Azo-Coupling Reaction - Practice Questions & MCQ

Benzene diazonium salts react with highly reactive compounds such as phenols and amines to form brightly coloured azo compounds. This reaction is called coupling reaction. Coupling with phenols occurs in the basic medium(pH 9-10) and that of amines occur in a fairly acidic medium(pH 4-5) at 273-298K. It is to be noted that diazo coupling reaction is an example of Electrophilic aromatic substitution reaction and happens on activated Benzene rings.

Mechanism of diazotisation

For example,

Reduction in acidic medium
Both aliphatic and aromatic nitro compounds are reduced to corresponding primary amines with Sn/HCl or Fe/HCl by a combination of some active metals like tin, iron or zinc and conc. HCl or catalytic reduction with Ni, Pt or Pd/C. The reaction occurs as follows:

$\mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}_2+6 \mathrm{H} \xrightarrow[\text { or } \mathrm{Sn} / \mathrm{HCl}]{\mathrm{Fe} / \mathrm{HCl}} \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2+2 \mathrm{H}_2 \mathrm{O}$

Reduction in basic medium
Both aliphatic and aromatic nitro compounds are reduced to corresponding hydroxylamines in the neutral medium with zinc dust and NH₄Cl or Al-Hg couple.

Reduction in neutral medium
Both aliphatic and aromatic nitro compounds are reduced to corresponding hydroxylamines in the neutral medium with zinc dust and NH₄Cl or Al-Hg couple. The reaction occurs as follows:

$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NO}_2+4 \mathrm{H} \xrightarrow{\mathrm{Zn} / \mathrm{NH}_4 \mathrm{Cl}, \mathrm{H}_2 \mathrm{O}} \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NHOH}+\mathrm{H}_2 \mathrm{O}$ 

Mulliken barker test
The test is based on the fact that a neutral agent, -NO₂ is reduced to the NHOH group. The formed hydroxylamine reduces the Tollen's reagent and gets oxidised to a nitroso compound. The reaction occurs as follows:

1. By nitration of benzene with a mixture of concentrated nitric acid and concentrated sulphuric acid at temperature below 330K. The temperature should not increase otherwise m-dinitrobenzene is formed. So, nitration of benzene depends upon temperature and nature of the nitrating agent used. The reaction occurs as follows:
   
2. Nitrobenzene can be prepared by carrying our diazotization of aniline and then reacting with nitrous acid(HNO₂) in the presence of cuprous oxide. The reaction occurs as follows:
3. $\mathrm{C}_6 \mathrm{H}_5 \mathrm{~N}_2 \mathrm{Cl}+\mathrm{HNO}_2 \xrightarrow{\mathrm{Cu}_2 \mathrm{O}} \mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}_2+\mathrm{N}_2+\mathrm{HCl}$
4. Nitrobenzene can be prepared from the oxidation of aniline by trifluoro acetic acid. The reaction occurs as follows:$\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2+3[\mathrm{O}] \xrightarrow{\mathrm{CF}_3 \mathrm{COOH}} \mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}_2+\mathrm{H}_2 \mathrm{O}$
5. Nitrobenzene can be prepared by the action of acetyl nitrate on benzene.$\mathrm{C}_6 \mathrm{H}_5 \mathrm{H}+\mathrm{CH}_3 \mathrm{COONO}_2 \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}_2+\mathrm{CH}_3 \mathrm{COOH}$

Aniline and other aromatic amines are far less basic than ammonia and aliphatic amines. Aniline is a weak base as it forms salts with strong mineral acids. Weaker basic nature of aniline as compared to aliphatic amines can be explained on the basis of resonance. In aliphatic amines, the non-bonding electron pair of N is localized and is fully available for coordination with a proton. On the other hand, in aniline or other aromatic amines, the non-bonding electron pair is delocalized into benzene ring by resonance and the electron-donating capacity of nitrogen for protonation is considerably decreased to that of NH₃ and aliphatic amines.

Lower stability of anilinium ion than aniline
Anilinium ion formed by aniline on accepting a proton is less resonance stabilized than aniline.

Thus, the electron density is less on N-atom due to which aniline or other aromatic amines are less basic than aliphatic amines.