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

Solution

Chemistry ma’am · 4 steps

Read the reagents in order: diazotise, swap in CN, then Stephen reduction turns the nitrile into an aldehyde.

1. Diazotisation first. With NaNO₂/HCl at 0 to 5 °C, aniline becomes benzenediazonium chloride, C₆H₅N₂⁺Cl⁻. This salt is only an intermediate, because the same step also has KCN.
2. Cyanide replaces nitrogen. The cyanide ion replaces the -N₂⁺ group, and nitrogen gas leaves. So A is benzonitrile, C₆H₅CN.

   C₆H₅CN →(SnCl₂/HCl) C₆H₅CH=NH →(H₃O⁺) C₆H₅CHO

3. Stephen reduction. SnCl₂/HCl reduces the nitrile only as far as the imine, which comes out as its salt. Acid hydrolysis then turns the C=NH group into C=O, giving benzaldehyde.
4. So. A is benzonitrile and B is benzaldehyde, which is option (d).

The idea to keep. Nitrile with SnCl₂/HCl and then H₃O⁺ is Stephen reduction, and it always stops at the aldehyde. Nitriles are a neat way to add one carbon and land on -CHO.

Answer from the NTA final answer key. Solution by PrepWiser.