The Gas-Phase Acidities of Very Strong Neutral Brønsted Acids

Ilmar A. Koppel, Robert W. Taft, Frederick Anvia, Shi Zheng Zhu, Li Quing Hu, Kuang Sen Sung, Darryl D. DesMarteau, Lev M. Yagupolskii, Yurii L. Yagupolskii, Nikolai V. Ignat'ev, Natal'ya V. Kondratenko, Andre Yu Volkonskii, Vladislav M. Vlasov, Rafael Notario, Pierre Charles Maria

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Abstract

An interlocking ladder of relative gas-phase acidities of a large number of very strong CH (substituted phenylmalononitriles, 2, 4, 6-(CF3SO2-)3C6H2CH3, CF3SO2- and FSO2-substituted methanes, TNT, etc.), NH (bis(per-fluoroalkyl)sulfonyl imides, 2, 4, 6-(CF3SO2)3C6H2NH2, etc.), OH (triflic acid, picric acid, 2, 4, 6-(CF3SO2)3C6H2OH, CH3SO3H, etc.) and SH (CF3COSH) Brønsted acids has been obtained using the pulsed FT ion cyclotron resonance (ICR) equilibrium constant method. The new intrinsic acidity scale covers a wide range from (CF3)2NH (ΔGacid = 324.3 kcal/mol) to (C4F9SO2)2NH (ΔGacid = 284.1 kcal/mol) and is anchored to the thermodynamic ΔGacid value (318.1 kcal/mol) of HBr. In several cases, the gas-phase acidity of compounds which make up the scale exceeds the acidity of such traditionally strong mineral acids as HCl, HBr, HI, or H2SO4 by more than 30 powers of 10. The roles of the acidity sites (CH, NH, OH, SH) and the structural factors (i) field/inductive effects (F), (ii) π-electron-acceptor resonance effects (R), and (iii) substituent polarizability (P) effects on increasing the gas-phase acidity of Bronsted acids are discussed. The effects of multiple substitution in families of Brønsted acids have been measured and discussed. The strong and extensive chains of conjugation in the resonance-stabilized planar conjugate anion of (p-nitrophenyl)- malononitrile lead to the same gas-phase CH acidity (ΔGacid = 299.5 kcal/mol) as for OH superacid CF3SO3H. A single para substituent in aniline has been found that exerts such a powerful “electron-withdrawing” effect that this aniline has a stronger gas-phase acidity than CH3SO3H. The substituent is -S(O)(═NSO2CF3)CF3, one of the family of Yagupolskii superacceptor substituents that is generated by replacing ═O by ═NSO2CF3 at a bonded S, P, or I (including in addition to the above -S(═NSO2CF3)2CF3, -P(═NSO2CF3)(C3F7)2 and -I═NSO2CF3). Perfluoro-aromatic acids have been identified as well-behaved compounds for gas-phase acidity determinations. Their moderately strong inherent acidifying effects are illustrated by the fact that a ΔGvalue of 302 ± 1 kcal/mol applies to all of the following: (4-C5F4N)2CHCN, (p-CF3C6F4)2CHCN, β-C10F7CH(CN)2, and (CF3SO2)2CH2. The introduction of more bulky strong electron-acceptor substituents in CH4 or NH3 has been found to be accompanied by an especially strong nonadditive increase in gas-phase acidity. The nonadditivity is least in CN-substituted compounds, such as for the above compounds. These results are discussed as being associated with saturation and particularly with steric repulsion in the anions. In spite of the nonadditivity of the acidities for disubstituted NH3 and CH4 (as well as X3CH), the acidifying R, F, and P effects tend still to be quite significant for strongly electron-withdrawing substituents. It will be noted that the acid (C4F9SO2)2NH presently holds the record as the strongest measured gas-phase superacid. Extensive ab initio calculations are reported for several basis sets which have been found useful for the study of structures and electronic and vibrational energies, as well as entropy effects. A major tool is provided for the prediction of new extremely acidic compounds. The present experimental and theoretical results set the grounds for preparation and measurement of new even stronger gas-phase superacids. Such results hold promise not only for extending basic knowledge but also in providing many new practical applications for condensed-phase chemistry, including those that utilize the acidic substructures.

Original languageEnglish
Pages (from-to)3047-3057
Number of pages11
JournalJournal of the American Chemical Society
Volume116
Issue number7
DOIs
Publication statusPublished - 1994 Apr 1

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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