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In chemistry, protonation is the addition of a proton (H+) to an atom, molecule, or ion, forming the conjugate acid.[1] Some examples include

\mathrm{H_2SO_4\ +\ H_2O\ \rightleftharpoons \ H_3O^+\ +\ HSO_4^-}
\mathrm{(CH_3)_2C}=\mathrm{CH_2\ +\ HBF_4\ \rightleftharpoons \ (CH_3)_3C^+\ +\ BF_4^-}
\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[edit]

Protonations are often rapid, in part because of the high mobility of protons in water. 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.[2]

Reversibility and catalysis[edit]

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. Proton transfer to hydridic hydrogens in transition metal hydrides with H2 elimination plays a key role in various chemical and biochemical catalytic processes[3] in solution and the solid state.[4] The protonation is starting from dihydrogen-bonded adducts[5] and H-bonded contact ion pairs Intimate ion pair and ending in dihydrogen complexes ([M(η2-H2)]+) as solvent-separated ion pairs or free ions.[6] Thus the particulars of proton transfer to hydride ligands and to conventional organic bases are similar. The difference between them is apparent in the contact ion pair formation step.

See also[edit]


  1. ^ Zumdahl, S. S. “Chemistry” Heath, 1986: Lexington, MA. ISBN 0-669--04529-2.
  2. ^ Kramarz, K. W.; Norton, J. R. (1994). "Slow Proton Transfer Reactions in Organometallic and Bioinorganic Chemistry". Progress in Inorganic Chemistry 42: 1–65. doi:10.1002/9780470166437.ch1. 
  3. ^ X. Zhao , I. P. Georgakaki , M. L. Miller , J. C. Yarbourh , M. Y. Darensbourg , J. Am. Chem. Soc. 2001 , 123 , 9710 [1]
  4. ^ R. Custelcean , J. E. Jackson , J. Am. Chem. Soc. 1998 , 120 , 12935.[ ]
  5. ^ Bakhmutov, Vladimir. I. Dihydrogen bonds: Principles, Experiments and Applications; John Wiley & Sons, Inc.: Hoboken, NJ, 2008. ISBN 9780470180969[2]
  6. ^ Vladimir I. Bakhmutov, Proton Transfer to Hydride Ligands with Formation of Dihydrogen Complexes: A Physicochemical View. Eur. J. Inorg. Chem. 2005, 245-25 [3]