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Aniline when diazotized in cold and then treated with dimethyl aniline gives a coloured product. Its structure would be :-
A
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B
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C
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D
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Detailed Solution
Diazotisation: aniline reacts with nitrous acid ($NaNO_2 + HCl$) at 273–278 K to give benzenediazonium chloride:
$C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278\,K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$
Coupling: the diazonium ion is an electrophile. It attacks the para position of N,N-dimethylaniline, which is strongly activated by the $-N(CH_3)_2$ group:
$C_6H_5N_2^+Cl^- + C_6H_5N(CH_3)_2 \rightarrow C_6H_5-N=N-C_6H_4-N(CH_3)_2\,(p) + HCl$
The product, p-(dimethylamino)azobenzene (butter yellow), is a coloured azo dye because of the extended conjugation through the $-N=N-$ group.
So its structure is $(CH_3)_2N-C_6H_4-N=N-C_6H_5$.
$C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278\,K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$
Coupling: the diazonium ion is an electrophile. It attacks the para position of N,N-dimethylaniline, which is strongly activated by the $-N(CH_3)_2$ group:
$C_6H_5N_2^+Cl^- + C_6H_5N(CH_3)_2 \rightarrow C_6H_5-N=N-C_6H_4-N(CH_3)_2\,(p) + HCl$
The product, p-(dimethylamino)azobenzene (butter yellow), is a coloured azo dye because of the extended conjugation through the $-N=N-$ group.
So its structure is $(CH_3)_2N-C_6H_4-N=N-C_6H_5$.