The proton affinities of toluene

The reactions of trichloromethylium (CCl3+) with benzene and the lower alkyl aromatics (ArH) have been studied by high pressure mass spectrometry at pressures in the range 2–4 Torr and temperatures from 300 to 560 K. The only two primary products are the adduct ArHCCl3+ and ArCCl2+, which is formed by loss of HCl from the adduct. The relative yields of adduct increase with increasing number of methyl substituents on the aromatic ring (benzene → mesitylene). The disappearance of CCl3+ is kinetically second order with specific rate constants increasing from benzene to mesitylene, the latter reacting essentially at every ion–molecule collision. All rate constants are fairly large (>1010 cc molecule−1 s−1) and show negative temperature coefficients. ArCCl2+ is unreactive but ArHCCl3+ reacts further by proton transfer to ArH.

Section: Ch3 Isupporting

confidence: 88%

“…The stabilization afforded by the methyl group of toluene protonated at different ring positions relative to protonated benzene has been estimated (17). The values are (kcal mol-I) ipso 0.3, ortho 6.5, meta 2.0, and para 8.5.…”

Section: Ch3 Imentioning

confidence: 99%

The gas phase reactions of the trichloromethylium ion with alkyl aromatics, studied by high pressure mass spectrometry

Stone¹,

Moote²,

Wojtyniak³

1985

“…The computed values, in conjunction with the available experimental (15,17) and theoretical (18) results, can be used to derive reasonably accurate estimates of the energetics of the processes of interest, reported in Table 8.…”

Section: The Ionic Reactions and Their Energeticsmentioning

confidence: 99%

A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles

Cacace¹,

Crestoni²,

Petris³

et al. 1988

Chem. 66, 3099 (1988).Aromatic desilylation and detertbutylation by CH5+, CzH5+, i-C3H7+, and t-C4H9+ ions have been studied in the gas phase by mass spectrometric and radiolytic techniques. The higher rate of desilylation than of dealkylation is traced to the step involving the formation of ipso arenium ions, protonated at the ring carbon bearing the SiMe, or the CMe3 substituent. The latter has been shown by a b initio calculations at the SCF STO-3G level to selectively depress the basicity of the ipsoposition, hence the stability of the corresponding arenium ion relative to its protomers, e.g. the basicity of the ipso position of PhCMe, is lower by ca. 1 1 kcal mol-' than of the para position. The %Me3 group has no comparable effect, very close basicities being computed for the para and the ipso position of PhSiMe,. The different effects of the CMe3 and SiMe3 groups are traced to electronic and steric factors.FULVIO CACACE, M. ELISA CRESTONI, GIULIA DE PETRIS, SIMONETTA FORNARINI et FELICE GRANDINETTI. Can. J. Chem. 66, 3099 (1988).Operant en phase gazeuse et faisant appel 2 des techniques de spectromttrie de masse et de radiolyse, on a ttudiC les rCactions de dt-tert-butylation et de dCsilylation aromatiques par les ions CH5+, C2H5+, i-C3H7+, et tert-C4H9+. Le fait que la vitesse de dCsilylation est plus rapide que celle de ladealkylation est attribue a l'Ctape impliquant la formation des ions ipso arCnium qui sont protones au niveau du cycle carbone portant les substituants SiMe, ou CMe3. Faisant appel a des calculs ab initio, en champ auto-cohdrent, au niveau STO-3G, on a dCmontre que, par rapport A ses protomeres, le dernier abaisse la basicitt de la position ipso et, par constquent, la stabilitC de l'ion arCnium correspondant; par exemple, la basicitt de la position ipso du PhCMe, est environ 11 kcal mol-' plus faible que celle de la position para. Le groupement SiMe3 n'a pas d'effet semblable et les calculs fournissent des valeurs trks comparables pour les positions para et ipso du PhSiMe3. On attribue les effets difftrents des groupements CMe3 et SiMe, a des facteurs Clectronique et sttrique. [Traduit par la revue] IntroductionElectrophilic cleavage of aryl-silicon bonds, observed as early as in 1901 (I), has been characterized by the systematic work of Eaborn (2, 3) as a typical aromatic substitution, whose rate-determining step is the formation of the ipso u complex.We report here extension of the work to the gas phase by an integrated approach based on the coordinated application of mass spectrometric, radiolytic, and computational techniques (4). The same approach has recently allowed us to demonstrate the first example of the inverse process, the direct aromatic substitution by SiMe3+ ( 5 , 6), unprecedented in solution, nor previously characterized in the gas phase.In order to provide a proper perspective, and in view of the close correlations between carbon and silicon chemistry, a parallel investigation has been carried out of gas-phase aromatic detertb~t~lation, the subject of earlier detaile...

“…Toluene protonates in thepara position although ortho protonation is energetically only slightly less favoured (15). It is therefore not surprising that p-F-toluene has the lowest proton affinity since the protonation site will necessarily change and become ortho to methyl but meta to fluorine.…”

Section: Proton Afjnities Of Toluene and Thefiuorotoluenesmentioning

confidence: 99%

“…The energetically favoured protonation site in toluene is para to CH, (15). The location of a substituent on the toluene ring is therefore expected to have a marked effect on proton affinity whereas for a n complex, whose binding site involves at least two ring C atoms, a different effect of the same substituent might be expected.…”

Section: Introductionmentioning

confidence: 99%

A comparison of the relative binding energies of H⁺ and NO⁺ to aromatic and haloaromatic bases by high pressure mass spectrometry

Stone¹,

Splinter²,

Kong³

1982

Proton transfer equilibria (BIH+ + B, e B , + B,H+)and NO+ transfer equilibria(B,NO+ + B, *B, + B,NO+) have been studied for the following bases B, benzene, toluene, o -, m -, and p-xylene. NO+ transfer equilibria for fluoro-and chlorobenzene have also been studied. Pulsed electron beam, high-pressure ion source mass spectrometry has been used to obtain the equilibrium constant K and hence the free energy changes AGO and from van't Hoff plots, AH0 and AS0. Entropy changes are in general much smaller for NO+ transfer than for H+ transfer but the magnitude of the changes in the proton affinity and NO+ affinity of toluene caused by a fluorine substituent is about the same, even though the absolute value of the proton affinity is greater by a factor of4. The position of the F substituent on toluene has a marked effect on proton affinity but noeffect on NO+ affinity. The latter appears to be responsive only to the inductive effect. [Traduit par le journal]