Nitronium ion

Nitronium ion
Systematic name Dioxidonitrogen(1+)[1]
Identifiers
CAS number	12269-46-4
PubChem	3609161
ChemSpider	2844644 Y
ChEBI	CHEBI:29424
Jmol-3D images	Image 1
SMILES O=[N+]=O
InChI InChI=1S/NO2/c2-1-3/q+1 Key: OMBRFUXPXNIUCZ-UHFFFAOYSA-N
Properties
Molecular formula	NO+ 2
Molar mass	46.0055 g mol-1
Exact mass	45.992903249 g mol-1
Thermochemistry
Standard molar entropy So298	233.86 J K-1 mol-1
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

The nitronium ion, or sometimes the nitryl ion (incorrect because it is not a radical), NO+
2, 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:

2 H₂SO₄ + HNO₃ → 2 HSO−
4 + NO+
2 + 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, N₂O₅, which is an ionic solid formed from nitronium and nitrate ions. Its liquid and gaseous 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 (NO+
2ClO−
4), nitronium tetrafluoroborate (NO+
2BF−
4), nitronium hexafluorophosphate (NO+
2PF−
6), nitronium hexafluoroarsenate (NO+
2AsF−
6), and nitronium hexafluoroantimonate (NO+
2SbF−
6). These are all very hygroscopic compounds.^[2]

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.

Related species

The compounds nitryl fluoride, 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).^[3]

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−
2, the nitrite ion.

References

1. ^ "dioxidonitrogen(1+) (CHEBI:29424)". Chemical Entities of Biological Interest (ChEBI). UK. http://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:29424. 
2. ^ Kenneth Schofield (1980). Aromatic nitration. CUP Archive. p. 88. ISBN 0521233623. 
3. ^ F.A.Cotton and G.Wilkinson, Advanced Inorganic Chemistry, 5th edition (1988), Wiley, p.333