nitrosobenzene

Nitrosobenzene and other nitrosoarenes typically participate in a monomer-dimer equilibrium. The dimers are often favored in the solid state, whereas the deeply colored monomers are favored in dilute solution or at higher temperatures. The dimers can be formulated as Ar(O⁻)N⁺=N⁺(O⁻)Ar. They exist as cis- and trans-isomers due to the presence of the N–N double bond. The dimers are sometimes called azobenzenedioxides. The cis-trans isomerization occurs via the intermediacy of the monomer.{{cite journal|author1=Beaudoin, D.|author2=Wuest, J. D.|year=2016|title=Dimerization of Aromatic C-Nitroso Compounds|journal=Chemical Reviews|volume=116|issue=1|pages=258–286|doi=10.1021/cr500520s|pmid=26730505}}

In the case of nitrosobenzene itself, the metastable monomeric form could be prepared by sublimation onto a cold finger. The monomeric material is selectively sublimed due to its lower molecular weight and is collected on a cold finger as lustrous, dark green crystals. Over time, the monomeric material dimerizes to give the parent azobenzene dioxide as a pale yellow solid. As dictated by Le Chatelier's principle, nitrosobenzene exists in the solution phase as a mixture of monomer and dimer in dynamic equilibrium whose composition is dependent on temperature (monomer favored at higher temperature) and concentration (monomer favored at low concentration), as well as the identity of the medium (gas phase or solvent).{{Cite journal |last1=Varga |first1=Katarina |last2=Biljan |first2=Ivana |last3=Tomišić |first3=Vladislav |last4=Mihalić |first4=Zlatko |last5=Vančik |first5=Hrvoj |date=8 March 2018 |title=Quantum Chemical Calculations of Monomer–Dimer Equilibria of Aromatic C -Nitroso Compounds |url=https://pubs.acs.org/doi/10.1021/acs.jpca.7b12179 |journal=The Journal of Physical Chemistry A |language=en |volume=122 |issue=9 |pages=2542–2549 |doi=10.1021/acs.jpca.7b12179 |pmid=29381362 |bibcode=2018JPCA..122.2542V |issn=1089-5639|url-access=subscription }}

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Nitrosobenzene was first prepared by Adolf von Baeyer by the reaction of diphenylmercury and nitrosyl bromide:{{cite journal | author = Baeyer, A. | title = Nitrosobenzol und Nitrosonaphtalin | journal = Chemische Berichte | date = 1874 | volume = 7 | issue = 2 | pages = 1638–1640 | doi=10.1002/cber.187400702214| url = https://zenodo.org/record/1425062}}

:(C₆H₅)₂Hg + BrNO → C₆H₅NO + C₆H₅HgBr

A modern synthesis entails reduction of nitrobenzene to phenylhydroxylamine (C₆H₅NHOH) which is then oxidized by sodium dichromate (Na₂Cr₂O₇).{{cite journal| author = G. H. Coleman, C. M. McCloskey, F. A. Stuart | title = Nitrosobenzene | journal=Org. Synth.|volume = 25 | pages = 80| year = 1945 | doi=10.15227/orgsyn.025.0080}}

Nitrosobenzene can also be prepared by oxidation of aniline using peroxymonosulfuric acid (Caro's acid){{cite journal |last1=Caro |first1=Heinrich |title=Hauptversammlung des Vereins deutscher Chemiker am 1. bis 4. Juni 1898 zu Darmstadt |journal=Zeitschrift für Angewandte Chemie |date=1898 |volume=11 |issue=36 |page=845ff |doi=10.1002/ange.18980113602|bibcode=1898AngCh..11..815. }} or potassium peroxymonosulfate under biphasic conditions.{{Cite journal|last1=Priewisch|first1=Beate|last2=Rück-Braun|first2=Karola|date=March 2005|title=Efficient Preparation of Nitrosoarenes for the Synthesis of Azobenzenes†|journal=The Journal of Organic Chemistry|language=en|volume=70|issue=6|pages=2350–2352|doi=10.1021/jo048544x|pmid=15760229|issn=0022-3263}} It is usually purified by sublimation or by steam distillation, where it comes over as a green liquid that solidifies to a colorless solid.

Nitrosobenzene undergoes Diels–Alder reactions with dienes.{{cite journal |last1=Yamamoto |first1=Hisashi |last2=Momiyama |first2=Norie |title=Rich chemistry of nitroso compounds |journal=Chemical Communications |date=2005 |issue=28 |pages=3514–3525 |doi=10.1039/b503212c |pmid=16010311}} Condensation with anilines affords azobenzene derivatives in a reaction known as the Mills reaction or Baeyer-Mills reaction.{{OrgSynth | author = H. D. Anspon | title = p-Phenylazobenzoic Acid | collvol = 3 | collvolpages = 711| year = 1955 | prep = CV3P0711}} Reduction of nitrosobenzene produces aniline.

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Most characteristically, nitrosobenzene condenses with active methylene groups, such as those of malonic esters and phenylacetonitrile. Phenylacetonitrile (PhCH₂CN) gives the imine (PhC(CN)=NPh) in a reaction known as the Ehrlich-Sachs reaction:{{cite book | author = H. Feuer | editor = S. Patai | title = The Chemistry of the Nitro and Nitroso Groups Part 1 | publisher = Wiley | location = New York | pages = 278–283}}

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Sometimes condensation with active methylene compounds gives products of O-nitroso-aldol reaction:{{cite web | url = http://www2.chemistry.msu.edu/faculty/wulff/myweb26/Literature_pdf/2004-05-27%20Lu.pdf | title = Asymmetric O− and N− Nitroso Aldol Reaction – an efficient access to a-oxy and a-amino carbonyl compound}}

:R–CH₂-CHO + Ph–NO → R–CH(CHO)–O–NHPh (aminoxylation adduct)

{{main|Transition metal nitroso complexes}}

Nitrosobenzene (and many nitrosoarenes) are good Lewis bases. They serve as ligands in many kinds of complexes, almost always with the nitrogen as the basic site. In {{chem2|Ru(acac)2(C6H5NO)2}}, nitrosobenzene is monodentate whereas in {{chem2|[Ru(acac)2(μ-C6H5NO)]2}} the nitrosobenzene is a bridging ligand.{{cite journal |doi=10.1021/acs.inorgchem.8b03191 |title=Electronically Triggered Switchable Binding Modes of the C -Organonitroso (ArNO) Moiety on the {Ru(acac)₂} Platform |date=2019 |last1=Dey |first1=Sanchaita |last2=Panda |first2=Sanjib |last3=Ghosh |first3=Prabir |last4=Lahiri |first4=Goutam Kumar |journal=Inorganic Chemistry |volume=58 |issue=2 |pages=1627–1637 |pmid=30615436 }}

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Category:Phenyl compounds

Category:Nitroso compounds