Nitronium ion
Encyclopedia
The nitronium ion, or sometimes the nitryl ion (incorrect because it is not a radical), , is a generally reactive cation created by the removal of an electron from the paramagnetic nitrogen dioxide
molecule
, or the protonation
of nitric acid
.
It is reactive enough to exist in normal conditions, but it is used extensively as an electrophile
in the nitration
of other substances. The ion
is generated in situ
for this purpose by mixing sulfuric acid
and nitric acid
according to the equilibrium
:
Historically, the nitronium ion was detected by Raman Spectroscopy, since its symmetric stretch is Raman active but Infrared inactive. (The nitronium ion is linear like carbon dioxide and their vibrational spectra are similar). The nitronium ion also exists in the solid form of dinitrogen pentoxide
, 25, which is an ionic solid
formed from nitronium and nitrate
ions. Its liquid
and gas
eous forms, however, are molecular
and do not contain nitronium ions.
A few stable nitronium salts with anions of weak nucleophilicity
can be isolated. These include nitronium perchlorate
(NOClO), nitronium tetrafluoroborate
(NOBF), nitronium hexafluorophosphate
(NOPF), nitronium hexafluoroarsenate (NOAsF), and nitronium hexafluoroantimonate
(NOSbF). These are all very hygroscopic compounds.
The nitronium ion is isoelectronic with carbon dioxide
and like that molecule has a linear structure with an ONO bond angle of 180°. For this reason it has a similar vibrational spectrum to carbon dioxide: the Raman active symmetrical stretch was first use to identify the ion in nitrating mixtures.
, NO2F, and nitryl chloride, NO2Cl, are not nitronium salts but rather molecular compounds, as shown by their low boiling points (−72 °C and −6 °C respectively) and short N-X bond lengths (N-F 135 pm, N-Cl 184 pm).
Addition of one electron forms the neutral nitryl
radical
, ·NO2; in fact, this is fairly stable and known as the compound nitrogen dioxide
.
The related negatively charged species is NO, the nitrite
ion.
Nitrogen dioxide
Nitrogen dioxide is the chemical compound with the formula it is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent...
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
, or the protonation
Protonation
In chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...
of nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
.
It is reactive enough to exist in normal conditions, but it is used extensively as an electrophile
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
in the nitration
Nitration
Nitration is a general chemical process for the introduction of a nitro group into a chemical compound. The dominant application of nitration is for the production of nitrobenzene, the precursor to methylene diphenyl diisocyanate...
of other substances. The ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
is generated in situ
In situ
In situ is a Latin phrase which translated literally as 'In position'. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may...
for this purpose by mixing sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
and nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
according to the equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
:
- 2 H2SO4 + HNO3 → 2 HSO + NO + H3O+
Historically, the nitronium ion was detected by Raman Spectroscopy, since its symmetric stretch is Raman active but Infrared inactive. (The nitronium ion is linear like carbon dioxide and their vibrational spectra are similar). The nitronium ion also exists in the solid form of dinitrogen pentoxide
Dinitrogen pentoxide
Dinitrogen pentoxide is the chemical compound with the formula N2O5. Also known as nitrogen pentoxide, N2O5 is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen...
, 25, which is an ionic solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
formed from nitronium and nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
ions. Its liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
and gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
eous forms, however, are molecular
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
and do not contain nitronium ions.
A few stable nitronium salts with anions of weak nucleophilicity
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
can be isolated. These include nitronium perchlorate
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 strong oxidizing and nitrating agent. It may become...
(NOClO), nitronium tetrafluoroborate
Nitronium tetrafluoroborate
Nitronium tetrafluoroborate is an inorganic compound with formula NO2BF4. It is a salt of nitronium cation and tetrafluoroborate anion. It is a colorless crystalline solid, which reacts heavily with water to form the corrosive acids HF and HNO3. As such, it must be handled under water-free conditions...
(NOBF), nitronium hexafluorophosphate
Hexafluorophosphate
Hexafluorophosphate is an anion with chemical formula of . This octahedral species is isoelectronic with sulfur hexafluoride, SF6, and is valence isoelectronic with the highly stable superacid anion fluoroantimonate . As a non-coordinating anion, it is a poor nucleophile...
(NOPF), nitronium hexafluoroarsenate (NOAsF), and nitronium hexafluoroantimonate
Fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination forms the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2....
(NOSbF). These are all very hygroscopic compounds.
The nitronium ion is isoelectronic with carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and like that molecule has a linear structure with an ONO bond angle of 180°. For this reason it has a similar vibrational spectrum to carbon dioxide: the Raman active symmetrical stretch was first use to identify the ion in nitrating mixtures.
Related species
The compounds nitryl fluorideNitryl fluoride
Nitryl fluoride, NO2F, is a colourless gas and strong oxidizing agent, which is used as an oxidizer in rocket propellants and as a fluorinating agent. It is a molecular species, not ionic, consistent with its low boiling point...
, NO2F, and nitryl chloride, NO2Cl, are not nitronium salts but rather molecular compounds, as shown by their low boiling points (−72 °C and −6 °C respectively) and short N-X bond lengths (N-F 135 pm, N-Cl 184 pm).
Addition of one electron forms the neutral nitryl
Nitryl
Nitryl is the nitrogen dioxide moiety when it occurs in a larger compound. Examples include nitryl fluoride and nitryl chloride ....
radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
, ·NO2; in fact, this is fairly stable and known as the compound nitrogen dioxide
Nitrogen dioxide
Nitrogen dioxide is the chemical compound with the formula it is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent...
.
The related negatively charged species is NO, the nitrite
Nitrite
The nitrite ion has the chemical formula NO2−. The anion is symmetric with equal N-O bond lengths and a O-N-O bond angle of ca. 120°. On protonation the unstable weak acid nitrous acid is produced. Nitrite can be oxidised or reduced, with product somewhat dependent on the oxidizing/reducing agent...
ion.