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 compounds shows unusual reactivity towards protonation. In the chemical ionization mass spectra of nitrobenzoic acids and their esters and amides, and of nitrophenols and their ethers, protonations on the carboxyl, ester, amide, hydroxyl or alkoxyl groups are highly suppressed by that on the nitro group. As a result, fragmentations based on protonation on these groups unexpectedly become negligible. Ortho effects were observed for all the ortho isomers where the initial protonation on the nitro group is followed by an intramolecular proton transfer reaction, which leads to the expected ‘normal’ fragmentations. Protonation on the nitro substituent is much more favourable in energy than on any of the other substituents. The interaction of the two substituents through 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 other substituent; although the COR (R OH, OMe, OEt, NH 2 ) groups are also electron‐withdrawing, their effects are weaker than that of NO 2 ; thus protonation on the latter group produces more‐stable MH + ions. On the other hand, an electron‐releasing group OR (R H, Me, Et) stabilizes the nitro‐protonated species; the stronger the electron‐donating effect of this group the more stable the nitro‐protonated ions.