What is 'C' in the given sequence of reactions?





In this reaction sequence, an aromatic compound undergoes a reaction with NaNO2/HCl, followed by hydrolysis with H2O/H3PO4, leading to the formation of a carboxyl group (COOH) as product 'C'.
Convert Ethanal to But-2-enal
Write structure of the products of the following reactions: 