Question

Write the correct order of rate of reaction of following with PhN$_2$Cl

 

• A. R>P>Q
• B. P>R>Q
• C. Q>P>R
• D. P>Q>R

Hint:

Electron-donating groups increase the nucleophilicity of the aromatic ring, speeding up the reaction.

Answer 1

The Correct Option is A

Step 1: Understanding Azo Coupling The reaction of aromatic compounds with benzenediazonium chloride (PhN$_2$Cl) is an electrophilic aromatic substitution reaction known as azo coupling. In this reaction, the diazonium ion acts as a weak electrophile. Therefore, the aromatic ring must be highly nucleophilic for the reaction to occur efficiently.

Step 2: Effect of Substituents The nucleophilicity of the aromatic ring depends on the substituents attached to it: 
1. Electron-Donating Groups (EDG) like -NH$_2$, -OH, and -CH$_3$ increase electron density on the ring, thereby increasing the rate of reaction. 2. Electron-Withdrawing Groups (EWG) like -NO$_2$, -CN, and -CHO decrease electron density, thereby slowing down or preventing the reaction.

Step 3: Comparing the given compounds Compound R: Contains an amino group (-NH$_2$). This is a very strong activating group due to resonance (+M effect), making the ring highly nucleophilic. Compound P: Contains a methyl group (-CH$_3$). This is an activating group due to inductive effect (+I) and hyperconjugation, but it is much weaker than -NH$_2$. Compound Q: Contains a nitro group (-NO$_2$). This is a strong deactivating group due to resonance (-M effect), making the ring very poor in electrons. Consequently, the order of reactivity is R (strongest EDG)>P (weak EDG)>Q (strong EWG).