Nitronium ion

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Nitronium ion
Skeletal formula of nitronium with some dimensions added Spacefill model of nitronium
Ball and stick model of nitronium
Identifiers
CAS number 12269-46-4
PubChem 3609161
ChemSpider 2844644 YesY
ChEBI CHEBI:29424
Jmol-3D images Image 1
Properties
Molecular formula NO+
2
Molar mass 46.0055 g mol-1
Thermochemistry
Std 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 (77 °F), 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 stable 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 concentrated sulfuric acid and concentrated 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+
2
ClO
4
), nitronium tetrafluoroborate (NO+
2
BF
4
), nitronium hexafluorophosphate (NO+
2
PF
6
), nitronium hexafluoroarsenate (NO+
2
AsF
6
), and nitronium hexafluoroantimonate (NO+
2
SbF
6
). These are all very hygroscopic compounds.[2]

The nitronium ion is isoelectronic with carbon dioxide and nitrous oxide, and like those molecules 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 used to identify the ion in nitrating mixtures.

Related species[edit]

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[edit]

  1. ^ "dioxidonitrogen(1+) (CHEBI:29424)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. 
  2. ^ Kenneth Schofield (1980). Aromatic nitration. CUP Archive. p. 88. ISBN 0-521-23362-3. 
  3. ^ F.A.Cotton and G.Wilkinson, Advanced Inorganic Chemistry, 5th edition (1988), Wiley, p.333