Benzene reacts with methanoyl chloride (HCOCl) in the presence of a catalyst - AQA - A-Level Chemistry - Question 8 - 2022 - Paper 2

The catalyst required for this reaction is aluminum chloride (AlCl₃). This catalyst is used to activate the methanoyl chloride and form the electrophile, [HCO]⁺.

An equation to illustrate how the catalyst is used is:

$HCOCl + AlCl_3 \rightarrow [HCO]^{+} + [AlCl_4]^{-}$

The mechanism for the electrophilic aromatic substitution reaction involves the following steps:

1. Formation of the Electrophile: The electrophile [HCO]⁺ is formed in the presence of aluminum chloride.

2. Attack of the Electrophile: The π electrons from the benzene ring attack the electrophile, leading to the formation of a sigma complex.

   

3. Deprotonation: The sigma complex loses a proton (H⁺), regenerating the aromaticity of the benzene ring.

   This gives the final product, benzaldehyde.