Preparation of Aldehydes and Ketones: Methods, Reactions and Examples

Aldehydes and ketones are important classes of organic compounds containing the carbonyl group (>C=O). These compounds are widely studied in organic chemistry and are important for Class 12 Chemistry, CBSE, Bihar Board, NEET, and other competitive examinations.

In this article, we will discuss the important methods of preparation of aldehydes and ketones, their chemical equations, examples, and key points for examinations.

1. What Are Aldehydes and Ketones?

Aldehydes

Aldehydes are organic compounds in which the carbonyl group is attached to at least one hydrogen atom. Their general formula is R–CHO.

Example: Ethanal (CH₃CHO).

Ketones

Ketones are organic compounds in which the carbonyl group is attached to two carbon-containing groups. Their general formula is R–CO–R′.

Example: Propanone (CH₃COCH₃), commonly known as acetone.

2. Preparation of Aldehydes

2.1 Oxidation of Primary Alcohols

Primary alcohols can be oxidised to aldehydes under controlled conditions. Pyridinium chlorochromate (PCC) is commonly used to prevent further oxidation to carboxylic acids.

General reaction:

R–CH₂OH + [O] → R–CHO + H₂O

Example:

CH₃CH₂OH + [O] → CH₃CHO + H₂O

Ethanol is oxidised to ethanal under suitable controlled conditions.

Important point: Strong oxidising agents may further oxidise aldehydes to carboxylic acids.

2.2 Dehydrogenation of Primary Alcohols

When primary alcohol vapours are passed over heated copper at about 573 K, they undergo dehydrogenation to form aldehydes.

General reaction:

R–CH₂OH → R–CHO + H₂

Example:

CH₃CH₂OH → CH₃CHO + H₂

Condition: Heated copper, approximately 573 K.

2.3 Rosenmund Reduction

Rosenmund reduction is used to prepare aldehydes from acid chlorides. The acid chloride is reduced using hydrogen in the presence of palladium supported on barium sulfate. The catalyst is poisoned to prevent further reduction of the aldehyde.

General reaction:

R–COCl + H₂ → R–CHO + HCl

Reagents: H₂, Pd/BaSO₄.

Example:

CH₃COCl + H₂ → CH₃CHO + HCl

Exam tip: Rosenmund reduction converts an acid chloride into an aldehyde.

2.4 Stephen Reaction

The Stephen reaction is a method of preparing aldehydes from nitriles. A nitrile is reduced using stannous chloride and hydrochloric acid, followed by hydrolysis of the intermediate.

General reaction:

R–C≡N → R–CHO

Reagents: SnCl₂/HCl, followed by hydrolysis.

Example: Ethanenitrile can be converted into ethanal by the Stephen reaction.

Important point: The carbon atom of the nitrile group becomes the carbonyl carbon of the aldehyde.

2.5 Etard Reaction

The Etard reaction is used to prepare aromatic aldehydes by oxidising a suitable methyl group attached to an aromatic ring. Chromyl chloride (CrO₂Cl₂) is the commonly used reagent.

Example:

Toluene → Benzaldehyde

Reagent: Chromyl chloride (CrO₂Cl₂), followed by hydrolysis.

Exam tip: Etard reaction is associated with the conversion of toluene into benzaldehyde.

2.6 Gattermann–Koch Reaction

The Gattermann–Koch reaction is used to introduce a formyl group (–CHO) into an aromatic ring.

Example:

C₆H₆ + CO → C₆H₅CHO

Reagents: CO, HCl, anhydrous AlCl₃, and CuCl.

Benzene is converted into benzaldehyde under suitable reaction conditions.

2.7 Preparation of Aldehydes from Esters Using DIBAL-H

Diisobutylaluminium hydride (DIBAL-H) can reduce suitable esters to aldehydes under controlled conditions.

General reaction:

R–COOR′ → R–CHO

Reagent: DIBAL-H, generally at low temperature, followed by hydrolysis.

Important point: Careful control of reaction conditions is required to avoid further reduction to an alcohol.

3. Preparation of Ketones

3.1 Oxidation of Secondary Alcohols

Secondary alcohols are oxidised to ketones using suitable oxidising agents.

General reaction:

R–CH(OH)–R′ + [O] → R–CO–R′ + H₂O

Example:

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

Propan-2-ol is oxidised to propanone (acetone).

3.2 Dehydrogenation of Secondary Alcohols

When secondary alcohol vapours are passed over heated copper at approximately 573 K, ketones are formed.

General reaction:

R–CH(OH)–R′ → R–CO–R′ + H₂

Example:

CH₃CH(OH)CH₃ → CH₃COCH₃ + H₂

3.3 Ozonolysis of Alkenes

Ozonolysis involves the cleavage of a carbon-carbon double bond using ozone, followed by a suitable work-up. Depending on the structure of the alkene and the work-up conditions, aldehydes and/or ketones may be formed.

Example:

CH₃CH=CHCH₃ → 2 CH₃CHO

But-2-ene gives two molecules of ethanal on reductive ozonolysis.

Another example:

(CH₃)₂C=CH₂ → (CH₃)₂CO + HCHO

2-Methylpropene gives propanone and methanal on reductive ozonolysis.

3.4 Friedel–Crafts Acylation

Friedel–Crafts acylation is used to prepare aromatic ketones by introducing an acyl group (RCO–) into an aromatic ring.

General reaction:

Ar–H + RCOCl → Ar–CO–R

Reagents: An acyl chloride and anhydrous AlCl₃.

Example:

C₆H₆ + CH₃COCl → C₆H₅COCH₃

Benzene reacts with acetyl chloride to form acetophenone under suitable conditions.

3.5 Hydration of Alkynes

Hydration of alkynes in the presence of suitable catalysts generally produces carbonyl compounds after tautomerisation of the initially formed enol.

Example: Hydration of ethyne

HC≡CH + H₂O → CH₃CHO

In the presence of HgSO₄ and dilute H₂SO₄, ethyne forms ethanal.

Example: Hydration of propyne

CH₃C≡CH + H₂O → CH₃COCH₃

Under the usual mercury(II)-catalysed acidic conditions, propyne forms propanone.

3.6 Reaction of Acid Chlorides with Dialkylcadmium

Acid chlorides react with dialkylcadmium reagents to form ketones.

General reaction:

2 RCOCl + R′₂Cd → 2 R–CO–R′ + CdCl₂

Important point: This is a classical method of preparing ketones. Organocadmium compounds are highly toxic and are not suitable for routine laboratory use.

4. Difference Between Preparation of Aldehydes and Ketones

Method	Aldehydes	Ketones
Oxidation of alcohols	From primary alcohols under controlled conditions	From secondary alcohols
Dehydrogenation	Primary alcohols over heated copper	Secondary alcohols over heated copper
Reduction of acid chlorides	Rosenmund reduction	Not the usual Rosenmund product
Ozonolysis of alkenes	May be formed, depending on alkene structure	May be formed, depending on alkene structure
Aromatic compounds	Etard and Gattermann–Koch reactions	Friedel–Crafts acylation
Hydration of alkynes	Ethyne produces ethanal	Many other terminal alkynes produce methyl ketones under HgSO4/H2SO4 conditions

5. Important Named Reactions for Examinations

• Rosenmund reduction: Acid chloride to aldehyde.
• Stephen reaction: Nitrile to aldehyde.
• Etard reaction: Toluene to benzaldehyde.
• Gattermann–Koch reaction: Introduction of the –CHO group into an aromatic ring.
• Friedel–Crafts acylation: Preparation of aromatic ketones.
• Ozonolysis: Cleavage of a carbon-carbon double bond to form carbonyl compounds under suitable work-up conditions.

6. Important MCQs on Preparation of Aldehydes and Ketones

Question 1

Which reagent is commonly used in Rosenmund reduction?

• A. NaBH₄
• B. H₂/Pd-BaSO₄
• C. LiAlH₄
• D. KMnO₄

Correct answer: B. H₂/Pd-BaSO₄

Question 2

Which compound is obtained by oxidation of propan-2-ol?

• A. Propanal
• B. Propanoic acid
• C. Propanone
• D. Ethanal

Correct answer: C. Propanone

Question 3

Which reaction converts toluene into benzaldehyde?

• A. Cannizzaro reaction
• B. Etard reaction
• C. Aldol condensation
• D. Haloform reaction

Correct answer: B. Etard reaction

Question 4

Which reaction is used to prepare aromatic ketones from benzene and an acyl chloride?

• A. Wurtz reaction
• B. Stephen reaction
• C. Friedel–Crafts acylation
• D. Rosenmund reduction

Correct answer: C. Friedel–Crafts acylation

Question 5

What is the product of oxidation of a primary alcohol under controlled conditions?

• A. Ketone
• B. Aldehyde
• C. Ether only
• D. Alkene only

Correct answer: B. Aldehyde

7. Conclusion

The preparation of aldehydes and ketones is an important topic in organic chemistry. Students should understand the reactions of primary and secondary alcohols, Rosenmund reduction, Stephen reaction, Etard reaction, Gattermann–Koch reaction, Friedel–Crafts acylation, and ozonolysis.

Learning the reagents, reaction conditions, and products of these reactions will help students solve organic chemistry problems in board examinations and competitive tests such as NEET.

Revision tip: Practise writing every general reaction and one example for each named reaction to improve your understanding and recall.