Preparation of Aldehydes and Ketones

Full Notes

Simple methods of aldehyde and ketone preparation have been covered in previous units, please see below.

By Oxidation of Alcohols

Primary and secondary alcohols can be oxidised to form aldehydes and ketones, respectively.

See class 12, Unit 7

By Dehydrogenation of Alcohols

Alcohol vapours passed over heavy metal catalysts undergo dehydrogenation.

See class 12, Unit 7

From Hydrocarbons

Aldehydes and ketones can be obtained by selective oxidation (ozonolysis) of alkenes and by the hydration of alkynes.

See class 11, unit 9

Preparation of Aldehydes

More advanced methods of preparation are outlined below.

From Acyl Chloride (Acid Chloride)

Acid chlorides react with hydrogen in presence of Pd/BaSO₄ (Rosenmund reduction).

Reagents: H₂/Pd–BaSO₄ poisoned catalyst

From Nitriles

Nitriles are partially reduced to aldehydes using mild reducing agents. Such as the use of SnCl₂ and HCl in the Stephen reaction.

Stephen Reaction:

Nitriles (RCN) are reduced using SnCl₂ + HCl, forming imines (RCH=NH), which on hydrolysis give aldehydes (RCHO).

Selective Reduction using AlH(i-Bu)₂ (Diisobutylaluminium hydride):
Nitriles and esters can be selectively reduced to aldehydes using DIBAL-H followed by hydrolysis.

For Example: RCN + DIBAL-H → R-CHO

RCOOR → RCHO (from ester)

Preparation of Aromatic Aldehydes

Aromatic aldehydes, such as benzaldehyde and its derivatives, can be synthesized from aromatic hydrocarbons through several key methods:

By Oxidation of Methylbenzene (Toluene)

Strong oxidising agents convert toluene into benzoic acid. However, with specific reagents, the oxidation can be stopped at the aldehyde stage, avoiding further oxidation.

Etard Reaction (using Chromyl Chloride, CrO₂Cl₂):
Toluene reacts with chromyl chloride in carbon disulfide (CS₂) to form a chromium complex. Hydrolysis of this complex gives benzaldehyde.

Equation: C₆H₅CH₃ + CrO₂Cl₂ → [C₆H₅CH(O₂CrOHCl)₂] → C₆H₅CHO (benzaldehyde)

Using Chromic Oxide (CrO₃):
Toluene reacts with CrO₃ and acetic anhydride ((CH₃CO)₂O) at 273–283 K to form benzylidene diacetate, which on acid hydrolysis gives benzaldehyde.

Equation: C₆H₅CH₃ + CrO₃ + (CH₃CO)₂O → C₆H₅CH(OCOCH₃)₂ → C₆H₅CHO (benzaldehyde)

By Side Chain Chlorination Followed by Hydrolysis

Chlorination of toluene in presence of light gives benzal chloride (C₆H₅CHCl₂), which on hydrolysis at 373 K gives benzaldehyde.

Equation: C₆H₅CH₃ + Cl₂ (in light) → C₆H₅CHCl₂
C₆H₅CHCl₂ + H₂O (373 K) → C₆H₅CHO (benzaldehyde)

By Gattermann–Koch Reaction

Benzene reacts with carbon monoxide (CO) and hydrogen chloride (HCl) in the presence of anhydrous aluminium chloride (AlCl₃) and cuprous chloride (CuCl) to form benzaldehyde.

Equation: C₆H₆ + CO + HCl →(AlCl₃/CuCl)→ C₆H₅CHO (benzaldehyde)

Preparation of Ketones

From Acyl Chlorides (Using Dialkylcadmium)

Ketones are formed when acyl chlorides react with dialkylcadmium, which is prepared by the reaction of Grignard reagent with cadmium chloride.

Equations:
2 R–MgX + CdCl₂ → R₂Cd + 2 Mg(X)Cl
2 R′–COCl + R₂Cd → 2 R′–CO–R + CdCl₂

From Nitriles (Using Grignard Reagent)

When a nitrile is treated with a Grignard reagent and then hydrolysed, a ketone is formed.

Equation: R–C≡N + R₁MgBr (ether) → R–C(MgBr)–R₁

From Benzene or Substituted Benzenes (Friedel–Crafts Acylation)

Ketones are also synthesized by reacting benzene or substituted benzene with an acid chloride in the presence of anhydrous aluminium chloride (AlCl₃).

This is known as the Friedel–Crafts acylation reaction.

General Equation: Ar–H + R–COCl →(Anhyd. AlCl₃)→ Ar–CO–R (Ketone)

Summary

• Oxidation or dehydrogenation of alcohols gives aldehydes and ketones.
• Rosenmund reduction and selective reductions of nitriles or esters give aldehydes.
• Dialkylcadmium from Grignard reagents converts acyl chlorides to ketones.
• Grignard addition to nitriles and Friedel–Crafts acylation provide ketones.