Aprotic

Aprotic is a term used in chemistry to describe substances that do not donate protons in acid–base reactions. In practice, an aprotic solvent or reagent lacks O–H or N–H bonds that can act as Brønsted acids under typical conditions. The absence of these labile protons distinguishes aprotic substances from protic ones, such as water, alcohols, and carboxylic acids, which can donate protons.

Aprotic solvents can be polar or nonpolar. Polar aprotic solvents, such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetonitrile, and acetone, dissolve salts well and have high dielectric constants. Nonpolar aprotic solvents include certain ethers and chlorinated solvents. The key feature is that these solvents do not donate hydrogen ions, even though they may engage in other interactions, such as hydrogen-bond accepting or solvating ions.

In organic synthesis, aprotic solvents are particularly important for certain reaction types. They often accelerate nucleophilic substitution (SN2) reactions because their lack of O–H or N–H bonds minimizes hydrogen-bonding with nucleophiles, thereby preserving nucleophilicity. Polar aprotic solvents can stabilize cations effectively while leaving anions relatively free, which contrasts with polar protic solvents that strongly solvate and stabilize anions through hydrogen bonding.

The label “aprotic” does not imply that a substance is inert; many aprotic solvents are highly reactive under suitable conditions. It is primarily a descriptor of acid–base behavior and hydrogen-bonding potential, rather than a comprehensive measure of reactivity.