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Reactions of Aromatic Compounds - Examples of Ar-SE Reaction

Diazonium Coupling (Diazo Coupling, Azo Coupling)

Due to their positive charge, diazonium cations, which are generated by treatment of aromatic amines with nitrous acid and a stronger mineral acid, may participate in an electrophilic aromatic substitution as an electrophile. The electrophilic reaction center is the terminal nitrogen of the -N=N+ group. As a result, two aromatic compounds are coupled by a -N=N- group. This is known as the azo group (diazo group). The corresponding reaction is called diazonium coupling (diazo coupling, azo coupling). However, the electrophilicity of diazonium ions is only relatively weak, as their positive charge is delocalized. The unsubstituted benzenediazonium cation may react only with strongly activated aromatic compounds, such as phenolates and amines.

Fig.1
Diazonium coupling with a phenolate.

By introducing electron-withdrawing substituents in ortho or para position regarding the azo group, the diazonium ions' electrophilicity may be increased to such a degree that diazonium coupling also occurs with phenols and phenolic ethers, such as anisole.

Fig.2
Increased diazonium cations' electrophilicity by electron-withdrawing substituents.
Fig.3

Electrostatic potential surfaces of the benzenediazonium cation.

Fig.4

Electrostatic potential surfaces of the p-nitrobenzenediazonium cation.

Comparison of the electrostatic potential surfaces illustrates the stronger diazonium group's positive polarization by the electron-withdrawing nitro group (red means a more negative potential, blue means a more positive potential).

Due to the fact that diazonium cations are poor electrophiles and relatively bulky species, mainly para substitution usually takes place in diazonium coupling. In the case of para substitution, steric hindrance is at its weakest, while the positive charge's stabilization is at its largest, in the σ complex (and, thus, in the transition state). If the para position is already occupied by another substituent, ortho substitution occurs.

When aromatic amines are coupled with diazonium cations, diazonium coupling competes with a nucleophilic attack of the amine's nitrogen on the diazonium cation's terminal nitrogen, as the strength of the resulting N-N single bond does not considerably differ from that of the C-N single bond. As a result, coupling of diazonium cations with secondary amines partially yields N-azo compounds, otherwise known as triazenes. When primary amines are applied in diazonium coupling, even almost only triazenes are obtained. In contrast, such a competition does not occur when phenols or phenolates are applied in diazonium coupling, since the N-O and C-O single bond strengths differ noticeably.

Fig.5
Reaction of diazonium cations with primary amines.

However, triazenes can be isomerized to the corresponding, thermodynamically more stable C-azo compounds through heating under acidic conditions. The reaction takes an intermolecular course and may be promoted by an amine excess.

Fig.6
Conversion of triazenes into C-azo compounds.

Diazonium coupling is decisively influenced by the pH value of the reaction mixture. Strong basic solutions must be prevented, as diazonium cations are stable only under acidic and moderately basic conditions. Diazonium coupling in connection with amines is carried out in faintly acidic solutions in order to ensure an as high as possible diazonium cation's as well as unprotonated aromatic amine's concentration. The result of low pH values is a largely protonated and, thus, deactivated aromatic amine, while higher pH values would cause lower diazonium cation concentration. In contrast, if a phenol is applied to diazonium coupling, the reaction is carried out in a weak basic solution in order to increase the concentration of the phenolate anion that is considerably more reactive.

The importance of diazonium coupling is that aromatic diazo compounds are usually intensively colored, as they possess an extensive conjugated π electron system. Therefore, many aromatic diazo compounds are utilized as dyes. These dyes are called azo dyes. Azo dyes that are applied to dyeing textiles often contain a sulfo group (SO3H-) in order to increase their water solubility and additional functional groups that are used for binding them to the textile fibre.

Fig.7
Congo red - an azo dye.

Some azo dyes are well-known acid-base indicators, as their colour is altered by protonation and deprotonation, respectively.

Fig.8
Methyl orange - an azo dye and acid-base indicator.
Diazotization of aniline and subsequent diazonium coupling with dimethylaniline and β-naphthol.
Some diazonium couplings.
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