Acidity measurements from protonation of aromatics. Protonation of hexamethylbenzene by trifluoromethanesulfonic acid and its use to relate acidities of strong superacids to acidities of acids included in the traditional Hammett scale

Fǎrcaşiu, D.; Marino, Gaye; Miller, Glen; Kastrup, Rodney V.

doi:10.1021/ja00200a047

Cited by 25 publications

(20 citation statements)

References 2 publications

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“…A demonstration of the strength of carborane acids is their ready protonation of benzene. Triflic acid does not protonate benzene 26 and the strongest known neat liquid acid, HFSO 3 , (Hammett acidity function H 0 5 215.1) does so only to a very small extent. 27 Previously, mixed Brønsted/Lewis acids such as HF/SbF 5 were necessary to attain acidity high enough to protonate benzene, but this comes at a price.…”

Section: Protonated Arenesmentioning

confidence: 99%

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

2005

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Icosahedral carborane anions such as CHB 11 Cl 11 2 are amongst the least coordinating, most chemically inert anions known. They are also amongst the least basic, so their conjugate acids, H(carborane), are superacids (i.e. stronger than 100% H 2 SO 4 ). Acidity scale measurements indicate that H(CHB 11 Cl 11 ) is the strongest pure Brønsted acid presently known, surpassing triflic and fluorosulfuric acid. Nevertheless, it is also an extremely gentle acid-because its conjugate base engages in so little chemistry. Carborane acids separate protic acidity from anion nucleophilicity and destructive oxidative capacity in the conjugate base, to a degree not previously achieved. As a result, many long-sought, highly acidic, reactive cations such as protonated benzene (C 6 H 7 + ), protonated C 60 (HC 60 +), tertiary carbocations (R 3 C + ), vinyl cations (R 2 CLC + -R), silylium ions (R 3 Si + ) and discrete hydronium ions (H 3 O + , H 5 O 2 + etc.) can be readily isolated as carborane salts and characterized at room temperature by X-ray crystallography.

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Section: Protonated Arenesmentioning

confidence: 99%

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

2005

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“…It was noted, however, that the Hammett approach was developed for acids-as-solvents, and it is not generally applicable even for acids in solutionn. , It was shown that acidity function studies are theoretically inapplicable to solids, which explains the widely different H o values obtained for the same material in different studies ( e.g . Nafion-H: ≤−12,16ab −10 to −12, and −6.5; sulfated zirconia: <−16 and −12 11 ), as well as the lack of correlation of catalytic activity with H o 15b…”

Section: Introductionmentioning

confidence: 99%

Strength of Solid Acids and Acids in Solution. Enhancement of Acidity of Centers on Solid Surfaces by Anion Stabilizing Solvents and Its Consequence for Catalysis

et al. 1997

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A comparison of acidity of two solids, a poly(styrenesulfonic acid) (Amberlyst 15) and a perfluoroinated ion exchange polymer (Nafion-H, PFIEP) with the structurally related liquid acids methanesulfonic, sulfuric, and trifluoromethanesulfonic acid (TFMSA), was conducted with mesityl oxide as probe base (determination of the Δδ1 parameter) and for the fluorinated materials also with hexamethylbenzene as the probe base. It was found that Nafion-H is similar in strength to 85% sulfuric acid, whereas Ambelyst 15 is much weaker than 80% methanesulfonic acid or 60% sulfuric acid. Thus, the solids are much weaker acids than their liquid structural analogs. This seems to be a general property, because the rigidity of the solids prevents the acid groups/sites from cooperating in the transfer of a hydron, an essential feature in the manifestation of superacidity. The postulation of superacidity for a number of solid acids appears to have no basis in fact. On the other hand, the acidity of the groups/sites on the surface can be increased by the interaction with a nonbasic solvent, capable of forming strong hydrogen bonds with the anion of the site (anion-stabilizing solvent). The anion-stabilizing solvent generates a new liquid phase around the acid site; for appropriate structures of the solid acid and solvent this phase can be superacidic. The acidity-enhancing effect of the anion-stabilizing solvent was found to have an important effect in boosting the catalytic activity of the solid for carbocationic reactions.

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“…Examination of the organic phase by 19 F NMR showed a very small signal which did not change in time, indicative of extracted acid, rather than an ester. The total amount of ester is limited, however, to the amount formed in the initial oxidation by the catalyst.…”

Section: Resultsmentioning

confidence: 99%

“…Trifluoromethanesulfonic acid (TFMSA) was more appropriate for our purpose. As a pure compound, it is a weak superacid 19 (H o Ϫ14.2, 20 Ϫ14.1 21 ). Even the commercially available 99-99.5% acid is superacidic.…”

Section: Resultsmentioning

confidence: 99%

The kinetics of isomerization of 3-methylpentane catalyzed by trifluoromethanesulfonic acid 1

Fǎrcaşiu¹,

Lukinskas²

1999

J. Chem. Soc., Perkin Trans. 2

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Reaction of 3-methylpentane (3MP) was conducted in a two-phase liquid phase system, with the weak superacid trifluoromethanesulfonic acid (TFMSA) as catalyst, below 40 ЊC. The reaction pattern depended upon the way in which the process was conducted. If the acid layer was homogenized periodically, isomerization to 2-methylpentane occurred, with very little cracking. If the acid layer was left undisturbed, a yellow zone (indicating unsaturated organic species) at the interface with the hydrocarbon layer was formed, the reaction showed an induction period after which the overall conversion was greater than for the other reaction mode, and the extent of cracking (direct or following alkylation) increased significantly. For the same quantities of TFMSA and 3MP, increasing the diameter of the reaction tube increased the reaction rate, indicating that the reaction takes place at the interface. The reaction kinetics for the clean isomerization mode (dispersion of the unsaturated organic species formed in the acid layer from the interface) were analyzed by the rate equations reported previously, allowing for catalyst deactivation. Rate measurements at five temperatures between 14 and 32 ЊC gave the activation parameters ∆H ‡ = 19 kcal mol Ϫ1 and ∆S ‡ ≈ Ϫ16 cal mol Ϫ1 deg Ϫ1 , which did not change when the ratio of catalyst to substrate was varied by a factor of two. These values are not compatible with a mass-transfer controlled reaction as found for the HF-SbF 5 catalyst. Instead, the ionization of an alkyl trifluoromethanesulfonate intermediate or a methyl group shift in the cation could be ratedetermining. No intermediate could be evidenced by NMR in either layer, but the 13 C NMR spectrum of the acid layer at the end of the reaction showed the presence of several alkenyl cations (polyalkylcyclopentenyl and possibly even some polyalkylcyclohexenyl cations).

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Acidity measurements from protonation of aromatics. Protonation of hexamethylbenzene by trifluoromethanesulfonic acid and its use to relate acidities of strong superacids to acidities of acids included in the traditional Hammett scale

Cited by 25 publications

References 2 publications

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

Strength of Solid Acids and Acids in Solution. Enhancement of Acidity of Centers on Solid Surfaces by Anion Stabilizing Solvents and Its Consequence for Catalysis

The kinetics of isomerization of 3-methylpentane catalyzed by trifluoromethanesulfonic acid 1

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