unprotonated

Unprotonated is a descriptor used in chemistry to refer to a molecule or a functional group that lacks a bound proton (H+). It is often contrasted with its protonated form, in which an additional proton is attached. Whether a site is considered unprotonated depends on the specific functional group and the pH of the environment.

In acid–base chemistry, unprotonated forms are typically the conjugate bases produced by deprotonation. For example, deprotonation of an amine yields an unprotonated amine (R–NH2) from the protonated form (R–NH3+); deprotonation of a carboxylic acid yields a carboxylate anion (R–COO−).

In biological systems and spectroscopy, protonation states influence charge, reactivity, and spectra. The fraction of molecules that are unprotonated at a given pH is governed by the pKa and described by Henderson–Hasselbalch equation: [unprotonated]/[protonated] ≈ 10^(pH−pKa).

Because many molecules contain multiple ionizable sites, 'unprotonated' is often context-specific: it refers to a particular site not bearing a proton, rather than the entire molecule. Proper use clarifies which group is unprotonated and under which conditions.