Concept:
The given reaction involves the reaction of a benzene diazonium chloride with phenol in an alkaline medium. Such reactions are known as
azo coupling reactions.
Azo coupling is an electrophilic aromatic substitution reaction in which a diazonium salt acts as an electrophile and couples with activated aromatic rings such as phenol or aniline to form coloured azo compounds containing the azo linkage
\[
-N=N-
\]
These compounds are known as azo dyes and are extensively used in the dye industry.
Step 1: Identifying the reactants.
The first reactant is benzene diazonium chloride:
\[
C_6H_5N_2^{+}Cl^{-}
\]
The second reactant is phenol:
\[
C_6H_5OH
\]
The reaction is carried out in the presence of
\[
OH^-
\]
which converts phenol into the more reactive phenoxide ion.
Step 2: Understanding the role of alkaline medium.
Phenol reacts with hydroxide ion to form phenoxide ion:
\[
C_6H_5OH + OH^-
\rightarrow
C_6H_5O^- + H_2O
\]
The phenoxide ion has a higher electron density on the aromatic ring due to resonance.
As a result, the ortho and para positions become highly activated towards electrophilic attack.
Step 3: Attack of the diazonium ion on the activated ring.
The diazonium ion
\[
C_6H_5N_2^+
\]
acts as an electrophile.
Because the para position of phenoxide ion is less sterically hindered than the ortho position, coupling occurs predominantly at the para position.
The azo linkage
\[
-N=N-
\]
is formed between the two benzene rings.
Step 4: Formation of the major product.
The major product obtained is
\[
p\text{-Hydroxyazobenzene}
\]
having the structure
\[
\boxed{
C_6H_5-N=N-C_6H_4-OH
}
\]
where the hydroxyl group is present para to the azo linkage.
Step 5: Writing the completed reaction.
\[
\boxed{
C_6H_5N_2^{+}Cl^{-}
+
C_6H_5OH
\xrightarrow{OH^-}
C_6H_5-N=N-C_6H_4-OH
+
HCl
}
\]
This compound is an azo dye and generally exhibits an intense yellow-orange colour.
Final Answer:
\[
\boxed{
C_6H_5-N=N-C_6H_4-OH
}
\]
\[
\boxed{\text{p-Hydroxyazobenzene}}
\]
