Question

Write the correct order of rate of reaction of following compounds with $PhN_2Cl$
P: $N,N$-dimethylaniline
Q: $N,N$-dimethyl-3-methylaniline
R: $N,N$-dimethyl-2,6-dimethylaniline

• (1) P > Q > R
• (2) P > R > Q
• (3) Q > P > R
• (4) R > P > Q

Answer 1

The Correct Option is A

Reaction of diazonium salts with activated aromatic rings is an Electrophilic Aromatic Substitution (EAS) called diazo-coupling. The rate depends on the nucleophilicity of the benzene ring.
1. Electronic effects: The $-N(CH_3)_2$ group is highly activating due to resonance (+M effect).
2. Steric effects: For the resonance (+M) to be effective, the lone pair on nitrogen must be delocalized into the ring, requiring the $-N(CH_3)_2$ group to be in the same plane as the benzene ring.
3. Steric Inhibition of Resonance (SIR): In compound R (2,6-dimethyl), the two ortho-methyl groups hinder the $-N(CH_3)_2$ group, forcing it out of the ring plane. This drastically reduces the resonance effect, making the ring less nucleophilic.
4. Comparing P and Q: In Q, there is a meta-methyl group. While $+I$ effect of methyl adds density, its presence compared to P is negligible. However, usually, the unsubstituted $N,N$-dimethylaniline is most reactive in coupling due to minimal steric hindrance to the incoming electrophile.

Order: P > Q > R. Option (1).