CHEMICAL-IONIZATION MASS-SPECTROMETRY OF SOME NITRO-CONTAINING BIFUNCTIONAL AROMATIC-COMPOUNDS - LOCATION OF PROTONATION SITE

It is found that the nitro substituent of some aromatic bifunctional c ompounds shows unusual reactivity towards protonation. In the chemical ionization mass spectra of nitrobenzoic acids and their esters and am ides, and of nitrophenols and their ethers, protonations on the carbox yl, ester, amide, hydroxyl or alkoxyl groups are highly suppressed by that on the nitro group. As a result, fragmentations based on protonat ion on these groups unexpectedly become negligible. Ortho effects were observed for all the ortho isomers where the initial protonation on t he nitro group is followed by an intramolecular proton transfer reacti on, which leads to the expected 'normal' fragmentations. Protonation o n the nitro substituent is much more favourable in energy than on any of the other substituents. The interaction of the two substituents thr ough the conjugating benzene ring is found to be responsible for this 'unfair' competitive protonation. The electron-attracting nitro group strongly destabilizes the MH+ ions formed through protonation on the o ther substituent; although the COR (R = OH, OMe, OEt, NH2) groups are also electron-withdrawing, their effects are weaker than that of NO2; thus protonation on the latter group produces more-stable MH+ ions. On the other hand, an electron-releasing group OR (R = H, Me, Et) stabil izes the nitro-protonated species; the stronger the electron-donating effect of this group the more stable the nitro-protonated ions.