protonatie

Protonatie, or protonation, is the process by which a molecule, ion, or functional group gains a proton (H+), resulting in the formation of its conjugate acid. It is the chemical opposite of deprotonation and is a central concept in acid-base chemistry, influencing structure, reactivity, solubility, and charge.

In aqueous solution, protonation equilibria depend on pH and the acidity (or basicity) of the site. The tendency of a species to be protonated is described by its pKa: at pH well below the pKa, the protonated form dominates; at pH well above the pKa, the deprotonated form dominates. The Henderson–Hasselbalch equation provides a practical relation between pH, pKa, and the relative amounts of protonated and deprotonated species.

Protonation sites are typically heteroatoms with lone pairs, such as nitrogen, oxygen, and sulfur. Common examples include ammonia protonating to ammonium, aniline to the anilinium cation, and pyridine to pyridinium. Some functional groups can also be protonated at carbonyl oxygens or other centers, altering reactivity and charge distribution.

Protonation plays a crucial role in biology, where the protonation state of amino and carboxyl groups influences protein structure, enzyme activity, and molecular interactions. It is also important in catalysis, electrochemistry, and analytical techniques, where pH and protonation determine reaction pathways, charge state in mass spectrometry, and spectral properties.

Measurement of protonation behavior relies on pKa values, which provide a quantitative handle on how readily a species gains or loses a proton under given conditions.