aprotic

Meaning: A protic solvent has a hydrogen on an electronegative atom (O–H or N–H, as in water, alcohols, amines) and can donate hydrogen bonds. An aprotic solvent lacks this, so it cannot. Polar aprotic solvents such as DMSO, DMF, acetone and acetonitrile still have a strong dipole. They dissolve salts by solvating cations through lone pairs on O or N, but they leave anions relatively bare. Nonpolar aprotic solvents include hexane and benzene; diethyl ether and THF are weakly polar aprotic.

On the MCAT: A passage compares substitution rates or yields in different solvents. Moving an SN2 reaction with an anionic nucleophile from ethanol or water to DMSO or acetone usually speeds it up, because the nucleophile is no longer caged by hydrogen bonds. Polar protic solvents favor SN1, since they stabilize the carbocation and the leaving group. Nucleophilicity also shifts. In protic solvents, larger halides are better nucleophiles (I⁻ > Br⁻ > Cl⁻ > F⁻). In polar aprotic solvents the order follows basicity and reverses (F⁻ > Cl⁻ > Br⁻ > I⁻).

Common confusion: Aprotic does not mean nonpolar, and it does not mean the molecule has no hydrogens. Acetone has C–H bonds, but none on O or N that can hydrogen-bond as a donor, so it is aprotic. Check for O–H or N–H: if neither is present, the solvent is aprotic.