCN\).
• Safety Note: We rarely use pure \(HCN\) gas directly because it is an extremely toxic, volatile gas. Instead, we generate it safely in situ by mixing \(KCN\) with acid.

Step-by-Step Reaction:

1. Attack: The cyanide ion (\(:CN^-\)), acting as a nucleophile, attacks the partially positive carbon atom (\(C^{\delta+}\)). The \(\pi\)-electrons from the \(C=O\) bond move onto the oxygen atom, forming an intermediate with a negatively charged oxygen atom (\(O^-\)).
2. Protonation: The intermediate oxygen ion (\(:O^-\)) uses a lone pair to bond with an \(H^+\) ion (from water or acid), forming an \(-OH\) group.

General Equation:
\(RCHO + HCN \to RCH(OH)CN\)
Example with ethanal:
\(CH_3CHO + HCN \to CH_3CH(OH)CN\) (2-hydroxypropanenitrile)

Why is this reaction useful?
1. It extends the carbon chain by one carbon atom.
2. The resulting \(-CN\) group can be converted to a carboxylic acid (\(-COOH\)) by hydrolysis or an amine (\(-CH_2NH_2\)) by reduction.

Optical Activity and Racemic Mixtures

When hydrogen cyanide adds to an unsymmetrical aldehyde (such as ethanal) or an unsymmetrical ketone, the product has a chiral carbon centre (a carbon bonded to 4 different groups).

• The carbonyl group (\(C=O\)) is planar around the carbonyl carbon.
• The nucleophile (\(:CN^-\)) has an equal probability of attacking from above or below the plane.
• This produces an equimolar mixture (50:50) of both enantiomers, known as a racemic mixture (racemate).
• Therefore, the resulting reaction mixture is optically inactive because the optical rotation from one enantiomer cancels out the optical rotation from the other.

Key Takeaway: Nucleophilic addition of \(HCN\) forms hydroxynitriles and increases chain length. Attack on a planar carbonyl group from above or below occurs with equal chance, yielding an optically inactive racemic mixture when a chiral centre is created.

6. Identification Reactions

1. Brady's Reagent: 2,4-Dinitrophenylhydrazine (2,4-DNP)

2,4-DNP is used to detect the presence of any carbonyl group (both aldehydes and ketones).

• Observation: Forms a bright orange or yellow precipitate (a 2,4-dinitrophenylhydrazone derivative).
• Identification of the specific compound:
1. Filter off the orange/yellow precipitate.
2. Recrystallize the solid from a suitable solvent to purify it.
3. Dry the purified crystals and measure their melting point using capillary melting point apparatus.
4. Compare the measured melting point value with known data book values to identify the exact aldehyde or ketone.

2. The Triiodomethane (Iodoform) Reaction

The triiodomethane test identifies the presence of a methyl carbonyl group (\(CH_3C=O\)) in aldehydes and ketones (and also ethanol or secondary alcohols with a \(CH_3CH(OH)-\) group).

• Reagents: Iodine (\(I_2\)) and aqueous sodium hydroxide (\(NaOH\)) warmed gently.
• Observation: Formation of a pale yellow precipitate of triiodomethane (\(CHI_3\)), which has a distinct, sweet, antiseptic smell.
• Which carbonyls give a positive test?
- The only aldehyde that gives a positive test is ethanal (\(CH_3CHO\)).
- All methyl ketones (such as propanone, \(CH_3COCH_3\), and butan-2-one, \(CH_3COCH_2CH_3\)) give a positive test.

Key Takeaway: 2,4-DNP gives an orange precipitate with all aldehydes and ketones; derivative melting points allow exact identification. The triiodomethane test gives a yellow precipitate (\(CHI_3\)) specifically for compounds with a \(CH_3C=O\) group (ethanal and all methyl ketones).

Chapter Summary & Quick Revision Checklist

Before moving on, make sure you can:

• Draw and name aldehydes (\(-CHO\)) and ketones (\(-CO-\)).
• Explain why the \(C=O\) bond is polar (\(C^{\delta+} = O^{\delta-}\)).
• Compare boiling points and explain water solubility via hydrogen bonding.
• Describe oxidation tests (Tollens', Fehling's, \(K_2Cr_2O_7 / H^+\)) to distinguish aldehydes from ketones.
• Write reduction equations using \(NaBH_4\) / \([H]\).
• Outline the nucleophilic addition mechanism with \(HCN\) / \(:CN^-\) and explain the formation of racemic mixtures due to the planar \(C=O\) group.
• Describe testing with 2,4-DNP (orange precipitate + melting point analysis) and the triiodomethane test for \(CH_3C=O\) groups.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the structural difference between an aldehyde and a ketone?

An aldehyde has its carbonyl group (\(C=O\)) at the end of the carbon chain (\(RCHO\)), whereas a ketone has its carbonyl group in the middle between two carbon atoms (\(RCOR'\)).

Why is the carbonyl group polar and susceptible to nucleophilic attack?

Oxygen is significantly more electronegative than carbon, drawing electron density towards itself and creating a permanent dipole (\(C^{\delta+} = O^{\delta-}\)) where the \(C^{\delta+}\) atom attracts electron-rich nucleophiles.

How do the boiling points of aldehydes and ketones compare to alkanes and alcohols of similar molecular mass?

Aldehydes and ketones have higher boiling points than alkanes due to permanent dipole–dipole attractions, but lower boiling points than alcohols because they cannot form intermolecular hydrogen bonds with themselves.

Why are short-chain aldehydes and ketones soluble in water?

They can form hydrogen bonds with water molecules using the lone pairs of electrons on the carbonyl oxygen atom (\(O^{\delta-}\)) and the partially positive hydrogen atoms (\(H^{\delta+}\)) of water.

How can aldehydes and ketones be distinguished using Tollens' reagent and Fehling's solution?

Aldehydes are easily oxidized, forming a silver mirror with Tollens' reagent and a brick-red precipitate with Fehling's solution, whereas ketones do not react with either.

What are the reduction products when aldehydes and ketones react with \(NaBH_4\)?

Aldehydes are reduced to primary (\(1^\circ\)) alcohols, while ketones are reduced to secondary (\(2^\circ\)) alcohols.

What type of reaction occurs when hydrogen cyanide (\(HCN\)) reacts with an aldehyde or ketone, and what is formed?

It is a nucleophilic addition reaction that produces a hydroxynitrile (cyanohydrin) and increases the carbon chain length by one carbon atom.

Why does the nucleophilic addition of \(HCN\) to an unsymmetrical carbonyl compound produce an optically inactive racemic mixture?

The planar carbonyl group (\(C=O\)) can be attacked by the \(:CN^-\) nucleophile with equal probability from above or below the plane, producing an equimolar (50:50) mixture of enantiomers.

How is Brady's reagent (2,4-DNP) used to detect and identify specific aldehydes and ketones?

2,4-DNP forms a bright orange/yellow precipitate with any aldehyde or ketone; purifying the derivative and determining its melting point allows precise identification by comparison with data tables.

What structural feature is detected by the triiodomethane (iodoform) reaction, and what is the positive observation?

It detects a methyl carbonyl group (\(CH_3C=O\)), producing a pale yellow precipitate of triiodomethane (\(CHI_3\)) with an antiseptic smell.

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