Acids and Bases: Solvent Effects on Acid-Base Strength
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In short:. The relative values are reliable, but their absolute magnitudes are arbitrary. The relative values are robust within the medium, as are approximations used in other nonpolar media, and depend less on impurities than the absolute values and are thus useful for comparing acids, e. The values are dependent on the medium and thus are strictly non-comparable between different media. It is derived experimentally from the p K ip,rel values in Table 2 at the buffer point BP , corrected to relative p K a,rel and anchored to the scale via the experimental procedure delineated above.
The values correspond to the thermodynamic proton activity in water at the same pH values, and are comparable between different media. In the course of direct p K a measurements, when increasing the acidity of the solution, traces of H 2 O, rather than DCE molecules, will be protonated.
However, do acids with such p K a values exist in DCE? Halogenated derivatives of closo -dodecaborane or 1-carba-undecaborane acids may have such acidities. From the p K a,rel of sulfuric acid —2. The main reason is that a conjugate acid anion here: [HSO 4 ] — is much less solvated and thus much more basic in DCE than in protic solvents. Even our strongest acid buffer , including the molecular superacid CN-TCNP, is more than 9 orders of magnitude away from showing medium superacidity as defined above. But, with the above delineated calculated p K a,DCE values of the carborane acids, it is clear that medium superacidity may be reached in DCE, yet only with special and difficult to prepare acids, and is awaiting experimental realization.
This indirectly supports the quality of the results and provides a convenient tool to predict the acidities of strong acids in MeCN, which cannot be directly measured in that solvent. The intercept at 33 is just the difference between the standard Gibbs solvation energies of the proton in the two media, i.
Relative Strengths of Acids and Bases | Chemistry
This difference of kJ mol —1 converted into the log-scale by division through 5. Given the inertness of DCE as a solvent, one could expect a good correlation between the acidities in DCE and the gas phase. The reality is very different. The correlation across all acid families is next to non-existent. It thus turns out that even such a low-polarity and inert solvent as DCE is by its influence on acid ionization much more similar to polar aprotic solvents MeCN than to the gas phase. Better correlations are obtained when acidities within families are compared see ESI, Fig. Thus, the relation between medium p K a,S values and GA values can be derived as:.
A linear correlation would require that the Gibbs solvation energy difference between a certain acid and its anion is identical for all acids. Obviously, this is not at all the case. Using acids from Table 2 , buffer solutions of a well-defined composition can be prepared spanning an acidity range of over 28 pH units or kJ mol —1 , which is double the pH window of water. This corresponds to the difference between one millisecond and the age of the universe! The quality of the derived values mainly depends on the quality of the extrathermodynamic assumption in our case the TATB assumption and the accuracy of the so obtained single ion transfer thermodynamics.
This approach is general and can be transferred to any solvent S, given that at least one acid is known for which the transfer energies from water to S exist for all particles. The general relation between aqueous acidities and the range of our buffer acidities measured is shown in Fig. One can easily see that with our used buffer systems, the protolytic window of water is by far exceeded, if compared on the unified acidity scale.
The latter would correspond to 1. This clearly shows that for measurable acid thermodynamics, solvated protons do not need to exist in the medium. Rather, the bound proton in the solvated neutral acid, being in equilibrium with the ionic solvated proton, has the same chemical potential and accounts for the protonation event. However, especially for non-polar media and in unbuffered solutions, impurities may determine the pH S. Thus, chemical reactions influenced by acidity may proceed in an unpredictable way, if not run in a carefully selected buffer, for example selected from the 87 systems collected in Table 2.
Quantum chemical calculations on the SMD model were done with the same procedure as in ref. As the main reason for this, we assume that the desolvation of the proton is accompanied by a desolvation of the protonated indicator base, which dampens the H 0 curve.
Strengths of Acids and Bases
National Center for Biotechnology Information , U. Journal List Chem Sci v. Chem Sci. Published online Aug 7. Koppel , a Ingo Krossing , b and Ivo Leito a. Ilmar A. Author information Article notes Copyright and License information Disclaimer.
Corresponding author. Received Mar 30; Accepted Aug 3. This article has been cited by other articles in PMC. Abstract The most comprehensive solvent acidity scale spanning 28 orders of magnitude of acidity was measured in the low-polarity solvent 1,2-dichloroethane DCE. Introduction The proton is undoubtedly one of the most important particles in the universe. Definitions and concepts Acidity in non-aqueous media, and especially the comparison of acidity across media or even phase boundaries, is not common knowledge and quantitative views remain sparse.
Unified acidity scale Following the seminal work of Bartmess, 13 we introduced the ideal proton gas at one bar as a unified — and medium-independent — reference state for acidity and as a thermodynamic zero point. Acidity measurements in low polarity media Acidity measurements in low polarity media become increasingly difficult and only limited examples are known. Superacidity Superacidity refers to the highest acidity and can be used in relation to media or molecules. The equilibrium acidity scale in DCE Buffer solutions are a superior means to measure the self-consistent equilibrium acidity scale in DCE, both for reasons of the measurable concentrations of ions as well as the stability of the protochemical potential.
To construct a self-consistent ladder of relative acidities, protonation equilibria between a large number of acid pairs involving all acids on the scale have to be evaluated 24 in the solvent S according to equilibrium 3 , HA 1 solv.
Table 2 Acidity scale in 1,2-dichloroethane; see the text for in-depth explanations of the data. Open in a separate window. Cross validation: quantum-chemical solvation models vs. Does one need solvated protons for protonation of bases? Is it possible to obtain true medium p K a values in DCE? The DCE acidity scale: explanation and discussion of the entries in Table 2 The columns p K ip,rel , p K a,DCE and of Table 2 illustrate the different levels of complexity in data treatment and the corresponding depth of information that can be obtained.
Do acids exist that allow for direct p K a determination in DCE? Can we reach superacidity in DCE? Medium superacidity From the p K a,rel of sulfuric acid —2. Conclusions Using acids from Table 2 , buffer solutions of a well-defined composition can be prepared spanning an acidity range of over 28 pH units or kJ mol —1 , which is double the pH window of water.
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Relation between the medium acidities in water and DCE and the scale and the limiting activities of the solvated proton in these media. Quantum chemical methods Quantum chemical calculations on the SMD model were done with the same procedure as in ref.
Conflicts of interest There are no conflicts to declare. It may be a molecular solvent like DCE or a strong acid itself. As we solve for the equilibrium concentrations in such cases, we will see that we cannot neglect the change in the initial concentration of the acid or base, and we must solve the equilibrium equations by using the quadratic equation.
What is the pH of a 0. Solution We need to determine the equilibrium concentration of the hydronium ion that results from the ionization of so that we can use to determine the pH.
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As in the previous examples, we can approach the solution by the following steps:. We need the quadratic formula to find x. Solving for x gives a negative root which cannot be correct since concentration cannot be negative and a positive root:. In solvents less basic than water, we find HCl, HBr, and HI differ markedly in their tendency to give up a proton to the solvent.
The inability to discern differences in strength among strong acids dissolved in water is known as the leveling effect of water. Water also exerts a leveling effect on the strengths of strong bases.