Amino radical

{{Short description|Chemical group (NH2)}} {{About|the neutral compound|other compounds with the formula NH₂ but different charges|Azanide|and|Nitrenium ion}} {{Redirect|Amidogen|the chemical compound also called amidogen|Ammonia}} {{Chembox | Name = | ImageFileL1 = Amino radical.svg | ImageFileR1 = Amino-radical-3D-vdW.png | verifiedrevid = 448993879 | SystematicName = Azanyl<ref name = "aminyl (CHEBI:29318)">{{Cite web|url = https://www.ebi.ac.uk/chebi/searchId.do?chebiId=29318|title = aminyl (CHEBI:29318)|work = Chemical Entities of Biological Interest (ChEBI)|location = UK|publisher = European Bioinformatics Institute|at = IUPAC Names}}</ref> (substitutive)<br /> Dihydridonitrogen(•)<ref name = "aminyl (CHEBI:29318)" /> (additive) | OtherNames = Amidogen; Amino radical | IUPACName = Azanyl; Aminyl | Section1 = {{Chembox Identifiers | CASNo_Ref = {{cascite|correct|??}} | CASNo = 13770-40-6 | PubChem = 123329 | ChemSpiderID = 109932 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChEBI = 29318 | ChEBI_Ref = {{ebicite|correct|EBI}} | SMILES = [NH2] | StdInChI = 1S/H2N/h1H2 | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = MDFFNEOEWAXZRQ-UHFFFAOYSA-N | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} }} | Section2 = {{Chembox Properties | Formula = {{Chem|NH|2}}^• | MolarMass = 16.0226 g mol⁻¹ }} | Section3 = {{Chembox Thermochemistry | DeltaHf = 190.37 kJ mol⁻¹ | Entropy = 194.71 J K⁻¹ mol⁻¹}} | Section4 = | Section5 = | Section6 = }}

In chemistry, the '''amino radical''', '''{{chem2|*NH2}}''', also known as the '''aminyl''' or '''azanyl''', is the neutral form of the amide ion ({{chem2|NH2-}}). Aminyl radicals are highly reactive and consequently short-lived, like most radicals; however, they form an important part of nitrogen chemistry. In sufficiently high concentration, amino radicals dimerise to form hydrazine. While {{chem2|NH2}} as a functional group is common in nature, forming a part of many compounds (e.g. the phenethylamines), the radical cannot be isolated in its free form.<ref name="die.net">{{cite web|url=http://dictionary.die.net/amidogen |archive-url=https://archive.today/20130221094825/http://dictionary.die.net/amidogen |url-status=dead |archive-date=February 21, 2013 |title=Amidogen |author=die.net |access-date=May 16, 2012 }}</ref>

== Synthesis == Amino radicals can be produced in controlled fashion via radiochemistry or single-electron redox.<ref name="NetaMaruthamuthu1978">{{cite journal|last1=Neta|first1=P.|last2=Maruthamuthu|first2=P.|last3=Carton|first3=P. M.|last4=Fessenden|first4=R. W.|year=1978|title=Formation and reactivity of the amino radical|journal=The Journal of Physical Chemistry|volume=82|issue=17|pages=1875–1878|doi=10.1021/j100506a004|issn=0022-3654}}</ref>

Irradiation aqueous ammonia solution generates the hydroxyl radical, which then abstracts hydrogen from ammonia: :{{chem2 | NH3 + '''*'''OH -> '''*'''NH2 + H2O }} The rate constant (''k₁'') for this reaction is about {{nowrap|(9 + 1){{e|7}} M⁻¹ s⁻¹}}. The reaction is suppressed in acidic solutions, as {{chem2|NH4+}} undergoes the corresponding reaction undetectably slowly.<ref name="NetaMaruthamuthu1978"/>

Aqueous electrons reduce hydroxylamine ({{chem2|NH2OH}}) to hydroxide and amino radicals. In the simplest case, such electrons are produced from titanium(III) salt solutions:<ref name="NetaMaruthamuthu1978" /> :{{chem2 | Ti^{III} + NH2OH -> Ti^{IV} + '''*'''NH2 + HO- }} Unlike the radiative reaction, a parallel reaction is expected to occur at pH&nbsp;3–7. One presumed intermediate is ammoniumyl ({{chem2|NH3+}}):<ref name="NetaMaruthamuthu1978" /> :{{chem2 | '''*'''NH3+ <-> '''*'''NH2 + H+ }}

== Properties ==

=== Electronic states === The amino radical has two characteristic electronic states:

328x328px|The electronic states of the amino radical

The more stable electronic state is ²B₁, where the unpaired electron is in the p-orbital perpendicular to the plane of the molecule (π type radical). The high energy electronic state, ²A₁, has the two electrons in the p-orbital and the unpaired electron in the sp² orbital (σ type radical).<ref name="NIST">{{Cite web |url=https://webbook.nist.gov/cgi/cbook.cgi?ID=C13770406&Mask=6FF |title=Amino Radical |date=2017 |work=NIST Chemistry WebBook |publisher=National Institute of Science and Technology |access-date=15 June 2018}}</ref><ref name="KoenigHoobler1974">{{cite journal |last1=Koenig |first1=T. |last2=Hoobler |first2=J. A. |last3=Klopfenstein |first3=C. E. |last4=Hedden |first4=G. |last5=Sunderman |first5=F. |last6=Russell |first6=B. R. |title=Electronic configurations of amido radicals |journal=Journal of the American Chemical Society |volume=96 |issue=14 |year=1974 |pages=4573–4577 |issn=0002-7863 |doi=10.1021/ja00821a036}}</ref>

Nitrogen centered compounds, such as amines, are nucleophilic in nature. This character is also seen in amino radicals, which can be considered to be nucleophilic species.<ref name="NIST" /><ref name="KoenigHoobler1974" />

=== Spectral properties === The amino radical only exhibits a very low optical absorption in the visible region (''λ''_max = 530&nbsp;nm, ''ε''_max = {{nowrap|81 M⁻¹ s⁻¹}}), while its absorption in the UV (<260&nbsp;nm) is similar to that of OH. Due to this, it is impractical to determine the rate of reaction of the amino radical with organic compounds by following the decay of the amino radical.

== Reactivity == In general, amino radicals are highly reactive and short lived; however, this is not the case when reacted with some organic molecules. Relative reactivities of the amino radical with several organic compounds have been reported, but the absolute rate constants for such reactions remain unknown. In reaction 1, it was hypothesized that the amino radical might possibly react with NH₃ more rapidly than OH and might oxidize {{chem|NH|4|+}} to produce the amino radical in acid solutions, given that radicals are stronger oxidants than OH. In order to test this, sulfate and phosphate radical anions were used. The sulfate and phosphate radical anions were found to react more slowly with NH₃ than does the amino radical and they react with ammonia by hydrogen abstraction and not by electron transfer oxidation.<ref name="NetaMaruthamuthu1978" />

When the amino radical is reacted with benzoate ions, the rate constant is very low and only a weak absorption in the UV spectra is observed, indicating that amino radicals do not react with benzene rapidly. Phenol, on the other hand, was found to react more rapidly with the amino radical. In experiments at pH&nbsp;11.3 and 12, using 1.5&nbsp;M NH₃ and varying concentrations of phenol between 4 and 10&nbsp;mM, the formation of the phenoxyl radical absorption was observed with a rate constant of {{nowrap|(3 + 0.4){{e|6}} M⁻¹ s⁻¹}}. This reaction can produce phenoxyl radicals via two possible mechanisms:<ref name="NetaMaruthamuthu1978" />

# Addition to the ring followed by elimination of NH₃, or # Oxidation by direct electron transfer

755x755px|thumb|center|Rate constants for reaction of NH₂ radicals. These rate constants for the amino radical reactions were measured in a 1978 study by Neta et al. by following the kinetics of formation of the resultant radicals. The observations were made at the absorption maxima of these radicals.<ref name="NetaMaruthamuthu1978" />

While the amino radical is known to be weakly reactive, the recombination process of two amino radicals to form hydrazine appears to be one of the fastest. As a result, it often competes with other NH₂ reactions.

:NH₂ + NH₂ → N₂H₄

At low pressures, this reaction is the fastest and therefore the principal mode of NH₂ disappearance.<ref name=KheSoulingnac1977>{{cite journal |last1=Khe |first1=P. V. |last2=Soulignac|first2=J. C.|last3=Lesclaux |first3=R. |title=Pressure and temperature dependence of amino radical recombination rate constant |journal=The Journal of Physical Chemistry |date=1977 |volume=81 |issue=3 |pages=210–214 |doi=10.1021/j100518a006}}</ref>

== See also == * Amide * Amine * Radical (chemistry) * Hydrazine&nbsp;&mdash; dimer of amino radicals

== References == {{Reflist}}

==Further reading== * {{Cite journal|last=Davies|first=P|date=2008|title=Detection of the amino radical NH₂ by laser magnetic resonance spectroscopy|journal=The Journal of Chemical Physics|volume=62|issue=9|pages=3739–3742|doi=10.1063/1.430970}} * {{Cite journal|last=Buttner|first=T|date=2005|title=A stable aminyl radical metal complex|pmid=15653498|journal=Science|volume=307|issue=5707|pages=235–8|doi=10.1126/science.1106070|bibcode=2005Sci...307..235B|s2cid=6625217}} * {{Cite journal|last=John|first=Seely|date=1977|title=Temperature and Pressure Dependence of the Rate Constant for the HO₂ + NO Reaction|journal=The Journal of Physical Chemistry|volume=81|issue=10|pages=210–214|doi=10.1021/jp952553f}} * {{Cite journal|last=Koenig|first=Hoobler|title=Electronic configurations of amino radicals|journal=Journal of the American Chemical Society|volume=96|issue=14|pages=4573–4577|doi=10.1021/ja00821a036|year=1974}}

{{Molecules detected in outer space}}

Category:Inorganic compounds Category:Free radicals