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Compound X in below reaction is-
${C_6}{H_6} + CO + HCl\xrightarrow{{Anhy\,AlC{l_3}}}X + HCl$

Last updated date: 14th Jul 2024
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Hint: From the given chemical reaction, we can see that the reactants are benzene, carbon monoxide and hydrochloric acid. This reaction must be similar to Friedel-Crafts reaction and it must be a substitution reaction. In this reaction, anhydrous aluminum trichloride plays the role of Lewis acid catalyst.

Complete answer:
We have to know that a chemical reaction where aromatic compounds are formulated by combination of carbon monoxide and hydrogen chloride in the presence of Lewis acid catalysts like aluminum trichloride is the Gattermann-Koch reaction.
The mechanism for this reaction proceeds in three steps,
First step: The first step of the reaction is formation of formyl chloride by carbon monoxide and hydrogen chloride.
Second step: The second step of the reaction is reaction of formyl chloride with anhydrous aluminum trichloride to form an electrophile.
Third step: The third step is reaction of benzene with electrophile, to form benzaldehyde. Hydrogen chloride and aluminum trichloride is removed.
So, from the above steps of mechanism, we can conclude that benzene reacts with carbon monoxide in acid medium in the presence of anhydrous aluminum chloride to form benzaldehyde as a product. The role of catalyst is played by anhydrous aluminum trichloride and this reaction is an example of electrophilic substitution reaction. We can write complete reaction as,
\[{C_6}{H_6} + CO + HCl\xrightarrow{{Anhy.AlC{l_3}}}{C_6}{H_5}CHO + HCl\]
So, we have identified compound X as benzaldehyde and its chemical formula is ${C_6}{H_5}CHO$.

We have to know that phenol and substrates of phenol ether do not undergo Gattermann-Koch reaction. We should not get confused between the Gattermann reaction and Gattermann-Koch reaction. In the Gattermann reaction, the reactants are a mixture of hydrogen cyanide and hydrogen chloride along with benzene whereas in the Gattermann-Koch reaction, the reactants are a mixture of carbon monoxide and hydrogen chloride with benzene. Anhydrous aluminum trichloride is used as a Lewis acid catalyst in Gattermann-Koch reaction and Gattermann reaction.