Question

Arrange the following compounds in decreasing order of coupling with benzene diazonium chloride.

A. IV > I > II > III
B. II > I > IV > III
C. II > IV > I > III
D. II > III > IV > I

Answer 1

Hint: Benzene diazonium chloride couples with highly reactive aromatic compounds containing electron-releasing groups such as $-OH,-N{{H}_{2}}$ etc. Bright-colored azo compounds are found as a product. The rate of coupling depends upon the strength of electron donating groups present in the systems.

Complete step by step solution:
The benzene diazonium chloride or diazonium salts have one diazo group ($-{{N}^{+}}\equiv N:$) and have a formula $ArN_{2}^{+}C{{l}^{-}}$. The structure of benzene diazonium chloride is:

 This type of coupling reaction is known as azo coupling. Here diazonium cation with a positive charge on the nitrogen atom acts as an electrophile and electron-rich arene compounds act as a nucleophile. The overall reaction process is an example of an electrophilic substitution reaction.

Consider a general azo-coupling reaction:

Here R = electron donating group (-OH,$-N{{H}_{3}}$)
X= halogen ion

In the given questions, there are four aromatic compounds with different groups attached to them. To answer the given question, first, we have to see the order of electron-donating ability of the given compounds. Except $-NH_{3}^{+}$, all three ($-OH,-{{O}^{-}}and-N{{H}_{2}}$) are electron-releasing groups and we know that diazonium salts react with electron-rich compounds more effectively. A positive charge on the nitrogen atom increases its electron-withdrawing power and undergoes a coupling reaction with the diazonium ion very slowly.

In compound (II), phenoxide is more reactive than (I)[phenol] and (IV) [aniline] because the electron density is concentrated onto a single atom of oxygen and it is very easy to donate.


Between phenol and aniline, aniline couples with diazonium salts at a faster rate, since the electronegativity of nitrogen is less than oxygen, and can act as a very good nucleophile in the coupling reaction. Therefore the decreasing order of coupling with diazonium chloride follows the order:
(II)>(IV)>(I)>(III)
Thus, option (C) is correct.

Note: Generally, coupling occurs at para position to electron releasing group. If the para position is blocked then coupling occurs at the ortho position. The presence of the electron-withdrawing group on the diazonium group also facilitates the coupling reaction as it increases the electrophilicity of the diazonium ion.