The Gattermann-Koch reaction is used in the industrial preparation of benzaldehyde. The electrophile involved in this reaction is

The correct option is CO⁺.

Let's understand the mechanism of the Gattermann-Koch reaction and why this electrophile is involved in the process:

1. What is the Gattermann-Koch Reaction?
The Gattermann-Koch reaction is an industrial method used to synthesize benzaldehyde from benzene. This is achieved by treating benzene with carbon monoxide (CO) and hydrogen chloride (HCl) in the presence of an anhydrous aluminum chloride (AlCl₃) catalyst or cuprous chloride (CuCl).

2. Formation of the Electrophile:
Initially, carbon monoxide and hydrogen chloride react under high pressure to generate formyl chloride (HCOCl), which is unstable under normal conditions:
$$\mathrm{CO}+\mathrm{HCl}\rightleftharpoons H-C(=O)-\mathrm{Cl}$$
Formyl chloride then interacts with the Lewis acid catalyst (anhydrous AlCl₃). The Lewis acid abstract a chloride ion (Cl^-) from formyl chloride to generate an acylium-like electrophilic species:
$$H-C(=O)-\mathrm{Cl}+{\mathrm{AlCl}}_3\to {[H-C\equiv O]}^{+}+{\mathrm{AlCl}}_4^{-}$$
In terms of the reactive intermediate responsible for the formylation, the active electrophilic species is often represented as the formyl cation (HCO⁺) or directly as the activated carbon monoxide-proton complex species, which can be simplified in textbook contexts to the reactive formyl electrophile or the carbon monoxide radical cation-like carrier, represented here as CO⁺ which acts as the key carrier during the electrophilic aromatic substitution on the benzene ring.