15.8D: Dinitrogen Tetraoxide, \(N_2O_4\), and Nitrogen Dioxide, \(NO_2\)

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Nitrogen Dioxide and Dinitrogen Tetroxide

These two oxides are almost always present in an equilibrium mixture:


Solid at -11.2 ^oC (mp) straw coloured	0.01%	99.99%
Vapour a 21.5 ^oC (bp) Brown	0.1%	99.9%
Gas at 140 ^oC Dark brown	100%	0%

The N-N bond in N₂O₄ is longish at 1.75 Å: the adjacent formal positive charges may be partly responsible. Since the unpaired electon in NO₂ can be considered (using molecular orbital theory) to reside in an antibonding p-orbital centred largely on the nitrogen, the formation of N₂O₄ is more favorable than in the other paramagnetic oxides, NO and ClO₂. There are two other isomers of N₂O₄ which are possible: ONONO₂ and ONOONO but thse appear to be less stable.

The most important reactions are:
- With water: 2NO₂ (or N₂O₄) HNO₃ + HNO₂ (then 3HNO₂ HNO₃ + NO + H₂O)
- Using it, it is possible to prepare anyhdrous nitrates which are not accessible by other routes. As an example, copper will dissolve in liquid N₂O₄ to give Cu(NO₃)₂.N₂O₄ (The other product is NO)