Question

What is the product formed when 4-methylaniline is treated with nitrous acid at $0^\circ C$ followed by coupling with N,N-dimethylaniline?

• A. 4-(4-(dimethylamino)phenylazo)toluene
• B. 4-methylbenzenediazonium chloride
• C. N,N-dimethyl-4-toluidine
• D. 4-methylaniline

Answer 1

Answer: (A)

4-(4-(dimethylamino)phenylazo)toluene

Answer 2

The reaction involves two main steps: diazotization and azo coupling.

1. Diazotization: 4-methylaniline reacts with nitrous acid ($HNO_2$) in the presence of a strong acid (like HCl) at a low temperature ($0-5^\circ C$) to form a diazonium salt, specifically 4-methylbenzenediazonium chloride.

   $CH_3C_6H_4NH_2 + HNO_2 + HCl \xrightarrow{0-5^\circ C} [CH_3C_6H_4N_2]^+ Cl^- + 2H_2O$

2. Azo Coupling: The diazonium salt formed is an electrophile. It then undergoes an electrophilic aromatic substitution reaction with an activated aromatic compound, N,N-dimethylaniline. The coupling typically occurs at the para position to the activating group (the dimethylamino group in this case) due to steric and electronic factors.

   $[CH_3C_6H_4N_2]^+ Cl^- + (CH_3)_2NC_6H_5 \xrightarrow{pH \approx 4-5} CH_3C_6H_4N=NC_6H_4N(CH_3)_2 + HCl$

The resulting product is 4-(4-(dimethylamino)phenylazo)toluene.