Nitric Dioxide (NO₂)

Nitrogen dioxide is the chemical compound with the formula NO₂. It exists as a radical in nature. One ofseveral nitrogen oxides, NO₂ is an intermediate in the industrial synthesis of nitric acid, millions oftons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is aprominent air pollutant. Nitrogen dioxide is paramagnetic bent molecule with C_2v point group symmetry.

Preparation and reactions

Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air:

2 NO + O₂ → 2 NO₂

In the laboratory, NO₂ can be prepared in a two step procedure by thermal decomposition of dinitrogenpentoxide, which is obtained by dehydration of nitric acid:

2 HNO₃ → N₂O₅ + H₂O
2 N₂O₅ → 4 NO₂ + O₂

The thermal decomposition of some metal nitrates also affords NO₂:

2 Pb(NO₃)₂ → 2 PbO + 4 NO₂ + O₂

Monomer-dimer equilibrium

NO₂ exists in equilibrium with N₂O₄:

2 NO₂ ↔ N₂O₄

The equilibrium is characterized by ΔH = -57.23 kJ/mol. Resulting from an endothermic reaction, theparamagnetic monomer is favored at higher temperatures. Colourless diamagnetic N₂O₄ can beobtained as a solid melting at m.p. –11.2 °C.

Main reactions

The chemistry of nitrogen dioxide has been investigated intensively. At 150 °C, NO₂ decomposes withrelease of oxygen via an endothermic process (ΔH = 114 kJ/mol):

2 NO₂ → 2 NO + O₂

As suggested by the weakness of the N-O bond, 2 NO₂ is a good oxidizer and will sustain the combustion,sometimes explosively, with many compounds, such as hydrocarbons.

It hydrolyzes with disproportionation to give nitric acid:

3 NO₂ + H₂O → NO + 2 HNO₃

This reaction is one step in the Ostwald process for the industrial production of nitric acid from ammonia. Nitricacid decomposes slowly to nitrogen dioxide, which confers the characteristic yellow color of most samples of thisacid:

4 HNO₃ → 4 NO₂ + 2H₂O + O₂

NO2 is used to generate anhydrous metal nitrates from the oxides:

MO + 3 NO₂ → 2 M(NO₃)₂ + NO

Similarly, alkyl and metal iodides give the corresponding nitrates:

2 CH₃I + 3 NO₂ → 2 CH₃NO₃ + NO + I₂
TiI₄ + 8 NO₂ → Ti(NO₃)₄ + 4 NO + 2 I₂

Safety and pollution considerations

Nitrogen dioxide is toxic by inhalation, but this could be avoided as the material is acrid and easily detected byour sense of smell. One potential source of exposure is fuming nitric acid, which is often contaminated withNO₂. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low butpotentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential foroverexposure.

Long-term exposure to NO₂ at concentrations above 40–100 µg/m³ causes adverse health effects.

Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogencombines with oxygen to form nitrogen dioxide:

2O₂ + N₂ → 2 NO₂

The most important sources of NO₂ are internal combustion engines, thermal power stations and, to alesser extent, pulp mills. Atmospheric nuclear tests are also a source of nitrogen dioxide, which is responsible forthe reddish colour of mushroom clouds The excess air required for complete combustion of fuels in these processesintroduces nitrogen into the combustion reactions at high temperatures and produces nitrous oxides (NOx). LimitingNOx production demands the precise control of the amount of air used in combustion.

The map shown below, depicting results of satellite measurements over Europe, illustrates nitrogen dioxide as largescale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthful levels. Nitrogen dioxide playsa role in atmospheric chemistry, including the formation of tropospheric ozone. A recent study by researchers at the University of California, SanDiego, suggests a link between NO₂ levels and Sudden Infant Death Syndrome.

Effects of Various NO₂ Levels