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JEE MainChemistryAldehydes and Ketones

Three para-substituted benzaldehydes have substituents X, Y, and Z at the para position. Their relative order of reactivity towards nucleophilic addition of HCN is observed to be: Z- C ₆ H ₄ -CHO > Y-C ₆ H ₄ -CHO > X-C ₆ H ₄ -CHO > Acetophenone If the three substituents are -CH ₃, -OCH ₃, and -NO ₂ (in no particular order), identify X, Y, and Z respectively.

Options

  1. AX = -NO ₂, Y = -CH ₃, Z = -OCH ₃
  2. BX = -CH ₃, Y = -OCH ₃, Z = -NO ₂
  3. CX = -CH ₃, Y = -NO ₂, Z = -OCH ₃
  4. DX = -OCH ₃, Y = -CH ₃, Z = -NO ₂

Correct answer

D. X = -OCH ₃, Y = -CH ₃, Z = -NO ₂

Step-by-step solution

Reactivity in nucleophilic addition reactions depends on the magnitude of the positive charge on the carbonyl carbon. Electron-withdrawing groups (EWG) increase this positive charge and thus increase reactivity, while electron-donating groups (EDG) decrease it. Among the given substituents: 1. -NO ₂ is a strong EWG due to its -I and -M effects. It will make the carbonyl carbon most electrophilic. Hence, Z is -NO ₂. 2. -OCH ₃ is a strong EDG due to its +M effect (which dominates over its -I effect). It will signific

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