ELECTROSPRAY MASS-SPECTRAL STUDIES OF AROMATIC DIAZONIUM CATIONS .2. THE SURPRISING STABILITY OF SOME SUBSTITUTED 2-NITROBENZENE DIAZONIUM CATIONS TO COLLISIONALLY ACTIVATED FRAGMENTATION

Electrospray mass spectra of a series of substituted 2-nitrobenzene diazonium cations reveal a surprising stability of the intact ion to the collisionally activated loss of nitrogen, contrary to the behaviour of monosubstituted benzenediazonium ions reported previously. Ions bearing electron-releasing groups at the para position, e.g. 4-CH3, 4-CH3O, and 4-C6H5O, all experience loss of this para substituent rather than loss of nitrogen. However, ions bearing electron-withdrawing para substituents, e.g. 4-NO2 4-CN, 4-CF3 and 4-Cl, all undergo nucleophilic displacement of the 2-nitro group by water to give the corresponding phenol derivative, which subsequently loses nitrogen, The exceptional stability of these substituted 2-nitrobenzene diazonium salts is attributed to a favourable coulombic interaction between the positive charge on the diazonium group and the partial negative charge on one of the oxygen atoms of the nearby nitro group. Once that nitro group has been removed, the resulting substituted 2-hydroxybenzenediazonium cation loses nitrogen in the normal way.