Nitrosobenzene

Nitrosobenzene is the organic compound C6H5NO. The compound can be viewed as hybrid of singlet O2 and azobenzene. This diamagnetic species exists in equilibrium with its dimer.

Preparation
C6H5NO was first reported by Adolf Baeyer in 1874, who prepared it by the reaction of diphenylmercury and nitrosylbromide, "Bringt man eine Loesung von NOBr in Benzol in eine Loesung von Quecksilberphenyl in Benzol …"
 * [C6H5]2Hg +  BrNO  →  C6H5NO  +  C6H5HgBr

The modern synthesis entails reduction of nitrobenzene to phenylhydroxylamine, C6H5NHOH, which is then oxidized by Na2Cr2O7. C6H5NO was also prepared by Caro via oxidation of aniline using peroxymonosulfuric acid, Caro's acid. It is usually purified by steam distillation, where it comes over as a green liquid that solidifes to a colorless solid.

Characteristic reactions
The monomer undergoes Diels-Alder reactions with dienes. Most characteristically nitrosobenzene condenses with "active" methylene and amine groups, e.g. of malonic esters. Condensation with anilines affords azobenzene derivatives (Mills reaction) and condensation with benzylcyanide PhCH2CN the imine PhC(CN)=NPh (The Ehrlich-Sachs Reaction, 1899)

Reduction of PhNO produces C6H5NH2.