Ortho Elimination in the Fragmentation of Nitroarenes

Nitroarenes are recognized from their characteristic neutral losses due to the NO2 substituent. Normally, all theoretically possible fragment ions, the plausible [M-N02] and [M-O] ions as well as the unexpected [M-NO] ion, are observed. It is worth noting that molecular ions are 1,2-distonic by definition, because nitroarene molecules are best represented as zwitterion (Chap. 6.3). The molecular ion may either dissociate directly by loss of an oxygen atom or a NO2 molecule or it may rearrange prior to loss of NO. For the latter process, two reaction pathways have been uncovered, one of them involving intermediate formation of a nitrite, and the other proceeding via a three-membered cyclic intermediate. [208] 

Fragmentation of the nitrite then yields a phenolic-type ion that can decompose further by elimination of CO. Thus, the characteristic series of [M-ld] (generally weak signal), [M-30] and [M-46] ions is obtained (Fig. 6.49a,b). 

Ort/io-substituted nitroarenes can be distinguished from meta- or para-substituted isomers due to a characteristic change in their mass spectra. In precence of a hydrogen-donating ortho subtituent, [M-OH] replaces the [M-O] the fragment ion, e.g., in case of the above nitrophenols, the ion at m/z 123 is replaced by one at m/z 122 (Fig. 6.49c). The mechanism of this OH loss is not quite clear, [209] because different proposals [210-212] are in good accordance with the experimental data. Here, the mechanism which resembles the previously discussed ortho eliminations is shown [212] 

Ortho eliminations are widely considered in the structure elucidation of aromatic nitro compounds such as nitroanilines [175], dinitrophenols [189], or trini-troaromatic explosives [190]. 

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