Question

Consider the three aromatic molecules (P, Q and R). The correct order regarding the reactivity of these compounds with \(Ph{-}N=N^+Cl^-\) under optimum but slightly acidic medium is:

• A. \(P>Q>R\)
• B. \(R>P>Q\)
• C. \(R>Q>P\)
• D. \(P>R>Q\)

Answer 1

The Correct Option is B

Concept: Reaction with \(Ph{-}N=N^+Cl^-\) is azo coupling, which is an electrophilic aromatic substitution. Electron donating groups increase reactivity of the aromatic ring.
Step 1:Analyze substituents} All three molecules contain a strongly activating \(-NMe_2\) group. However methyl groups (\(-CH_3\)) also donate electrons by hyperconjugation.
Step 2:Compare activation}

• \(R\): contains additional methyl groups → highest activation.
• \(P\): contains only the dimethylamino group.
• \(Q\): steric hindrance reduces reactivity.

Thus the reactivity order is: \[ R>P>Q \] \[ \boxed{R>P>Q} \]