Toulene when heated with Alkaline KMnO₄ followed by acidification gives ‘A’. A on decarboxylation gives B. B in presence of CO + HCl / CuCl gives C. The reaction B $\stackrel{ }{\to }$ C is

When benzene is treated with carbon monoxide in an acidic medium in presence of anhydrous aluminium chloride, it results in the production of benzaldehyde. This reaction is a name reaction, popularly known as Gattermann-Koch reaction..