The nitronium ion, , is a
cation. It is an
onium ion because its nitrogen atom has +1 charge, similar to
ammonium ion . It is created by the removal of an electron from the
paramagnetic nitrogen dioxide molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
, or the
protonation
In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
of
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
(with removal of ).
It is stable enough to exist in normal conditions, but it is generally reactive and used extensively as an
electrophile in the
nitration of other substances. The
ion is generated ''
in situ'' for this purpose by mixing concentrated
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
and concentrated
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
according to the
equilibrium:
:
Structure
The nitronium ion is
isoelectronic with
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
and
nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and ha ...
, and has the same linear structure and bond angle of 180°. For this reason it has a similar vibrational spectrum to carbon dioxide. Historically, the nitronium ion was detected by
Raman spectroscopy, because its symmetric stretch is Raman-active but infrared-inactive. The Raman-active symmetrical stretch was first used to identify the ion in nitrating mixtures.
Salts
A few stable nitronium salts with anions of weak
nucleophilicity
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
can be isolated. These include
nitronium perchlorate
Nitronium perchlorate, NO2ClO4, also known as nitryl perchlorate and nitroxyl perchlorate, is an inorganic chemical, the salt of the perchlorate anion and the nitronium cation. It forms colorless monoclinic crystals. It is hygroscopic, and is a s ...
,
nitronium tetrafluoroborate , nitronium
hexafluorophosphate
Hexafluorophosphate is an anion with chemical formula of . It is an octahedral species that imparts no color to its salts. is isoelectronic with sulfur hexafluoride, , and the hexafluorosilicate dianion, , and hexafluoroantimonate . In this an ...
, nitronium
hexafluoroarsenate
The hexafluoroarsenate (sometimes shortened to fluoroarsenate) anion is a chemical species with formula . Hexafluoroarsenate is relatively inert, being the conjugate base of the notional superacid hexafluoroarsenic acid ().
Synthesis
The fir ...
, and nitronium
hexafluoroantimonate
Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, formed when mixing liquid HF with liquid SbF5 in a 2:1 ratio. ...
. These are all very
hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...
compounds.
The solid form of
dinitrogen pentoxide, , actually consists of nitronium and
nitrate ions, so it is an
ionic compound, nitronium nitrate , not a
molecular solid
A molecular solid is a solid consisting of discrete molecules. The cohesive forces that bind the molecules together are van der Waals forces, dipole-dipole interactions, quadrupole interactions, π-π interactions, hydrogen bonding, halogen bo ...
. However, dinitrogen pentoxide in
liquid or
gaseous state is
molecular and does not contain nitronium ions.
[
]
Related species
The compounds nitryl fluoride
Nitryl fluoride, NO2F, is a colourless gas and strong oxidizing agent, which is used as a fluorinating agent and has been proposed as an oxidiser in rocket propellants (though never flown).
It is a molecular species, not ionic, consistent with it ...
, , and nitryl chloride
Nitryl chloride is a volatile inorganic compound with formula ClNO2. At standard conditions it is a gas.
Formation
Nitryl chloride can be formed in the reaction of dinitrogen pentoxide with chlorides or hydrogen chloride:
:N2O5 + 2HCl → 2ClNO2 ...
, , are not nitronium salts but molecular compounds, as shown by their low boiling points (−72 °C and −6 °C respectively) and short nitrogen–halogen bond lengths (N–F 135 pm, N–Cl 184 pm).[ F. A. Cotton and G.Wilkinson, Advanced Inorganic Chemistry, 5th edition (1988), Wiley, p.333]
Addition of one electron forms the neutral nitryl
Nitryl is the nitrogen dioxide (NO2) moiety when it occurs in a larger compound as a univalent fragment. Examples include nitryl fluoride (NO2F) and nitryl chloride (NO2Cl).
Like nitrogen dioxide, the nitryl moiety contains a nitrogen atom with ...
radical, ; in fact, this is fairly stable and known as the compound nitrogen dioxide.
The related negatively charged species is , the nitrite ion.
See also
* Nitrosonium
The nitrosonium ion is , in which the nitrogen atom is bonded to an oxygen atom with a bond order of 3, and the overall diatomic species bears a positive charge. It can be viewed as nitric oxide with one electron removed. This ion is usually ob ...
References
{{Nitrogen compounds
Oxycations
Nitronium compounds
The nitronium ion, , is a cation. It is an onium ion because its nitrogen atom has +1 charge, similar to ammonium ion . It is created by the removal of an electron from the paramagnetic nitrogen dioxide molecule , or the protonation of nitri ...
Functional groups
Nitrogen(V) compounds