Azoxy compounds

{{short description|1=Chemical compound of the form R–N=N(–O)–R}} [[File:Azoxy-group-2D.png|thumb|right|150px|The structure of the azoxy functional group, where R is a substituent.]]

In chemistry, '''azoxy compounds''' are a group of organic compounds sharing a common functional group with the general structure {{chem2|R\sN\dN+(\sO-)\sR}}.<ref>{{GoldBookRef | file = A00567 | title = azoxy compounds}}</ref> They are considered N-oxides of azo compounds. Azoxy compounds are 1,3-dipoles and cycloadd to double bonds. Most azoxy-containing compounds have aryl substituents.

==Preparation== Azoxybenzene and its derivatives are typically prepared by reduction of nitro compounds, such as the reduction of nitrobenzene with arsenous oxide.<ref>H. E. Bigelow and Albert Palmer "Azoybenzene" Org. Synth. 1931, 11, 16. {{doi|10.15227/orgsyn.011.0016}}</ref> Such reactions are proposed to proceed via the intermediacy of the nitroso compounds and hydroxylamines, e.g. phenylhydroxylamine and nitrosobenzene (Ph = phenyl, {{chem2|C6H5}}):<ref name=Patai1 />{{rp|p=445}} <chem display=block>PhNHOH + PhNO -> PhN(O)NPh + H2O</chem>

Nitrosocarbamate esters decarboxylate in strong base to an azotate susceptible to strong alkylation agents: :–N(H)CO2R + [[Nitrogen dioxide|[NO]+[NO3]−]] → –N(N=O)CO2R + HNO3 :–N(N=O)CO2R + KOR → –N=NO−K+ + CO2 + R2O :–N=NO−K+ + R3O+BF{{su|b=4|p=−}} → –N(N=O)R + R2O + KBF4

An alternative route involves oxidation of azobenzenes with peroxy acids.<ref name=Patai1 />{{rp|pp=96–100}}

== Structure == [[File:CSD CIF TIHTEK.png|thumb|Structure of azoxybenzene as determined by X-ray crystallography.<ref name=XRD/>]] Azoxybenzene compounds are more stable as their trans isomer isomer. In Ph2N2O, the N-N and N-O bond lengths are is 1.24 and 1.255 Å respectively, corresponding to some double bonds character. The CNNC and CNNO are near 176°.<ref name=XRD>{{cite journal |doi=10.1107/S1600536807035787 |title=''trans'' -Diphenyldiazene Oxide |date=2007 |last1=González Martínez |first1=Sandra Patricia |last2=Bernès |first2=Sylvain |journal=Acta Crystallographica Section E: Structure Reports Online |volume=63 |issue=8 |pages=o3639 |bibcode=2007AcCrE..63O3639G }}</ref>

''trans''-Azoxydibenzene's resonance form with a negative formal charge on oxygen (–N=N+(O−)–) corresponds to a theoretical 6{{Nbh}}D dipole moment. However, the observed moment is only 4.7 D, suggesting a substantial resonance contribution in which the other nitrogen bears negative charge (–N−–N+(=O)–).<ref name=Patai2 />{{rp|p=42}}

==Reactions== Under ultraviolet light transazobenzene compounds isomerize to their cis isomers, analogous to azobenzene. Similar reaction conditions can instead cause isomery to an ''ortho''-azophenol, or migration of the oxygen atom across the two nitrogens.<ref name=Patai1 />{{rp|pp=101,766,1008}}

Unlike azo compounds, azoxy compounds do not fragment thermally to lose nitrous oxide; the process is believed forbidden by orbital symmetries. Correspondingly, the reaction is possible under UV light with wavelength approximately 220 nm.<ref name=Patai1 />{{rp|pp=922,1007}}

The azoxy group is electron-withdrawing, but in non-oxidizing media, aliphatic α hydrogens must be situated between two azoxy groups to appreciably dissociate. Alkyllithia replace the hydrogens, but with reduction: :–N+(O−)=NC(H)< + 2LiR → –N=NC(R)< + Li2O↓ + RH Basic oxidants abstract α hydrogens in a free-radical process, leading to dimerization.<ref name=Patai1 />{{rp|pp=712–716}}

Azoxyarenes ''ortho'' to a benzylic carbon with good leaving group eliminate the leaving group to give an indazolone (the Davis-Beirut reaction).<ref name=Patai1 />{{rp|pp=835–836}}

Azoxy compounds are weak bases, and unstable to strong acids. In acids, azoxyarenes undergo the Wallach rearrangement to ''para''-azophenols; primary and secondary azoxyaliphates tautomerize to a hydrazamide.<ref name=Patai1 />{{rp|pp=173–174,621,837}}

The two aryl groups in azoxyarenes undergo electrophilic aromatic substitution differently. In the case of azoxybenzene, PhNN(O)– reacts at the ''meta'' position, while PhN(O)N– reacts at the ''ortho'' and ''para'' positions.<ref name=Patai1 />{{rp|p=247,712–713}}

Electrochemical reduction converts azoxyarenes to azo compounds. Single-electron reductants give a deep-blue radical anion, which dimerizes in aqueous solution to the corresponding azo compound and hydrogen peroxide. Strong reducing agents such as lithium aluminum hydride also hydrogenolyze electronegative ''ortho'' or ''para'' arene substituents, but catalytic hydrogenation selects out the azoxy link. Conversely, sodium borohydride preserves the azoxy group even as it reduces arene substituents.<ref name=Patai1 />{{rp|pp=452–455,616–617,619–620,924–925}}

==Safety== Alkyl azoxy compounds, e.g. azoxymethane are suspected to be genotoxic.<ref>Guideline On The Limits Of Genotoxic Impurities {{cite web|url=http://www.emea.eu.int/pdfs/human/swp/519902en.pdf |title=Archived copy |access-date=2007-07-11 |url-status=dead |archive-url=https://web.archive.org/web/20060904034322/http://www.emea.eu.int/pdfs/human/swp/519902en.pdf |archive-date=2006-09-04 }}</ref>

==See also== * azobenzene dioxide

== References == <references> <ref name=Patai1>{{Cite book |title=The Chemistry of the Hydrazo, Azo and Azoxy Groups |date=1975-01-01 |publisher=John Wiley & Sons, Ltd |isbn=978-0-471-66924-1 |editor-last=Patai |editor-first=Saul |series=The Chemistry of Functional Groups |volume=1 |location=Chichester, UK |language=en |doi=10.1002/0470023414}}</ref> <ref name=Patai2>{{Cite book |url=http://doi.wiley.com/10.1002/9780470771501 |title=Double-Bonded Functional Groups |date=1977-03-16 |publisher=John Wiley & Sons, Ltd. |isbn=978-0-470-77150-1 |editor-last=Patai |editor-first=Saul |series=The Chemistry of Functional Groups |volume=1 |location=Chichester, UK |language=en |doi=10.1002/9780470771501}}</ref> </references>

Category:Functional groups Category:Azo compounds Category:Azoxy compounds