There is no single simple measure of solvent polarity. This is because the possible interactions between different solvents and different solutes are too complex to be represented by a single, all-encompassing, number.
Two important numbers that are helpful in identifying a solvent's polarity are its:
- dielectric constant, ε
- dipole moment μ
A number of scientists have produced polarity indexes in an effort to define a solvent's polarity using a single number - for example, Lloyd Snyder's index, and Christian Reichardt's index.
A further distinction between solvents is whether they are protic or aprotic.
Protic solvents are potential proton donors, i.e. H+ donors; they have H bound to oxygen or nitrogen. Aprotic solvents are those that cannot donate H+.
Some polar solvents are protic, and some are aprotic. All non-polar solvents are aprotic.
List of Solvent Polarities
from High Polarity to Low Polarity
Key
| Solvent | Snyder Polarity | Reich Polarity | ε | μ/D | Protic |
|---|---|---|---|---|---|
| Water | 9.0 | 10.0 | 80.10 | 1.8546 | Y |
| Methanol | 6.6 | 7.62 | 33.0 | 1.7 | Y |
| Dimethyl Sulfoxide (DMSO) | 6.5 | 4.44 | 47.24 | 3.96 | N |
| Dimethylformamide | 6.4 | 3.86 | 38.25 | 3.82 | N |
| Acetonitrile | 6.2 | 4.6 | 36.64 | 3.92 | N |
| Acetic Acid | 6.2 | 6.48 | 6.20 | 1.70 | Y |
| Ethylene glycol | 5.4 | 7.9 | 41.4 | 2.28 | Y |
| Acetone | 5.4 | 3.55 | 21.01 | 2.88 | N |
| Pyridine | 5.3 | 3.02 | 13.260 | 2.21 | N |
| Ethanol | 5.2 | 6.54 | 25.3 | 1.69 | Y |
| Diethylene glycol | 5.0 | 7.13 | 30.2 | 2.3 | Y |
| Benzonitrile | 4.6 | 3.33 | 25.9 | 4.18 | N |
| Cyclohexanone | 4.5 | 2.81 | 16.1 | 2.87 | N |
| 2-Propanol | 4.3 | 5.46 | 20.18 | 1.56 | Y |
| Chloroform | 4.3 | 2.59 | 4.8069 | 1.04 | N |
| Ethyl acetate | 4.3 | 2.28 | 6.0814 | 1.78 | N |
| Tetrahydrofuran (THF) | 4.2 | 8.86 | 7.52 | 1.75 | N |
| Dichloromethane | 3.4 | 3.09 | 8.93 | 1.60 | N |
| Benzene | 3.0 | 1.11 | 2.2825 | 0 | N |
| Carbon disulfide | 1.0 | 0.65 | 2.6320 | 0 | N |
| Toluene | 1.0 | 0.99 | 2.379 | 0.37 | N |
| Hexane | 0 | 0.09 | 1.8865 | ≈ 0 | N |
Notes:
- Reichardt values multiplied ×10 for easier comparison with Snyder data.
- Snyder data sourced from Modern Methods of Pharmaceutical Analysis, Second Edition, Volume 2, Roger E. Schirmer, CRC Press, 1990.
- Reichardt data sourced from Solvents and Solvent Effects in Organic Chemistry, Third Edition, Christian Reichardt, Wiley-VCH, 2003.
- ε and μ data from CRC Handbook of Chemistry and Physics, 86th Edition, Edited by David R. Lide. CRC Press, 2005.