The
Hammett acidity function is a measure of acidity that is used for very concentrated solutions of strong acids, including superacids. In such solutions, simple approximations such as the
Henderson-Hasselbalch equationIn chemistry, the Henderson–Hasselbalch equation describes the derivation of pH as a measure of acidity in biological and chemical systems...
are no longer valid due to the variations of the
activity coefficientAn activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances. In an ideal mixture the interactions between each pair of chemical species are the same and, as a result, properties of the mixtures can be expressed...
s in highly concentrated solutions. The Hammett acidity function is used in fields such as
physical organic chemistryPhysical organic chemistry is the study of the interrelationships between structure and reactivity in organic molecules. It can be seen as the study of organic chemistry using tools of physical chemistry such as chemical equilibrium, chemical kinetics, thermochemistry, and quantum chemistry...
for the study of acid-catalyzed reactions, because some of these reactions use acids in very high concentrations, or even neat (pure).
The Hammett acidity function, H
0, is used as a
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
surrogate.
The
Hammett acidity function is a measure of acidity that is used for very concentrated solutions of strong acids, including superacids. In such solutions, simple approximations such as the
Henderson-Hasselbalch equationIn chemistry, the Henderson–Hasselbalch equation describes the derivation of pH as a measure of acidity in biological and chemical systems...
are no longer valid due to the variations of the
activity coefficientAn activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances. In an ideal mixture the interactions between each pair of chemical species are the same and, as a result, properties of the mixtures can be expressed...
s in highly concentrated solutions. The Hammett acidity function is used in fields such as
physical organic chemistryPhysical organic chemistry is the study of the interrelationships between structure and reactivity in organic molecules. It can be seen as the study of organic chemistry using tools of physical chemistry such as chemical equilibrium, chemical kinetics, thermochemistry, and quantum chemistry...
for the study of acid-catalyzed reactions, because some of these reactions use acids in very high concentrations, or even neat (pure).
The Hammett acidity function, H
0, is used as a
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
surrogate. It is defined as
where
a is the activity, and γ are the activity coefficients of a base B and its
conjugate acidWithin the Brønsted-Lowry theory of acids and bases, a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. A conjugate acid can also be seen as the chemical substance that releases, or donates, a proton in the forward...
BH
+. H
0 can be calculated using an equation analogous to the Henderson-Hasselbalch equation:
where p
KBH+ is −log(K) for the dissociation of BH
+. By using bases with very negative p
KBH+ values, the
H0 scale may be extended to negative values.
HammettLouis Plack Hammett was an American physical chemist. He is perhaps best known for the Hammett equation, which relates reaction rates to equilibrium constants for certain classes of organic reactions involving substituted aromatic compounds...
originally used a series of
anilineAniline, phenylamine or aminobenzene is an organic compound with the formula C
6H
7N. It is the simplest and one of the most important aromatic amines, being used as a precursor to more complex chemicals. Its main application is in the manufacture of polyurethane...
s with electron-withdrawing groups for the bases.
On this scale, pure
H2SO4Sulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
(18.4 M) has a
H0 value of −12, and pyrosulfuric acid has
H0 ~ −15. 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 H
2O), pH is very nearly equal to
H0. By using a solvent-independent quantitative measure of acidity, the implications of the
leveling effectThe term leveling effect refers to a solvent's ability to level the effect of a strong acid or base dissolved in it.-Process:When a strong acid is dissolved in water, it reacts with it to form H3O+ in the following reaction :HA + H2O -> A- +...
are eliminated, and it becomes possible to directly compare the acidities of different substances (e.g. using p
Ka, HF is weaker than HCl in water but stronger than HCl in glacial acetic acid; however, pure HF is "stronger" than HCl because the
H0 of pure HF is higher than that of pure HCl.)
H
0 for some concentrated acids:
- Fluoroantimonic acid
Fluoroantimonic acid HSbF
6 is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination affords the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H
2.The reaction of...
(1990): −31.3
- Magic acid (1974): −19.2
- Carborane superacid (1969): −18.0
- Fluorosulfuric acid
Fluorosulfuric acid is the inorganic compound with the formula FSO3H. It is one of the strongest acids commercially available and is a superacid. The molecule is better described by the formula HSFO3, which emphasizes its relationship to sulfuric acid,...
(1944): −15.1
- Triflic acid (1940): −14.9
- Sulfuric acid
Sulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
−12.0
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 H
0 vs
mole fractionIn chemistry, mole fraction x is a way of expressing the composition of a mixture...
can be found in the literature for many acids.
Although the Hammett acidity function is the best known
acidity functionAn acidity function is a measure of the acidity of a medium or solvent system, usually expressed in terms of its ability to donate protons to a solute . The pH scale is by far the most commonly used acidity function, and is ideal for dilute aqueous solutions...
, other acidity functions have been developed by authors such as Arnett, Cox, Katrizky, Yates, and Stevens.