Hammett acidity function

Source: Wikipedia, the free encyclopedia.

The Hammett acidity function (H0) is a measure of acidity that is used for very concentrated solutions of strong

aqueous solutions for which the pH
scale is useful.

In highly concentrated solutions, simple approximations such as the

acid-catalyzed reactions, because some of these reactions use acids in very high concentrations, or even neat (pure).[3]

Definition

The Hammett acidity function, H0, can replace the pH in concentrated solutions. It is defined using an equation[4][5][6] analogous to the Henderson–Hasselbalch equation:

where log(x) is the

conjugate acid of a very weak base B, with a very negative pKBH+. In this way, it is rather as if the pH scale has been extended to very negative values. Hammett originally used a series of anilines with electron-withdrawing groups for the bases.[3]

Hammett also pointed out the equivalent form

where a is the activity, and the γ are thermodynamic

protonating
ability equivalent to H3O+ at a fictitious (ideal) concentration of 1012 mol/L, as measured by its ability to protonate weak bases.

Although the Hammett acidity function is the best known acidity function, other acidity functions have been developed by authors such as Arnett, Cox, Katrizky, Yates, and Stevens.[3]

Typical values

On this scale, pure

pyrosulfuric acid has H0 ~ −15.[7] Take note that the Hammett acidity function clearly avoids water in its equation. It is a generalization of the pH scale—in a dilute aqueous solution (where B is H2O), pH is very nearly equal to H0. By using a solvent-independent quantitative measure of acidity, the implications of the leveling effect are eliminated, and it becomes possible to directly compare the acidities of different substances (e.g. using pKa, HF is weaker than HCl or H2SO4 in water but stronger than HCl in glacial acetic acid.[8][9]
)

H0 for some concentrated acids:[10]

For mixtures (e.g., partly diluted acids in water), the acidity function depends on the composition of the mixture and has to be determined empirically. Graphs of H0 vs mole fraction can be found in the literature for many acids.[3]

References

  1. ^ L. P. Hammett and A. J. Deyrup (1932) J. Am. Chem. Soc. 54, 2721
  2. ^ L. P. Hammett (1940). Physical Organic Chemistry. (McGraw-Hill)
  3. ^ .
  4. ^ a b William L. Jolly, Modern Inorganic Chemistry (McGraw-Hill 1984), p.202-3
  5. ^ F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (5th edition, Wiley-Interscience 1988), p.107-9
  6. ^ G. L. Miessler and D. A. Tarr, Inorganic Chemistry (2nd edition, Prentice-Hall 1999), p.170-1
  7. ^ What do you mean pH = -1? Super Acids Archived 2006-09-23 at the Wayback Machine
  8. ^ Liang, Jack Joan-Nan, "The Hammett Acidity Function for Hydrofluoric Acid and some related Superacid Systems" (1976). Open Access Dissertations and Theses. Paper 3850.
  9. OCLC 391334955.{{cite book}}: CS1 maint: others (link
    )
  10. ^ The Chemistry of Nonaqueous Solvents VB: Acid and Aprotic Solvents Ed J.J. Lagowski, pp139, Academic Press, London, 1978