Why is sulfuric acid a much stronger acid than ethanol? Determination of the contributions by inductive/field effects and electron-delocalization effects

Abstract

Two different and complementary computational methods were used to determine the contributions by inductive/field effects and by electron-delocalization effects toward the enhancement of the gas-phase deprotonation enthalpy of sulfuric acid over ethanol. Our alkylogue extrapolation method employed density functional theory calculations to determine the deprotonation enthalpy of the alkylogues of sulfuric acid, HOSO₂–(CH₂CH₂)_n–OH, and of ethanol, CH₃CH₂–(CH₂CH₂)_n–OH. The inductive/field effect imparted by the HOSO₂ group for a given alkylogue of sulfuric acid was taken to be the difference in deprotonation enthalpy between corresponding (i.e., same n) alkylogues of sulfuric acid and ethanol. Extrapolating the inductive/field effect values for the n = 1–6 alkylogues, we obtained a value of 51.0 ± 6.4 kcal mol⁻¹ for the inductive/field effect for n = 0, sulfuric acid, leaving 15.4 kcal mol⁻¹ as the contribution by electron-delocalization effects. Our block-localized wavefunction method was employed to calculate the deprotonation enthalpies of sulfuric acid and ethanol using the electron-localized acid and anion species, which were compared to the values calculated using the electron-delocalized species. The contribution from electron delocalization was thus determined to be 18.2 kcal mol⁻¹, which is similar to the value obtained from the alkylogue extrapolation method. The two methods, therefore, unambiguously agree that both inductive/field effects and electron-delocalization effects have significant contributions to the enhancement of the deprotonation enthalpy of sulfuric acid compared with ethanol, and that the inductive/field effects are the dominant contributor.