Nitrosobenzene

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Nitrosobenzene is an organic compound with the formula C6H5NO. It is a dark green solid that can exist in equilibrium with a pale yellow dimer. Both forms are diamagnetic (they don’t have unpaired electrons).

Monomer and dimer
- In solid form, the pale yellow dimer is usually favored. In dilute solution or at higher temperatures, the darker green monomer is favored.
- The dimer has the structure Ar(O−)N+=N+(O−)Ar and can exist as cis- and trans- isomers due to the N=N double bond.
- The monomer can be prepared by subliming the solid onto a cold surface; over time it converts back to the dimer.

How it’s made
- Traditional route: reduce nitrobenzene to phenylhydroxylamine, then oxidize to nitrosobenzene.
- Alternative route: oxidize aniline using Caro’s acid or potassium peroxymonosulfate under biphasic conditions.
- Purification is usually by sublimation or steam distillation, giving a green liquid that solidifies to a colorless solid.

Reactions and uses
- Nitrosobenzene reacts in Diels–Alder reactions with dienes.
- Condensation with anilines gives azobenzene derivatives (Mills or Baeyer–Mills reactions).
- Reduction of nitrosobenzene yields aniline.
- It can react with active methylene compounds (like malonic esters or phenylacetonitrile) to give various products; with phenylacetonitrile it forms an imine in the Ehrlich–Sachs type reaction.
- Nitrosobenzene and similar nitrosoarenes are good Lewis bases and can act as ligands in metal complexes; in some ruthenium complexes it can be monodentate or bridging.

Properties at a glance
- Formula and mass: C6H5NO, molar mass about 107.11 g/mol.
- Appearance: dark green solid (monomer), pale yellow solid (dimer); solutions are bright green and light-sensitive.
- Physical constants: melting point ≈ 65–69 °C; boiling point ≈ 59 °C (at 18 mmHg).
- Solubility: poorly soluble in water; soluble in organic solvents.
- Safety: toxic substance; handled with appropriate precautions.