Editing Protonation

{{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#Hydrogen ion|proton]] (or [[hydron (chemistry)|hydron]], or hydrogen cation), usually denoted by H<sup>+</sup>, to an [[atom]], [[molecule]], or [[ion]], forming a [[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–base theory|Brønsted–Lowry acid]], is [[deprotonation]].) Some examples include
* The protonation of [[water]] by [[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]] in the formation of a [[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]] in the formation of [[ammonium chloride]] from ammonia and [[hydrogen chloride]]:
*: NH<sub>3</sub>([[gas|g]]) + HCl([[gas|g]]) → NH<sub>4</sub>Cl([[solid|s]])

Protonation is a fundamental chemical reaction and is a step in many [[stoichiometry|stoichiometric]] and [[catalysis|catalytic processes]]. Some ions and molecules can undergo more than one protonation and are labeled polybasic, which is true of many biological [[macromolecule]]s. Protonation and deprotonation (removal of a proton) occur in most [[acid–base reaction]]s; they are the core of most acid–base reaction theories. A [[Brønsted–Lowry acid–base theory|Brønsted–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, making it an essential step in certain analytical procedures such as electrospray [[mass spectrometry]]. Protonating or deprotonating a molecule or ion can change many other chemical properties, not just the charge and mass, for example [[solubility]], [[hydrophile|hydrophilicity]], [[reduction potential]] or [[oxidation potential]], and [[ultraviolet–visible spectroscopy|optical properties]] can change.

==Rates==
Protonations are often rapid, partly because of the high mobility of protons in many solvents. The [[reaction rate|rate]] of protonation is related to the [[acidity]] of the protonating species: protonation by [[weak acid]]s is slower than protonation of the same base by [[strong acid]]s. The rates of protonation and [[deprotonation]] can be especially slow when protonation induces significant structural changes.<ref>{{cite journal|last1=Kramarz|first1=K. W.|last2=Norton|first2=J. R.|title=Slow Proton Transfer Reactions in Organometallic and Bioinorganic Chemistry|journal=Progress in Inorganic 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]].  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 }}</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]], for example ''cis''-[[alkene]]s can be converted to ''trans''-alkenes using a catalytic amount of protonating agent. Many enzymes, such as the [[serine hydrolase]]s, operate by mechanisms that involve reversible protonation of substrates.{{cn|date=May 2025}}

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

==References==
{{Reflist}}

[[Category:Chemical reactions]]
[[Category:Reaction mechanisms]]