Chemistry Chemical Reactions and Importance of Diazonium Salts
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Topics Covered :

● Chemical Reactions
● Reactions Involving Displacement of Nitrogen
● Reactions Involving Retention of Diazo Group Coupling Reactions
● Importance of Diazonium Salts in Synthesis of Aromatic Compounds

Chemical Reactions :

The reactions of diazonium salts can be broadly divided into two categories, namely (`A`) reactions involving displacement of nitrogen and (`B`) reactions involving retention of diazo group.

Reactions involving displacement of nitrogen :

`=>` Diazonium group being a very good leaving group, is substituted by other groups such as `color{red}(Cl^–, Br^–, I^–, CN^–)` and `color{red}(OH^–)` which displace nitrogen from the aromatic ring. The nitrogen formed escapes from the reaction mixture as a gas.

(i) `color{green}(text(Replacement by halide or cyanide ion ))` : The `color{red}(Cl^–, Br^–)` and `color{red}(CN^–)` nucleophiles can easily be introduced in the benzene ring in the presence of `color{red}(Cu(I))` ion. This reaction is called `color{green}(text(Sandmeyer reaction))`.

`color{red}(Ar overset(+)N_2 overset(-)X overset(CuCl//HCl)→ ArCl +N_2)`

`color{red}(Ar overset(+)N_2 overset(-)X overset(CuBr//HBr)→ ArBr+N_2)`

`color{red}(Ar overset(+)N_2 overset(-)X overset(CuCN//KCN)→ ArCN +N_2)`

`=>` Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating the diazonium salt solution with corresponding halogen acid in the presence of copper powder. This is referred as `color{green}(text(Gatterman reaction))`.

`color{red}(Aroverset(+)N_2 overset(-)X overset(Cu//HCl)→ ArCl+N_2+CuX)`

`color{red}(Aroverset(+)N_2 overset(-)X overset(Cu//HBr)→ ArBr +N_2+CuX)`

`=>` The yield in Sandmeyer reaction is found to be better than Gattermann reaction.

(ii) `color{green}(text(Replacement by iodide ion ))` : Iodine is not easily introduced into the benzene ring directly, but, when the diazonium salt solution is treated with potassium iodide, iodobenzene is formed.

`color{red}(Aroverset(+)N_2 C overset(-)l +KI → ArI+KCl+N_2)`

(iii) `color{green}(text(Replacement by fluoride ion ))` : When arenediazonium chloride is treated with fluoroboric acid, arene diazonium fluoroborate is precipitated which on heating decomposes to yield aryl fluoride.

`color{red}(Aroverset(+)N_2 C overset(-)l +HBF_4 → Ar - overset(+)N_2 B overset(-)F_4 overset(Delta)→ Ar-F +BF_3+N_2)`

(iv) `color{green}(text(Replacement by H))` : Certain mild reducing agents like hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanal, respectively.

`color{red}(Aroverset(+)N_2 C overset(-)l +H_3PO_2+H_2O →ArH+N_2+H_3PO_3+HCl)`

`color{red}(Ar overset(+)N_2 C overset(-)l +CH_3CH_2OH → ArH +N_2+CH_3CHO+HCl)`

(v) `color{green}(text(Replacement by hydroxyl group ))` : If the temperature of the diazonium salt solution is allowed to rise upto `283 K`, the salt gets hydrolysed to phenol.

`color{red}(Ar overset(+)N_2 C overset(-)l +H_2O → ArOH +N_2 +HCl)`

(vi) `color{green}(text(Replacement by))` `color{red}(–NO_2)` `color{green}(text(group ))` : When diazonium fluoroborate is heated with aqueous sodium nitrite solution in the presence of copper, the diazonium group is replaced by `color{red}(–NO_2)` group. See fig.

Reactions involving retention of diazo group coupling reactions :

`=>` The azo products obtained have an extended conjugate system having both the aromatic rings joined through the `color{red}(–N=N–)` bond.

`=>` These compounds are often coloured and are used as dyes.

`=>` Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to form p-hydroxyazobenzene. This type of reaction is known as coupling reaction.

`=>` Similarly the reaction of diazonium salt with aniline yields p-aminoazobenzene. This is an example of electrophilic substitution reaction.

Importance of Diazonium Salts in Synthesis of Aromatic Compounds :

`=>` From the above reactions, it is clear that the diazonium salts are very good intermediates for the introduction of `color{red}(–F, –Cl, –Br, –I, –CN, –OH, –NO_2)` groups into the aromatic ring.

`=>` Aryl fluorides and iodides cannot be prepared by direct halogenation.

● The cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene but cyanobenzene can be easily obtained from diazonium salt.

`=>` Thus, the replacement of diazo group by other groups is helpful in preparing those substituted aromatic compounds which cannot be prepared by direct substitution in benzene or substituted benzene.
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