# Protonation

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{{short description|Addition of a proton to an atom, molecule, or ion, forming the conjugate acid}}
In chemistry, '''protonation''' (or '''hydronation''') is the adding of a [proton](/source/proton) (or [hydron](/source/hydron_(chemistry)), or hydrogen cation), usually denoted by H<sup>+</sup>, to an [atom](/source/atom), [molecule](/source/molecule), or [ion](/source/ion), forming a [conjugate acid](/source/conjugate_acid).<ref>{{cite book|last=Zumdahl|first=S. S.|title=Chemistry|publisher=Heath|date=1986|location=Lexington, MA|isbn=0-669-04529-2}}</ref> (The complementary process, when a proton is removed from a [Brønsted–Lowry acid](/source/Br%C3%B8nsted%E2%80%93Lowry_acid%E2%80%93base_theory), is [deprotonation](/source/deprotonation).) Some examples include
* The protonation of [water](/source/water) by [sulfuric acid](/source/sulfuric_acid):
*: H<sub>2</sub>SO<sub>4</sub> + H<sub>2</sub>O {{eqm}} H<sub>3</sub>O<sup>+</sup> + {{chem|HSO|4|-}}
* The protonation of [isobutene](/source/isobutene) in the formation of a [carbocation](/source/carbocation):
*: (CH<sub>3</sub>)<sub>2</sub>C=CH<sub>2</sub> + HBF<sub>4</sub> {{eqm}} (CH<sub>3</sub>)<sub>3</sub>C<sup>+</sup> + {{chem|BF|4|-}}
* The protonation of [ammonia](/source/ammonia) in the formation of [ammonium chloride](/source/ammonium_chloride) from ammonia and [hydrogen chloride](/source/hydrogen_chloride):
*: NH<sub>3</sub>([g](/source/gas)) + HCl([g](/source/gas)) → NH<sub>4</sub>Cl([s](/source/solid))

Protonation is a fundamental chemical reaction and is a step in many [stoichiometric](/source/stoichiometry) and [catalytic processes](/source/catalysis). Some ions and molecules can undergo more than one protonation and are labeled polybasic, which is true of many biological [macromolecule](/source/macromolecule)s. Protonation and deprotonation (removal of a proton) occur in most [acid–base reaction](/source/acid%E2%80%93base_reaction)s; they are the core of most acid–base reaction theories. A [Brønsted–Lowry acid](/source/Br%C3%B8nsted%E2%80%93Lowry_acid%E2%80%93base_theory) is defined as a [chemical substance](/source/chemical_substance) that protonates another substance. Upon protonating a substrate, the mass and the charge of the species each increase by one unit, making it an essential step in certain analytical procedures such as electrospray [mass spectrometry](/source/mass_spectrometry). Protonating or deprotonating a molecule or ion can change many other chemical properties, not just the charge and mass, for example [solubility](/source/solubility), [hydrophilicity](/source/hydrophile), [reduction potential](/source/reduction_potential) or [oxidation potential](/source/oxidation_potential), and [optical properties](/source/ultraviolet%E2%80%93visible_spectroscopy) can change.

==Rates==
Protonations are often rapid, partly because of the high mobility of protons in many solvents. The [rate](/source/reaction_rate) of protonation is related to the [acidity](/source/acidity) of the protonating species: protonation by [weak acid](/source/weak_acid)s is slower than protonation of the same base by [strong acid](/source/strong_acid)s. The rates of protonation and [deprotonation](/source/deprotonation) can be especially slow when protonation induces significant structural changes.<ref>{{cite book|last1=Kramarz|first1=K. W.|last2=Norton|first2=J. R. |title=Progress in Inorganic Chemistry |chapter=Slow Proton-Transfer Reactions in Organometallic and Bioinorganic Chemistry |year=1994|volume=42|pages=1–65|doi=10.1002/9780470166437.ch1|isbn=978-0-471-04693-6 }}</ref>

Enantioselective protonations are under kinetic control, are of considerable interest in [organic synthesis](/source/organic_synthesis).  They are also relevant to various biological processes.<ref>{{cite journal |doi=10.1038/nchem.297 |title=Enantioselective protonation |date=2009 |last1=Mohr |first1=Justin T. |last2=Hong |first2=Allen Y. |last3=Stoltz |first3=Brian M. |journal=Nature Chemistry |volume=1 |issue=5 |pages=359–369 |pmid=20428461 |pmc=2860147 |bibcode=2009NatCh...1..359M }}</ref>

==Reversibility and catalysis==
Protonation is usually reversible, and the structure and bonding of the conjugate base are normally unchanged on protonation. In some cases, however, protonation induces [isomerization](/source/isomerization), for example ''cis''-[alkene](/source/alkene)s can be converted to ''trans''-alkenes using a catalytic amount of protonating agent. Many enzymes, such as the [serine hydrolase](/source/serine_hydrolase)s, operate by mechanisms that involve reversible protonation of substrates.{{cn|date=May 2025}}

==See also==
* [Acid dissociation constant](/source/Acid_dissociation_constant)
* [Deprotonation](/source/Deprotonation) (or dehydronation)
* [Molecular autoionization](/source/Molecular_autoionization)

==References==
{{Reflist}}

Category:Chemical reactions
Category:Reaction mechanisms

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Adapted from the Wikipedia article [Protonation](https://en.wikipedia.org/wiki/Protonation) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Protonation?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.