Question

Which of the following converts benzene diazonium chloride to benzene?
A. ${{H}_{3}}P{{O}_{2}}$
B. $CuCN$
C. ${{H}_{2}}O$
D. $HB{{F}_{4}}$

Answer 1

When a diazonium salt is heated with a reagent, the diazonium group is replaced by the negative part of the reagent i.e. if it is heated with a reducing agent, the diazonium group is replaced by H giving benzene as the product.

Complete step by step answer:
Diazonium salts are a group of organic compounds that share a common functional group $R-{{N}_{2}}^{+}{{X}^{-}}$ where, R denotes an organic residue i.e. an alkyl or aryl group and X denotes an organic or inorganic anion (for example a halogen). So, diazonium salts undergo two types of chemical reactions i.e. reactions which involve the replacement of nitrogen with nucleophiles and the other is the reactions which involve retention of the diazo group.
As per the given question, let’s see how benzene diazonium chloride with chemical formula $[{{C}_{6}}{{H}_{5}}{{N}_{2}}]Cl$reacts with the given reagents:

- With ${{H}_{3}}P{{O}_{2}}$ which is a reducing agent, the diazonium group present in the diazonium salt will be replaced by H forming benzene, a strong acid i.e. hydrogen chloride and nitrogen.
$[{{C}_{6}}{{H}_{5}}{{N}_{2}}]Cl\xrightarrow{{{H}_{3}}P{{O}_{2}}}{{C}_{6}}{{H}_{6}}+HCl+{{N}_{2}}$

- With $CuCN$, the diazonium group in the salt will be replaced by cyanide group showing the chemical following reaction,
$[{{C}_{6}}{{H}_{5}}{{N}_{2}}]Cl\xrightarrow{CuCN}{{C}_{6}}{{H}_{5}}CN+{{N}_{2}}$

- With ${{H}_{2}}O$, the benzene diazonium salt will get hydrolysed to phenol showing the respective reaction,
$[{{C}_{6}}{{H}_{5}}{{N}_{2}}]Cl\xrightarrow{{{H}_{2}}O}{{C}_{6}}{{H}_{5}}OH+HCl+{{N}_{2}}$

- With $HB{{F}_{4}}$, just like the reaction with $CuCN$the diazonium group will be replaced by the fluoroborate forming benzene fluoroborate. The respective chemical equation shows the reaction,
$[{{C}_{6}}{{H}_{5}}{{N}_{2}}]Cl\xrightarrow{HB{{F}_{4}}}{{C}_{6}}{{H}_{5}}{{N}_{2}}B{{F}_{4}}+HCl$
So, we can observe that benzene is formed when benzene diazonium chloride reacts with ${{H}_{3}}P{{O}_{2}}$. So, the correct answer is “Option A”.

Note: Possibly you can get confused with option C but when diazonium salt reacts with ${{H}_{2}}O$, the diazonium group is replaced by the hydroxyl group (i.e. OH group) forming phenol. Remember when any diazonium salt reacts with a reducing agent, it is reduced by H forming benzene.