In chemistry a protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group) or a nitrogen (as in an amine group). More generally, any molecular solvent which contains dissociable H⁺, is called a protic solvent. The molecules of such solvents can donate an H⁺ (proton). Conversely, aprotic solvents cannot donate hydrogen.

Polar protic solvents are solvents that share ion dissolving power with aprotic solvents but have an acidic hydrogen. These solvents generally have high dielectric constants and high polarity.

Common characteristics of protic solvents :

• solvents display hydrogen bonding
• solvents have an acidic hydrogen (although they may be very weak acids)
• solvents are able to stabilize ions
• • cations by unshared free electron pairs
  • anions by hydrogen bonding

Examples are water, methanol, ethanol, formic acid, hydrogen fluoride and ammonia.

Polar aprotic solvents are solvents that share ion dissolving power with protic solvents but lack an acidic hydrogen. These solvents generally have intermediate dielectric constants and polarity.

Common characteristics of aprotic solvents:

• solvents do not display hydrogen bonding
• solvents do not have an acidic hydrogen
• solvents are able to stabilize ions

Examples are dimethyl sulfoxide, dimethylformamide, dioxane and hexamethylphosphorotriamide, tetrahydrofuran.

Polar protic solvents are favorable forS_N1 reactions, while polar aprotic solvents are favorable for S_N2 reactions.

Apart from solvent effects, polar aprotic solvents may be essential for reactions which use strong bases, such as reactions involving Grignard reagents or n-butyllithium. If a protic solvent were to be used, the reagent would be consumed by a side reaction with the solvent.

An example of a dipolar aprotic solvent is methylpyrrolidone.

Properties of common solvents

The solvents are grouped into non-polar, polar aprotic, and polar protic solvents and ordered by increasing polarity. The polarity is given as the dielectric constant. The properties of solvents that exceed those of water are bolded.