Protonation

 ( Chemical Reactions ) 

In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), usually denoted by H⁺, to an atom, molecule, or ion, forming a conjugate acid.

(The complementary process, when a proton is removed from a Brønsted–Lowry acid, is deprotonation.) Some examples include

• The protonation of water by sulfuric acid:
• : H₂SO₄ + H₂O H₃O⁺ +
• The protonation of isobutene in the formation of a carbocation:
• : (CH₃)₂C=CH₂ + HBF₄ (CH₃)₃C⁺ +
• The protonation of ammonia in the formation of ammonium chloride from ammonia and hydrogen chloride:
• : NH₃(gas) + HCl(gas) → NH₄Cl(solid)

Protonation is a fundamental chemical reaction and is a step in many stoichiometry and catalysis. Some ions and molecules can undergo more than one protonation and are labeled polybasic, which is true of many biological . Protonation and deprotonation (removal of a proton) occur in most acid–base reactions; they are the core of most acid–base reaction theories. A 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, reduction potential or oxidation potential, and optical properties can change.

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

(1994). 9780471046936 ISBN 9780471046936

Enantioselective protonations are under kinetic control, are of considerable interest in organic synthesis. They are also relevant to various biological processes.

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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- can be converted to trans-alkenes using a catalytic amount of protonating agent. Many enzymes, such as the , operate by mechanisms that involve reversible protonation of substrates.

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See also

• Acid dissociation constant
• Deprotonation (or dehydronation)
• Molecular autoionization

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