Question

How will you obtain the following from benzene diazonium chloride ? Write chemical equations involved : (c) p-Aminoazobenzene

Hint:

Azo coupling is an electrophilic substitution reaction in which a diazonium ion couples with an activated aromatic ring. Aniline and phenol generally give para-substituted azo compounds as the major products.

Answer 1

Concept: Benzene diazonium chloride undergoes a very important reaction known as

azo coupling reaction. In this reaction, the diazonium ion acts as an electrophile and reacts with activated aromatic compounds such as phenol and aniline. The product formed contains the characteristic azo linkage: \[ -N=N- \] Compounds containing this linkage are known as azo compounds and many of them are intensely coloured dyes. Aniline is a highly activated aromatic compound because the amino group \(( -NH_2 )\) donates electron density to the benzene ring through resonance. As a result, electrophilic substitution occurs mainly at the para position.

Step 1: Coupling of benzene diazonium chloride with aniline. When benzene diazonium chloride is treated with aniline in a mildly acidic medium at low temperature, azo coupling occurs. \[ C_6H_5N_2^+Cl^- + C_6H_5NH_2 \rightarrow H_2NC_6H_4N=NC_6H_5 + HCl \]

Step 2: Orientation of substitution. The amino group present in aniline is an electron-donating group. It activates the benzene ring and directs incoming electrophiles towards the ortho and para positions. Among these two possibilities, the para product is formed predominantly because:

• Steric hindrance is minimum at the para position.

• The para product is more stable.

• Better resonance stabilization is possible.
Hence, the major product obtained is p-aminoazobenzene.

Step 3: Structure of the product. The structure of p-aminoazobenzene is: \[ \mathrm{H_2N-C_6H_4-N=N-C_6H_5} \] The molecule contains:

• One amino group \((-NH_2)\)

• One azo linkage \((-N=N-)\)

• Two benzene rings

Step 4: Significance of the reaction. Azo coupling reactions are extensively used in the manufacture of dyes, indicators and colouring agents. The extended conjugation present in azo compounds is responsible for their characteristic colour.

Conclusion: p-Aminoazobenzene is prepared by coupling benzene diazonium chloride with aniline in a mildly acidic medium. \[ \boxed{ C_6H_5N_2^+Cl^- + C_6H_5NH_2 \rightarrow p\text{-}H_2NC_6H_4N=NC_6H_5 + HCl } \] \[ \boxed{\text{Product : p-Aminoazobenzene}} \]