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Diazonium Salts and Coupling Reactions
Concepts tested here
- Diazotisation and azo coupling
All Questions
2008 AIPMT 1 question
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In a reaction of aniline a coloured product (C) was obtained.
The structure of (C) would be -Step 1 (diazotisation): aniline reacts with nitrous acid ($NaNO_2 + HCl$) in the cold to give benzenediazonium chloride.
$C_6H_5NH_2 \xrightarrow{NaNO_2,\ HCl} C_6H_5-N\equiv N^+Cl^-$ (B)
Step 2 (coupling): the benzenediazonium ion is a weak electrophile. It attacks the strongly activated ring of N,N-dimethylaniline in the cold.
The $-N(CH_3)_2$ group is ortho, para directing; because of its bulk the attack takes place at the para position.
$C_6H_5-N_2^+Cl^- + C_6H_5-N(CH_3)_2 \xrightarrow{\text{Cold}} C_6H_5-N=N-C_6H_4-N(CH_3)_2 + HCl$
The product (C) is p-dimethylaminoazobenzene, a yellow azo dye known as butter yellow.
The colour is due to the extended conjugation through the $-N=N-$ (azo) group.