Aprotics

Aprotics, or aprotic solvents, are substances that do not donate hydrogen atoms in acid–base reactions. In common usage, the term refers to solvents lacking O–H or N–H bonds and therefore unable to participate as hydrogen bond donors.

Aprotics are typically categorized as polar aprotic or nonpolar aprotic. Polar aprotic solvents have high dielectric constants and can solvate cations through their lone pairs without forming strong hydrogen bonds to anions. This leaves anions relatively unsolvated and often more nucleophilic, which can accelerate certain reactions, especially SN2 processes. Nonpolar aprotic solvents are less polar but still do not contain acidic hydrogens and are used when solubility of organic reagents is paramount or when reaction mechanisms favor low solvent polarity.

Representative examples include polar aprotic solvents such as acetonitrile, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), N,N-dimethylacetamide (DMA), acetone, and tetrahydrofuran (THF). Nonpolar examples include toluene and benzene; diethyl ether is also commonly used as a polar aprotic solvent in many contexts.

Applications and considerations: aprotics are widely used in organic synthesis, particularly for SN2 reactions, Grignard and organolithium reagent workups, and reactions involving strong anions such as enolates and carbanions. They do not donate protons, which helps avoid quenching highly reactive nucleophiles or bases. Handling typically requires dry, inert conditions, as many aprotic solvents absorb moisture and can be toxic or flammable. The choice of aprotic solvent depends on solubility, reactivity, and the need to minimize hydrogen-bonding effects.