Question

Why is meta diazonium anisole have more reactivity than diazonium?

Answer 1

Aryl diazonium salts are important intermediates. They are prepared in cold (0 º to 10 ºC) aqueous solution, and generally react with nucleophiles with loss of nitrogen. Some of the more commonly used substitution reactions are shown in the following diagram. Since the leaving group (N2) is thermodynamically very stable, these reactions are energetically favored. Those substitution reactions that are catalyzed by cuprous salts are known as Sandmeyer reactions. Fluoride substitution occurs on treatment with BF4(–), a reaction known as the Schiemann reaction. Stable diazonium tetrafluoroborate salts may be isolated, and on heating these lose nitrogen to give an arylfluoride product. The top reaction with hypophosphorus acid, H3PO2, is noteworthy because it achieves the reductive removal of an amino (or nitro) group. Unlike the nucleophilic substitution reactions, this reduction probably proceeds by a radical mechanism.

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Answer 2

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.