Gattermann Koch Reaction
Definition
The formation of reactive species with the help of the acid is the first step in Gattermann Koch's reaction mechanism. The reaction's overall goal is to attach a formyl group (-CHO group) to an aromatic system. The Gattermann - Koch reaction is illustrated below.
The Gattermann - Koch reaction does not work on phenol or phenol ether substrates. When using zinc chloride as a catalyst in the Gattermann - Koch reaction, traces of copper(I) chloride are frequently required because it acts as a co-catalyst.
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Gattermann Formylation |
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Named After |
Ludwig Gattermann |
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Reaction Type |
Substitution Reaction |
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Gattermann Koch Reaction Mechanism
Step 1 - The generation of reactive species that can later be used to react on the aromatic ring is the first step in the Gattermann Koch reaction mechanism. Carbon monoxide can accept a proton from hydrochloric acid because it is a Lewis base. This produces a positively charged molecule with various resonance structures. One such resonance structure has a positive charge on the carbon, which explains the hybrid's reactivity. While reacting with the aromatic ring, this species can act as an electrophile. It is more likely, however, to be the target of a nucleophilic attack by the chloride ion in hydrochloric acid.
Step 2 - When a Lewis acid (Aluminium Chloride) is added, a chloride ion is easily removed from the species. The species has now degenerated into the reactive formyl cation.
Step 3 - At the aromatic ring, an electrophilic aromatic substitution occurs. As a nucleophile, the aromatic ring donates an electron pair to the formyl cation. A proton expulsion quickly resolves the temporary loss of aromaticity.
As a result of the Gattermann - Koch reaction, the formyl group is attached to the aromatic ring. In the above example, benzaldehyde is formed by treating benzene with carbon monoxide and hydrochloric acid in the presence of Aluminium Chloride.
Limitations of Gattermann Koch Reaction
Due to a lack of suitable substrates, the Gattermann-Koch reaction is limited to alkylbenzenes; therefore, Gattermann devised a modification that allowed for the formylation of phenols, phenolic ethers, and heteroaromatic compounds such as pyrroles and indoles.
The Gattermann reaction had one major flaw: anhydrous hydrogen cyanide was required (HCN). To avoid handling HCN, R. Adams created it in-situ from zinc cyanide and hydrochloric acid; this method became known as the Adams modification, and it is now more commonly used in organic synthesis.
Things to Remember in Gattermann Koch Reaction
- In 1897, L Gattermann and J.A. Koch successfully inserted an aldehyde group on toluene using formyl chloride (HCOCl) as an acylating agent under Friedel-Crafts acylation conditions.
- The addition of a formyl group to electron-rich aromatic rings using CO/HCl/Lewis acid catalysts (AlX3, FeX3, where X = Cl, Br, I) to produce aromatic aldehydes was named Gattermann-Koch formylation.
- Carbon monoxide and hydrochloric acid are used as reactants in the Gattermann-Koch Reaction to produce an aryl aldehyde.
- In the presence of trace amounts of cuprous chloride, aluminium trichloride catalyses the reaction.
- The Gattermann-Koch Reaction does not apply to phenol and phenol ethers because they "could not be effectively formylated at atmospheric pressure in benzene as a solvent.""The failure to react was blamed on the insolubility of the cuprous chloride in the reaction mixture."
The Gattermann-Koch Formylation method is used to produce substituted benzaldehydes. The formyl cation is the key intermediate in this process, and once formed, it interacts similarly to the other acyl cations in Friedel-Crafts processes.
What are the uses of Gattermann Koch Reaction?
The uses of Gattermann Koch Reaction are as follows:
- The Gattermann Koch Reaction is used to Synthesize aldehydes from Alkylbenzenes.
- This reaction is also an industrial reaction to manufacturing Benzaldehyde.