Question

Write the structure of P and Q. Name the reaction that gives the product P.

Answer 1

Gattermann Koch reaction is a reaction that begins with the formation of the reactive species with the help of the acid. Basically, the aim of this reaction is to attach the aldehyde group to an aromatic system. To solve this question, we need to know the reaction mechanism of this reaction.

Complete step by step answer:
As we can see that there is a structure of benzene which reacts with carbon monoxide and HCl in presence of a catalyst i.e. $AlC{l_3}$ to synthesize the aromatic aldehyde. Here, $AlC{l_3}$ acts as a Lewis acid i.e. it is an electron pair acceptor. This type of reaction is further known as Gattermann Koch aldehyde reaction. Now, let’s discuss the mechanism of this reaction.
So basically, in the first step, the carbon monoxide acts as a Lewis base and accepts a pair of electrons from HCl. Due to this a positively charged molecule is formed which has a different resonance structure. Further, these species act as an electrophile while reacting with the aromatic ring. However, it is more likely to be the target of a nucleophilic attack from the chloride ion in the hydrochloric acid. Further, in the next step, when Lewis acid is added, it easily removes chloride ion from the species. Finally, in the last step an electrophilic aromatic substitution occurs at the aromatic ring and this ring acts as a nucleophile and donates an electron pair to the formyl cation. Thus, the formyl group is attached to the aromatic ring via Gattermann Koch reaction.
Now, the complete reaction according to the reaction mechanism is as shown:

In the first step, benzaldehyde is formed i.e. product P is benzaldehyde and further when it reacts with $\dfrac{{HN{O_3}}}{{{H_2}S{O_4}}}$, then product Q i.e. Para-nitro benzaldehyde is formed. The reaction is as shown:

Therefore, product P and Q are benzaldehyde and p-nitrobenzaldehyde respectively.

Note: The Gattermann Koch reaction is not applicable to phenols and phenol ether substrates. Moreover, if zinc chloride is used as a catalyst in this reaction then, the traces of copper (I) chloride are often necessary since they act as co-catalyst.