Nonprotic

Nonprotic describes substances that do not donate protons (H+) in acid–base reactions. In solvent terminology, nonprotic solvents are typically called aprotic solvents, meaning they lack acidic hydrogen atoms on heteroatoms such as oxygen or nitrogen. This contrasts with protic solvents, which contain O–H or N–H bonds and can donate protons, as seen in water, alcohols, and carboxylic acids.

In practice, nonprotic (aprotic) solvents are chosen for their ability to solvate ions without providing protons that can react with strong bases or nucleophiles. They often support reactions where a highly reactive anionic species is needed, such as SN2 substitutions, organometallic couplings, and certain deprotonation steps. Nonprotic solvents range from polar to nonpolar; many commonly used polar aprotic solvents include acetone, acetonitrile, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP). Nonpolar examples include toluene and chlorinated solvents like dichloromethane. Diethyl ether is also considered a nonprotic solvent, though it is relatively nonpolar compared with strongly polar aprotics.

Key characteristics of nonprotic solvents include the absence of easily ionizable hydrogen atoms, varying dielectric constants, and differing abilities to stabilize cations and anions. Their lack of hydrogen-bond donating capability can accelerate certain reactions by increasing the nucleophilicity or basicity of reagents in solution, while potentially hindering others that rely on hydrogen bonding for transition-state stabilization.

Note that some sources distinguish nonprotic from strictly aprotic, though the terms are often used interchangeably in practice. See also protic solvents and solvent polarity.