In chemistry, protonation (or hydronation) is the adding of a
proton
A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
(or
hydron, or hydrogen cation), usually denoted by H
+, to an
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
,
molecule
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
, 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
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
by
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
:
*: H
2SO
4 + H
2O H
3O
+ +
* The protonation of
isobutene in the formation of a
carbocation:
*: (CH
3)
2C=CH
2 + HBF
4 (CH
3)
3C
+ +
* The protonation of
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
in the formation of
ammonium chloride
Ammonium chloride is an inorganic chemical compound with the chemical formula , also written as . It is an ammonium salt of hydrogen chloride. It consists of ammonium cations and chloride anions . It is a white crystalline salt (chemistry), sal ...
from ammonia and
hydrogen chloride:
*: NH
3(
g) + HCl(
g) → 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 (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
A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be com ...
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
In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...
,
hydrophilicity,
reduction potential or
oxidation potential, and
optical properties can change.
Rates
Protonations are often rapid, partly because of the high mobility of protons in many solvents. The
rate of protonation is related to the
acidity of the protonating species: 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.
Enantioselective protonations are under kinetic control, are of considerable interest in
organic synthesis. They are also relevant to various biological processes.
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''-
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 protonation of substrates.
See also
*
Acid dissociation constant
*
Deprotonation (or dehydronation)
*
Molecular autoionization
References
{{Reflist
Chemical reactions
Reaction mechanisms