Aminoxyl group

{{Redirect|Nitroxide|the NO⁻ ion|Nitroxide anion}}

Aminoxyl denotes a radical functional group with general structure R₂N–O^•. It is commonly known as a nitroxyl radical or a nitroxide, however IUPAC discourages the use of these terms, as they erroneously suggest the presence of a nitro group.{{GoldBookRef|title=aminoxyl radicals| file = A00285}} Aminoxyls are structurally related to hydroxylamines and N-oxoammonium salts, with which they can interconvert via a series of redox steps.

Aminoxyl radical.svg|The general structure of the aminoxyl radical

2,2,6,6-Tetramethylpiperidinyloxyl.svg|TEMPO, a commonly encountered organic aminoxyl radical

Kaliumnitrosodisulfonat.svg|Fremy's salt, an inorganic aminoxyl radical

Sterically unhindered aminoxyls bearing α-hydrogens are unstable and undergo rapid disproportionation to nitrones and hydroxylamines.{{cite journal |last1=Nilsen |first1=Aaron |last2=Braslau |first2=Rebecca |title=Nitroxide decomposition: Implications toward nitroxide design for applications in living free-radical polymerization |journal=Journal of Polymer Science Part A: Polymer Chemistry |date=15 January 2006 |volume=44 |issue=2 |pages=697–717 |doi=10.1002/pola.21207|bibcode=2006JPoSA..44..697N |doi-access=free }} Sterically hindered aminoxyls without α-hydrogens, such TEMPO and TEMPOL, and are persistent (stable) radicals and find use in a range of applications, both on the laboratory scale and in industry. Their ability to reversibly bind to other radical compounds makes them important as both spin labels and spin traps. They are used to selectively oxidise carbonyl groups via oxoammonium-catalyzed oxidations. They are also used as polymer stabilizers such as hindered amine light stabilizers or as transient reactive species in p-phenylenediamine based antiozonants.{{cite journal |last1=Cataldo |first1=Franco |title=Early stages of p-phenylenediamine antiozonants reaction with ozone: Radical cation and nitroxyl radical formation |journal=Polymer Degradation and Stability |date=January 2018 |volume=147 |pages=132–141 |doi=10.1016/j.polymdegradstab.2017.11.020}} They are used both to form polymers via nitroxide-mediated radical polymerization and prevent their formation as polymerization inhibitors. Various other reagents, such as N-hydroxyphthalimide can also be converted into aminoxyl radicals as part of their chemistry.