Examples of polar solvents and their polarity
A polar solvent is made of molecules with an uneven distribution of charge, giving each molecule a slightly positive end and a slightly negative end. Polar solvents dissolve ionic and polar substances well, following the rule like dissolves like. Polarity is often compared using the dielectric constant (higher means more polar) and the dipole moment in debyes (D); the values below are approximate, near room temperature.
- Water (H₂O) — polar protic; dielectric constant about 80, dipole moment 1.85 D; dissolves salts, sugars, acids and bases.
- Formamide (HCONH₂) — polar protic; dielectric constant about 110, even higher than water's; dissolves many salts.
- Propylene carbonate (C₄H₆O₃) — polar aprotic; dielectric constant about 65, dipole moment about 4.9 D; used in lithium battery electrolytes.
- Formic acid (HCOOH) — polar protic; dielectric constant about 58; the simplest carboxylic acid.
- Dimethyl sulfoxide, DMSO ((CH₃)₂SO) — polar aprotic; dielectric constant about 47, dipole moment about 4.0 D; dissolves both polar and many nonpolar compounds.
- Glycerol (C₃H₈O₃) — polar protic; dielectric constant about 43; its three O–H groups make it thick and fully miscible with water.
- Ethylene glycol (HOCH₂CH₂OH) — polar protic; dielectric constant about 38; the main ingredient in car antifreeze.
- Acetonitrile (CH₃CN) — polar aprotic; dielectric constant about 37, dipole moment about 3.9 D; widely used in chromatography.
- Dimethylformamide, DMF (HCON(CH₃)₂) — polar aprotic; dielectric constant about 37, dipole moment about 3.8 D; used to make fibers and coatings.
- Methanol (CH₃OH) — polar protic; dielectric constant about 33; the simplest alcohol, and toxic to drink.
- Ethanol (C₂H₅OH) — polar protic; dielectric constant about 25; mixes with water in any proportion.
- Acetone ((CH₃)₂CO) — polar aprotic; dielectric constant about 21, dipole moment about 2.9 D; the solvent in many nail polish removers.
Protic and aprotic solvents
Polar protic solvents, such as water, methanol and formic acid, have hydrogen atoms bonded to oxygen or nitrogen, so they can form hydrogen bonds and donate hydrogen ions. Polar aprotic solvents, such as acetone, DMSO and acetonitrile, are polar but have no O–H or N–H bonds. The choice matters in the lab because the solvent affects how fast reactions go: some substitution reactions run much faster in polar aprotic solvents.
Polar does not always mean a high dielectric constant
Acetic acid has an O–H bond and a dipole moment similar to water's, yet its dielectric constant is only about 6, because its molecules pair up into dimers. For comparison, a nonpolar solvent such as hexane has a dielectric constant of about 2.