Consider the above reaction, compound B is:

When $$\mathrm{N,N\text{-}dimethylaniline}$$ is used instead of aniline in the coupling reaction with benzenediazonium chloride, an azo coupling reaction occurs.

The reaction is:

$$\mathrm{C_6H_5N_2^+Cl^- + C_6H_5N(CH_3)_2 \xrightarrow{H^+} C_6H_5-N=N-C_6H_4N(CH_3)_2 + HCl}$$

The diazonium ion acts as a weak electrophile and attacks the activated aromatic ring of $$\mathrm{N,N\text{-}dimethylaniline}$$.

The $$\mathrm{-N(CH_3)_2}$$ group is strongly activating and:

$$\mathrm{ortho/para\ directing}$$

Due to lower steric hindrance, substitution occurs mainly at the para position.

Thus, the product formed is:

$$\mathrm{p\text{-}(N,N\text{-}dimethylamino)azobenzene}$$

The final azo dye retains the two methyl groups attached to nitrogen.

This compound is commonly known as:

$$\mathrm{Butter\ Yellow}$$

which is a brightly coloured azo dye.