Gattermann Reaction - JEE PDF Download

Introduction

The Gattermann reaction is the formylation of aromatic ring compounds. Formylation is the process where the formyl group (-CH=O) is attached to the compound. It can be categorized as a substitution reaction which is very similar to the Friedel Crafts Reaction.

Reaction: Treatment of Benzene or its derivative with HCl and HCN in the presence of a Lewis acid catalyst like aluminium chloride, AlCl₃, followed by hydrolysis gives benzaldehyde or substituted benzaldehyde.

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Diazonium Salt

Diazonium salt is formed as a result of reaction of aromatic Amine with Nitrous Acid leaving water as a side product. This reaction is termed as Diazotization Reaction.

The reaction is given below:

ArNH₂ + HNO₂ + HX → RN₂ + X^- + H₂O

[Aromatic amine + Nitrous acid + Mineral acid → Diazonium salt + Water]

Diazotization of Aniline: It is done by treating aniline with sodium nitrate and HCl at the temperature of 273K.

Gattermann Reaction Mechanism

Let us look at the Gattermann reaction mechanism step-by-step.
Step 1: Formation of Formimino Chloride
HCN Reacts with HCl to form formimino chloride.
The reaction is given below:

Step 2: Formation of Electrophile
Formimino chloride reacts with Lewis acid catalyst (such as AlCl₃) and forms formimino cation.
Reaction is given below:
Step 3: Attack of Electrophile on Benzene Ring
Formimino cation (electrophile) attacks the benzene rings and forms benzylamine.
Reaction is given below: 

Step 4: Hydrolysis of Benzaldimine

Hydrolysis of benzylamine takes place in this step which results in the formation of benzaldehyde. 

Reaction is given below:

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Example of Gattermann Reaction 

This reaction is used for obtaining chlorobenzene or bromobenzene from benzene-diazonium chloride by treating it with Cu/HCl or Cu/HBr respectively.

Key points for the above reaction:

• The reaction can be simplified by replacing the HCN/AlCl₃ combination with zinc cyanide.
• Although it is extremely toxic, Zn(CN)₂ is a solid, making it safer to work with than gaseous HCN.
• The Zn(CN)₂ reacts with the HCl to form the key HCN reactant and Zn(Cl)₂ which serves as the Lewis-acid catalyst in-situ.
• An example of the Zn(CN)₂ method is the synthesis of mesitaldehyde from mesitylene.

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Gattermann-Koch Reaction

The Gattermann–Koch reaction, named after the German chemists Ludwig Gattermann and Julius Arnold Koch, is a variant of the Gattermann reaction in which carbon monoxide (CO) is used instead of hydrogen cyanide. The reaction can be represented as follows:

Gattermann Reaction & Gattermann-Koch Reaction: Differences

Applications of Gattermann Reaction

• It is used in obtaining Aromatic halides like Chlorobenzene and Bromobenzene.
• It is used in making Aromatic aldehydes like Benzaldehyde.
• Products of Gattermann reaction such as benzaldehydes, haloarenes, chlorobenzene and others are used in various fields such as pharmaceuticals, agricultural, medicinal etc.