A and B in the following reactions are: - DASH

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$A$ and $B$ in the following reactions are:

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Step-by-Step Solution Step 1: Formation of the Diazonium Salt An aniline derivative (aromatic amine) is treated with sodium nitrite ($NaNO_2$) and hydrochloric acid ($HCl$) at a low temperature (around $0^\circ C$ to $5^\circ C$). This step converts the amino group ($-NH_2$) into a diazonium salt ($-N_2^+$). This compound is referred to as A in the reaction sequence. Step 2: Substitution to Form the Nitrile The freshly prepared diazonium salt (A) is then reacted with potassium cyanide ($KCN$). The diazonium group ($-N_2^+$) is replaced by the cyano group ($-CN$). This substitution yields an aryl nitrile (phenyl cyanide or benzonitrile) and is referred to as B in the reaction. Step 3: Stephen Reduction to Get the Aldehyde The nitrile (B) is next subjected to partial reduction—commonly known as the Stephen reduction—using stannous chloride ($SnCl_2$) and hydrochloric acid, followed by hydrolysis. This step converts the nitrile group ($-CN$) into an aldehyde group ($-CHO$). Thus, the final product is the aldehyde derivative, matching the structure shown in the correct answer (Option 4).

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