Acidity in Nonaqueous Solutions
A solvent will be more likely to promote ionization of a dissolved acidic molecule in the following circumstances:
- It is a protic solvent, capable of forming hydrogen bonds.
- It has a high donor number, making it a strong Lewis base.
- It has a high dielectric constant (relative permittivity), making it a good solvent for ionic species.
pKa values of organic compounds are often obtained using the aprotic solvents dimethyl sulfoxide (DMSO) and acetonitrile (ACN).
| Solvent | Donor number | Dielectric constant |
|---|---|---|
| Acetonitrile | 14 | 37 |
| Dimethylsulfoxide | 30 | 47 |
| Water | 18 | 78 |
DMSO is widely used as an alternative to water because it has a lower dielectric constant than water, and is less polar and so dissolves non-polar, hydrophobic substances more easily. It has a measurable pKa range of about 1 to 30. Acetonitrile is less basic than DMSO, and, so, in general, acids are weaker and bases are stronger in this solvent. Some pKa values at 25 °C for acetonitrile (ACN) and dimethyl sulfoxide (DMSO) are shown in the following tables. Values for water are included for comparison.
| HA A− + H+ | ACN | DMSO | water |
|---|---|---|---|
| p-Toluenesulfonic acid | 8.5 | 0.9 | strong |
| 2,4-Dinitrophenol | 16.66 | 5.1 | 3.9 |
| Benzoic acid | 21.51 | 11.1 | 4.2 |
| Acetic acid | 23.51 | 12.6 | 4.756 |
| Phenol | 29.14 | 18.0 | 9.99 |
| BH+ B + H+ | |||
| Pyrrolidine | 19.56 | 10.8 | 11.4 |
| Triethylamine | 18.82 | 9.0 | 10.72 |
| Proton sponge | 18.62 | 7.5 | 12.1 |
| Pyridine | 12.53 | 3.4 | 5.2 |
| Aniline | 10.62 | 3.6 | 4.6 |
Ionization of acids is less in an acidic solvent than in water. For example, hydrogen chloride is a weak acid when dissolved in acetic acid. This is because acetic acid is a much weaker base than water.
- HCl + CH3CO2H Cl− + CH3C(OH)2+
- acid + base conjugate base + conjugate acid
Compare this reaction with what happens when acetic acid is dissolved in the more acidic solvent pure sulfuric acid
- H2SO4 + CH3CO2H HSO4− + CH3C(OH)2+
The unlikely geminal diol species CH3C(OH)2+ is stable in these environments. For aqueous solutions the pH scale is the most convenient acidity function. Other acidity functions have been proposed for non-aqueous media, the most notable being the Hammett acidity function, H0, for superacid media and its modified version H− for superbasic media.
In aprotic solvents, oligomers, such as the well-known acetic acid dimer, may be formed by hydrogen bonding. An acid may also form hydrogen bonds to its conjugate base. This process, known as homoconjugation, has the effect of enhancing the acidity of acids, lowering their effective pKa values, by stabilizing the conjugate base. Homoconjugation enhances the proton-donating power of toluenesulfonic acid in acetonitrile solution by a factor of nearly 800. In aqueous solutions, homoconjugation does not occur, because water forms stronger hydrogen bonds to the conjugate base than does the acid.
Read more about this topic: Acid Dissociation Constant
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