Determination of the site of protonation of substituted benzenes in water chemical ionization mass spectrometry

The site of protonation of a substituted benzene may be determined using chemical onization mass spectrometry with D₂O as a reagent gas. The observation of extensive exchange of the ring hydrogens for deuteriums is linked to protonation on the benzene ring. The lack of this exchange coupled with the formation of cluster ions (the association of the protonated species with one or more D₂O molecules) is evidence of protonation on the substituent rather than the ring. Aniline, benzaldehyde and nitrobenzene are observed to protonate at the substituent while toluene, bromobenzene, biphenyl and iodobenzene protonate on the ring. The dimethylbenzenes protonate on the ring while the diaminobenzenes protonate at one of the substituents. The dihydroxybenzenes, as well as a number of other compounds in which an oxygen is attached directly to the ring, protonate predominantly at the substituent although a small amount of exchange of one ring hydrogen is observed.