Studies of the exchange reaction between deuterium oxide and substituted benzenes and of the clustering reaction between water and substituted phenols in chemical ionization mass spectrometry

Evidence is presented for the protonation site in substituted benzenes. Benzene itself was found to undergo an exchange of ring hydrogens only when protonated. In addition, benzene did not undergo a clustering reaction with water. Based on similar behavior, benzene, toluene, bromobenzene, iodobenzene, fluorobenzene, the xylenes, mesitylene, and the di- and tri-fluorobenzenes were found to protonate on the ring. Nitrobenzene, anisole, phenol, benzaldehyde, and aniline were found to protonate on the substituent. These results are consistent with theoretical predictions for benzene, toluene, and the xylenes. However, discrepancies exist for anisole and phenol (in coomparison to calculations done on the cresols and methylanisoles). The result for anisole also disagrees with a determination by an ICR technique. Kinetics of the exchange reaction for benzene, toluene, the xylenes, and mesitylene were observed. Based on a simple model of consecutive pseudo-first order reactions, rate constants for each step in the reaction sequence were determined. The possibility of a more complicated mechanism is also discussed. The clustering reaction between water molecules and a chemical ionization sample has been studied for a series of substituted phenols. In the nitrophenols, protonation at the nitro group is favored whereas in the methylphenols, ethylphenols, chlorophenols, hydroxphenols, and aminophenols, protonation at the hydroxy group is favored. In the pulsed ionization mode equilibrium effects dominated the spectra obtained in the continuous ionization mode