Protonation

In chemistry, protonation is the addition of a proton (H⁺) to an atom, molecule, or ion, forming the conjugate acid. Some examples include

• the protonation of water by sulfuric acid:

$\mathrm{H_2SO_4\ +\ H_2O\ \rightleftharpoons \ H_3O^+\ +\ HSO_4^-}$

• the protonation of isobutene in the formation of a carbocation:

$\mathrm{(CH_3)_2C}$=$\mathrm{CH_2\ +\ HBF_4\ \rightleftharpoons \ (CH_3)_3C^+\ +\ BF_4^-}$

• the protonation of ammonia in the formation of ammonium chloride from ammonia and hydrogen chloride:

$\mathrm{NH_{3 (g)}\ +\ HCl_{(g)}\ \rightarrow \ NH_4Cl_{(s)}}$

Protonation is a fundamental chemical reaction and is a step in many stoichiometric and catalytic processes. Some ions and molecules can undergo more than one protonation and are labeled polybasic, which is true of many biological macromolecules. Protonation and deprotonation occur in most acid-base reactions; they are the core of most acid-base reaction theories. A Bronsted-Lowry acid is defined as a chemical substance that protonates another substance. Upon protonating a substrate, the mass and the charge of the species each increase by one unit. Protonating or deprotonating a molecule or ion alters many chemical properties beyond the change in the charge and mass: hydrophilicity, reduction potential, optical properties, among others. Protonation is also an essential step in certain analytical procedures such as electrospray mass spectrometry.

Rates of protonation and deprotonation

Protonations are often rapid, in part because of the high mobility of protons in many solvents. The rate of protonation is related to the acidity of the protonating species, in that protonation by weak acids is slower than protonation of the same base by strong acids. The rates of protonation and deprotonation can be especially slow when protonation induces significant structural changes.

Reversibility and catalysis

Usually, protonations are reversible and the conjugate base is unchanged by being protonated. In some cases protonation induces isomerization, however. Cis-alkenes can be converted to trans-alkenes using a catalytic amount of protonating agent. Many enzymes, such as the serine hydrolases, operate by mechanisms that involve reversible protonations of substrates.

See also

• Acid dissociation constant
• Deprotonation
• Molecular autoionization