Confusing Quantitative Descriptions of BrønstedLowry AcidBase Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators

Abstract

In chemistry textbooks, the pK a,H2O value of water in the solvent water at 25 8C is sometimes given as 14.0, sometimes as 15.7. This is confusing. The particular chemical reaction considered is the one in which water as BrønstedÀLowry acid reacts with water as BrønstedÀLowry base in water as solvent to yield equal concentrations of hydrated oxonium and hydroxide ions, H 3 O þ (aq) and HO À (aq), respectively. This reaction is also known as the self-ionization of water for which the equilibrium constant is abbreviated as K w with its known value of 10 À14.0 at 25 8C, i.e., pK w (25 8C) ¼ 14.0. Identical values for pK a,H2O and pK w at a fixed temperature appear reasonable, since K a,H2O and K w refer to one and the same reaction. Therefore, reasons for the apparent disagreement between the thermodynamically correct pK a value for water (14.0 at 25 8C) and the value reported in most organic chemistry textbooks (15.7) should be discussed when teaching acidÀbase chemistry. There are good arguments for introducing, from the very beginning, the concepts of activity and thermodynamic standard states when teaching quantitative aspects of chemical equilibria. This also explains in a straightforward way why all thermodynamic equilibrium constants, including K w , are dimensionless, and why pK H3O þ(25 8C) ¼ 0.

Computational Study of the Donor–Acceptor Interactions Underlying the Variable Oxygen Probe

“…It would then be interesting to establish whether the VOP slopes obtained from the gas phase structures would be similar in magnitude to the slopes from the condensed phase as obtained from the crystallographic data and whether these were better correlated to the calculated NBO interactions energies. To this end, calculations were performed for each of the model systems 1 – 13 at B3LYP/6-311++G­(d,p), M06-2X/6-311++G­(d,p), and MP2/6-31+G­(d) using the following oxygen substituents for which p K a ’s for the parent acids are given in parentheses: −OH (14), , −OCH 2 CF 3 (12.4), −OPh (9.98), −OPh- p -NO 2 (7.16), −OCOCH 3 (4.76), −OC­(O)­OH (3.6), −OCOCHCl 2 (1.3), and −OCOCF 3 (0.23) . These substituents provide a good spread of p K a values and include a variety of alcohols, acids, ethers and esters.…”

“…2,4−11 The values given represent updated values from those originally reported to include additional high-quality lowtemperature crystal data harvested from the Cambridge Structural Database (CSD) 12 and to ensure consistency with a pK a value of 14 for water rather than 15.7 used previously among other minor revisions (see the Supporting Information for the updated VOP plots). 13,14 A strong response of C−OR bond distance to the electron demand of the OR group is demonstrated for 1, 2 which has an oxygen lone pair (n O ) orbital antiperiplanar to the OR substituent (the n O −σ* C−OR interaction is one of the classical explanations for the structural anomeric effect, 15,16 though recent computational studies contest whether a larger proportion of the stabilization in these systems is due to electrostatic effects 17−19 or hyperconjugative interactions 20,21 ), and a strong response is also observed for 2, 4 which has a C−Si bond antiperiplanar to the OR substituent (this is the basis of the well-known silicon βeffect). 22−25 6 The stronger response for the 1°C −OR bond distance in cubylmethanol derivatives 8 compared to the simple 1°model system 7 was interpreted as evidence for strain-enhanced hyperconjugation involving the cubane antiperiplanar C−C bond.…”

“…Model systems which have been studied using the variable oxygen probe, the equations obtained from plots of C–OR bond distance (Å) vs p K a (ROH), and what were predicted to be the important donor–acceptor interactions involving the σ* C–O antibonding orbital are summarized in Figure . ,− The values given represent updated values from those originally reported to include additional high-quality low-temperature crystal data harvested from the Cambridge Structural Database (CSD) and to ensure consistency with a p K a value of 14 for water rather than 15.7 used previously among other minor revisions (see the Supporting Information for the updated VOP plots). , A strong response of C–OR bond distance to the electron demand of the OR group is demonstrated for 1 , which has an oxygen lone pair (n O ) orbital antiperiplanar to the OR substituent (the n O –σ* C–OR interaction is one of the classical explanations for the structural anomeric effect, , though recent computational studies contest whether a larger proportion of the stabilization in these systems is due to electrostatic effects − or hyperconjugative interactions , ), and a strong response is also observed for 2 , which has a C–Si bond antiperiplanar to the OR substituent (this is the basis of the well-known silicon β-effect). − However, a weaker response is present in 3 , which has two σ C–C bonding orbitals, which are weaker donor orbitals, antiperiplanar to the OR bond. Though similar responses for 3 and 4 were originally observed, inclusion of more points from the CSD now suggests that C–H bonding orbitals are slightly stronger donors than C–C bonding orbitals.…”

The Proton in Biochemistry: Impacts on Bioenergetics, Biophysical Chemistry, and Bioorganic Chemistry

“…8, to obtain a value of pK a (water) 15.74. However, because water is the solvent, its activity is 1, and using its molar concentration in an equilibrium constant expression is thermodynamically incorrect (Meister et al, 2014) (Silverstein and Heller, 2017). Similarly, the release of aqueous H + into water is sometimes written as: a Measured for the amino acid inserted in an alanine pentapeptide.…”

“…10 must be 1 because products and reactants are identical, so pK a (H + (aq) ) 0. We often see this value as −1.74 in the organic chemistry literature, but again, this value is incorrect, due to the mistaken use of the molar concentration of water, 55.3 M, rather than its activity, 1 (Meister et al, 2014) (Silverstein and Heller, 2017).…”