# Azoxy compounds

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{{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](/source/substituent).]]

In [chemistry](/source/chemistry), '''azoxy compounds''' are a group of [organic compound](/source/organic_compound)s sharing a common [functional group](/source/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](/source/Amine_oxide) of [azo compound](/source/azo_compound)s. Azoxy compounds are [1,3-dipole](/source/1%2C3-dipole)s and [cycloadd to double bonds](/source/1%2C3-dipolar_cycloaddition). Most azoxy-containing compounds have [aryl](/source/aryl) substituents.

==Preparation==
Azoxybenzene and its derivatives are typically prepared by [reduction](/source/Redox) of [nitro compound](/source/nitro_compound)s, such as the reduction of [nitrobenzene](/source/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](/source/nitroso) compounds and [hydroxylamines](/source/hydroxylamines), e.g. [phenylhydroxylamine](/source/phenylhydroxylamine) and [nitrosobenzene](/source/nitrosobenzene) (Ph = [phenyl](/source/phenyl), {{chem2|C6H5}}):<ref name=Patai1 />{{rp|p=445}} <chem display=block>PhNHOH  +  PhNO  ->  PhN(O)NPh  +  H2O</chem>

Nitrosocarbamate [ester](/source/ester)s decarboxylate in strong base to an [azotate](/source/diazo) susceptible to strong alkylation agents:
:&ndash;N(H)CO<sub>2</sub>R + [[Nitrogen dioxide|[NO]<sup>+</sup>[NO<sub>3</sub>]<sup>&minus;</sup>]] → &ndash;N(N=O)CO<sub>2</sub>R + HNO<sub>3</sub>
:&ndash;N(N=O)CO<sub>2</sub>R + K[OR](/source/Alkoxide) → &ndash;N=NO<sup>&minus;</sup>K<sup>+</sup> + CO<sub>2</sub> + R<sub>2</sub>O
:&ndash;N=NO<sup>&minus;</sup>K<sup>+</sup> + [R<sub>3</sub>O<sup>+</sup>BF{{su|b=4|p=&minus;}}](/source/Meerwein's_reagent) → &ndash;N(N=O)R + R<sub>2</sub>O + KBF<sub>4</sub>

An alternative route involves oxidation of [azobenzene](/source/azobenzene)s with [peroxy acid](/source/peroxy_acid)s.<ref name=Patai1 />{{rp|pp=96–100}}

== Structure ==
[[File:CSD CIF TIHTEK.png|thumb|Structure of azoxybenzene as determined by [X-ray crystallography](/source/X-ray_crystallography).<ref name=XRD/>]]
Azoxybenzene compounds are more stable as their [trans isomer](/source/trans_isomer) isomer. In Ph<sub>2</sub>N<sub>2</sub>O, the N-N and N-O bond lengths are is 1.24 and 1.255&nbsp;Å respectively, corresponding to some [double bonds character](/source/Double_bond).  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](/source/Azoxydibenzene)'s [resonance form](/source/resonance_form) with a negative [formal charge](/source/formal_charge) on oxygen (–N=N<sup>+</sup>(O<sup>−</sup>)–) corresponds to a theoretical 6{{Nbh}}[D](/source/Debye) [dipole moment](/source/Dipole_moments_of_molecules).  However, the observed moment is only 4.7&nbsp;D, suggesting a substantial resonance contribution in which the other nitrogen bears negative charge (–N<sup>−</sup>–N<sup>+</sup>(=O)–).<ref name=Patai2 />{{rp|p=42}}

==Reactions==
Under [ultraviolet light](/source/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 compound](/source/azo_compound)s, azoxy compounds do not fragment thermally to lose [nitrous oxide](/source/nitrous_oxide); the process is believed [forbidden by orbital symmetries](/source/selection_rule).  Correspondingly, the reaction is possible under UV light with wavelength approximately 220&nbsp;nm.<ref name=Patai1 />{{rp|pp=922,1007}}

The azoxy group is [electron-withdrawing](/source/electron-withdrawing), but in non-oxidizing media, aliphatic [α](/source/locant) hydrogens must be situated between two azoxy groups to appreciably dissociate. [Alkyllithia](/source/Organolithium_reagent) replace the hydrogens, but with reduction:
:&ndash;N<sup>+</sup>(O<sup>&minus;</sup>)=NC(H)< + 2LiR → &ndash;N=NC(R)< + Li<sub>2</sub>O↓ + RH
Basic [oxidant](/source/oxidant)s abstract α hydrogens in a [free-radical process](/source/radical_reaction), leading to dimerization.<ref name=Patai1 />{{rp|pp=712–716}}

Azoxyarenes ''ortho'' to a benzylic carbon with good [leaving group](/source/leaving_group) eliminate the leaving group to give an indazolone (the [Davis-Beirut reaction](/source/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](/source/Wallach_rearrangement) to ''para''-azophenols; primary and secondary azoxyaliphates [tautomerize](/source/tautomerize) to a [hydrazamide](/source/hydrazide).<ref name=Patai1 />{{rp|pp=173–174,621,837}}

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

Electrochemical reduction converts azoxyarenes to [azo compounds](/source/azo_compounds). [Single-electron reductant](/source/Single-electron_reductant)s give a deep-blue radical anion, which dimerizes in [aqueous solution](/source/aqueous_solution) to the corresponding azo compound and [hydrogen peroxide](/source/hydrogen_peroxide).  Strong reducing agents such as [lithium aluminum hydride](/source/lithium_aluminum_hydride) also [hydrogenolyze](/source/hydrogenolysis) electronegative ''ortho'' or ''para'' arene substituents, but [catalytic hydrogenation](/source/catalytic_hydrogenation) selects out the azoxy link.  Conversely, [sodium borohydride](/source/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](/source/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](/source/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>

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

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Adapted from the Wikipedia article [Azoxy compounds](https://en.wikipedia.org/wiki/Azoxy_compounds) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Azoxy_compounds?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.