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| In chemistry, '''protonation''' is the addition of a [[Proton#Hydrogen ion|proton]] (H<sup>+</sup>) to an [[atom]], [[molecule]], or [[ion]], forming the [[conjugate acid]].<ref>Zumdahl, S. S. “Chemistry” Heath, 1986: Lexington, MA. ISBN 0-669--04529-2.</ref> Some classic examples include
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| *the protonation of [[water]] by [[sulfuric acid]]:
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| :<math>\mathrm{H_2SO_4\ +\ H_2O\ \rightleftharpoons \ H_3O^+\ +\ HSO_4^-}</math>
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| *the protonation of [[isobutene]] in the formation of a [[carbocation]]:
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| :<math>\mathrm{(CH_3)_2C}</math>=<math>\mathrm{CH_2\ +\ HBF_4\ \rightleftharpoons \ (CH_3)_3C^+\ +\ BF_4^-}</math>
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| *the protonation of [[ammonia]] in the formation of [[ammonium chloride]] from ammonia and [[hydrogen chloride]]:
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| :<math>\mathrm{NH_3\ +\ HCl\ \rightleftharpoons \ NH_4^+\ +\ Cl^-}</math>
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| 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 occur in most [[acid-base reaction]]s; they are the core of most acid-base reaction theories. A [[Brønsted-Lowry acid-base theory|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: [[hydrophile|hydrophilicity]], [[reduction potential]], [[Ultraviolet-visible spectroscopy|optical properties]], among others. Protonation is also an essential step in certain analytical procedures such as electrospray [[mass spectrometry]].
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| ==Rates of protonation and deprotonation==
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| Protonations are often rapid, in part because of the high mobility of protons in water. The [[reaction rate|rate]] of protonation is related to the acidity of the protonated species, in that weak acids deprotonate more slowly than strong acids. The rates of protonation and deprotonation can be especially slow when protonation induces significant structural changes.<ref>{{cite journal|author1=Kramarz, K. W.|author2=Norton, J. R.|title=Slow Proton Transfer Reactions in Organometallic and Bioinorganic Chemistry|journal=Progress in Inorganic Chemistry|year=1994|volume=42|pages=1–65}}</ref>
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| ==Reversibility and catalysis==
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| Usually, protonations are reversible and the conjugate base is unchanged by being protonated. In some cases protonation induces [[isomerization]], however. 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 protonations of substrates.
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| == See also ==
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| *[[Deprotonation]]
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| *[[Molecular autoionization]]
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| *[[Acid dissociation constant]]
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| ==References==
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| {{reflist}}
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| [[Category:Chemical reactions]]
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| [[Category:Reaction mechanisms]]
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