protic

Meaning: A protic solvent has a hydrogen bonded to a strongly electronegative atom, usually O or N (O–H or N–H), so it can donate hydrogen bonds and can release H⁺. Water, methanol, ethanol, acetic acid and ammonia are protic. Aprotic solvents lack such a hydrogen; polar aprotic ones such as DMSO, DMF, acetone and acetonitrile have dipoles but cannot donate hydrogen bonds. Protic solvents surround anions with hydrogen bonds, stabilizing them and reducing their reactivity as nucleophiles.

On the MCAT: A passage compares a substitution reaction in different solvents and asks for the rate or the mechanism. Polar protic solvents tend to favor SN1 because they stabilize the carbocation and the leaving group. Polar aprotic solvents tend to favor SN2 because the nucleophile is poorly solvated and more reactive. In protic solvents, halide nucleophilicity rises down the group (I⁻ > Br⁻ > Cl⁻ > F⁻) because small F⁻ is most tightly solvated; in aprotic solvents the order roughly follows basicity.

Common confusion: Students call any solvent that contains hydrogen protic. Acetone and DMSO have hydrogen only on carbon (C–H), so they are aprotic. The test is an H on O or N. Protic also does not mean strongly acidic; ethanol is protic yet only very weakly acidic.