Electrophilic Aromatic Cl+ Addition And Cȯ+ Substitution In The Gas Phase

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Chlorine cation addition to benzene, aniline, anisole, styrene, chlorobenzene, and nitrobenzene was studied using NH3Cl+, ClC≡O+, protonated CH3Cl, and Cl+ as reagent ions. The reactions of protonated monochloramine were followed using a direct insertion membrane probe for sample introduction and a pentaquadrupole mass spectrometer for product characterization. The other reagent ions ClC≡O+, Cl+, and protonated CH3Cl were generated by electron ionization of acetyl chloride and carbon tetrachloride and by methane chemical ionization of CH3Cl, respectively. The main reactions of NH3Cl+ with aromatic compounds are electrophilic Cl+ and H+ addition and charge exchange to form the aromatic radical cation. Reactions of ClC≡O+ with aromatic compounds include (i) Cl+ addition, (ii) CȮ+ substitution for a hydrogen atom, and (iii) formation of the molecular radical cation of the substrate. The naked Cl+ ion does not chlorinate aromatic compounds but does undergo charge exchange. Protonated CH3Cl also fails to add Cl+ to the aromatic compounds, proton transfer being the main reaction observed. Ion/molecule reaction products were characterized by comparing sequential product ion mass spectra (MS/MS/MS) to the MS/MS product ion mass spectra of reference ions, generated by chemical ionization of appropriate chlorine-substituted compounds. The sequential product spectra collected with the pentaquadrupole instrument show that both the Cl+ addition products and the CȮ+ substitution products are σ-bonded to the aromatic compound. Comparisons with the MS/MS spectra of model ions suggest that both Cl+ and CȮ+ add principally to the para position in aniline. Reaction occurs at the same position for anisole, although contributions from reactions at other sites are not excluded. Substitution of hydrogen by CȮ+ in aniline and anisole also proceeds principally at the para position, although it also occurs at the nitrogen of aniline. Evidence is given for Cl+ binding to the β-carbon in styrene and to the ring in chlorobenzene. Nitrobenzene, the least reactive compound, gave only traces of a Cl+ addition product and did not undergo substitution of CȮ+ for hydrogen. However, it did display one unique reaction, the substitution of NO2 ̇ by Cl+. The evidence provided by the MS3 experiments for the site of Cl+ addition was tested against - and found to be consistent with - the sites predicted to have the highest Cl+ affinity by semiempirical AM1 molecular orbital calculations.115310041014202327/2013-2; CNPq; Conselho Nacional de Desenvolvimento Científico e Tecnológico; 302997/2013-0; CNPq; Conselho Nacional de Desenvolvimento Científico e Tecnológico; 304925/2010-1; CNPq; Conselho Nacional de Desenvolvimento Científico e Tecnológico; 484035/2013-4; CNPq; Conselho Nacional de Desenvolvimento Científico e Tecnológico; FAPESP; Conselho Nacional de Desenvolvimento Científico e TecnológicoFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Gas-phase Cl+ Affinities Of Pyridines Determined By The Kinetic Method Using Multiple-stage (ms3) Mass Spectrometry

The relative gas-phase halogen cation affinities of a group of substituted pyridines have been ordered, and absolute Cl+ affinity values have been estimated. The Cl+-bound dimer of two pyridines is generated in an ion/molecule reaction using mass-selected Cl-C≡O+ as the chlorinating agent, and its competitive fragmentations to yield the Cl+-pyridine monomers are monitored by multiple-stage (MS3) experiments. These data yield approximate Cl+ affinities which show an excellent linear correlation with literature proton affinity (PA) values. The relationship Cl+ affinity (kcal/mol) = 0.83PA - 42.5 between the two affinities is derived, and both slope and intercept are rationalized in terms of the greater polarizability of Cl+ ion. While proton affinities are unaffected by hindrance near the bonding site in the corresponding proton-bound dimers, the affinities for the larger Cl+ ion are significantly decreased by intramolecular steric effects in those Cl+-bound dimers which involve ortho-substituted pyridines. Electronic effects are separated from steric effects by comparing the fragmentation of the Cl+- and H+-bound dimers composed of a hindered and an unhindered pyridine. In this way, ortho substituents are ordered in terms of the magnitudes of their steric effects. The intramolecular steric effects of ortho substituents, defined here as a gas-phase steric parameter Sk, are found to increase, not only with the size of the substituent but also as the Cl+ affinity of the pyridine increases, due to shortening of the N-Cl+ bond. The Sk values are found also to fall in the same order as the corresponding S0 steric parameters obtained by solution kinetic measurements. Exceptions occur for 2-methoxypyridine and quinoline, where an additional, through-space electronic interaction between the electron-rich substituent and Cl+ is proposed. The methodology used to order Cl+ affinities can be extended to Br+ and I+ affinities, and, in the cases examined, the magnitude of the steric effect falls in the order Br+ > I+ ≃ Cl+ ≫ H+. The intramolecular steric effect in the I+-bound dimers is reduced because of the long N-I bond. The quality of the data obtained is such that it is possible to predict with an estimated uncertainty of 2 kcal/mol Cl+ affinities for compounds which were not examined. To check further on the experimental data and predictions, semiempirical AMI molecular orbital calculations are used to estimate absolute values of Cl+ affinities. An excellent correlation is obtained between the experimental values and the AM1 Cl+ affinities of unhindered pyridines. The calculation overestimates the Cl+ affinities of the hindered pyridines, and this confirms that steric, not electronic, effects are responsible for the decreases observed in the Cl+ affinities of ortho-substituted pyridines. Ab initio MP2/6-31G(d,p)//6-31G(d,p) molecular orbital calculations are used to confirm that Cl+ addition to pyridine occurs at the nitrogen and that the lowest energy structure of the Cl+-bound dimer is the N-Cl+-N-bound species.11662457246