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Reactions of Aromatic Compounds - Examples of Ar-SE Reaction


Much like in primary aliphatic amines, aminobenzene (aniline) is converted into a diazonium ion by treating it with a nitrous acid and mineral acid mixture. Formally, a nitrosyl cation (NO+) is then transferred to the amine.

Diazotization of aniline.

Aliphatic diazonium ions decompose spontaneously through an expulsion of nitrogen. In contrast, aromatic diazonium ions that are placed at below about 5 °C are relatively stable because the positive charge is delocalized and, thus, resonance-stabilized by the aromatic system.

Delocalization of the positive charge in aromatic diazonium ions.
Electrostatic potential surface and LUMO of an aromatic diazonium ion.

Results of a semiempirical-quantum-mechanical calculation. Left) Electrostatic potential surface (red means a more negative potential, blue means a more positive potential). Right) LUMO.

Nitrous acid, which is usually generated in situ by treating NaNO2 with a mineral acid, is further protonated by the mineral acid. Subsequently, water is expelled, thus yielding the active diazotizing nitrosyl reagent whose exact composition depends on the mineral acid that is applied.

Formation of the diazotizing nitrosyl reagent.

According to the mineral acid that is applied, the actually diazotizing reagent may be dinitrogen trioxide (X = NO2), a nitrosyl halide (X = Cl, Br), or ONOH2+ (X = H2O). These active species are nitrosyl-transferring reagents. In strong acidic solutions, the nitrosyl cation itself may act as a diazotizing reagent. Diazotization requires an unprotonated amine whose lone electron pair nucleophilically attacks the electrophilic nitrosyl reagent. Therefore, diazotization of aliphatic amines does not occur in strong acidic solutions, as almost all amino groups are protonated and do not possess a lone electron pair. However, aromatic amines can also be diazotized in strong acidic solutions, as they have a weaker basicity than aliphatic amines. Thus, sufficient unprotonated amine molecules remain for diazotization even in strong acidic solutions.

Nitrosyl transfer and diazonium ion formation.

The cation that has initially derived from the nitrosyl transfer converts into a nitrosamine through deprotonation. Consequently, the nitrosamine then tautomerizes to a diazohydroxide. Protonation of the diazohydroxide's hydroxy group and subsequent expulsion of water yield the diazonium ion.

Aromatic diazonium ions are relatively stable only in acidic and weak basic solutions, while they are transformed into diazohydroxides and, finally, into diazotate anions under moderately and strong basic conditions.

Conversion of a diazonium ion in basic solution.

Aromatic diazonium salts are even isolable. However, they are explosive in a dried state. Diazonium ions (salts) are starting products for both diazo coupling and Sandmeyer reaction, which enables the introduction of subsituents such as OH and F.

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