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| Names | |||
|---|---|---|---|
| IUPAC name Azanyl; Aminyl | |||
| Systematic IUPAC name | |||
| Other names Amidogen; Amino radical | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChEBI | |||
| ChemSpider |
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| Properties | |||
| NH 2• | |||
| Molar mass | 16.0226 g mol−1 | ||
| Thermochemistry | |||
Std molar entropy(S⦵298) | 194.71 J K−1 mol−1 | ||
Std enthalpy of formation(ΔfH⦵298) | 190.37 kJ mol−1 | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Inchemistry, theamino radical,·NH2, also known as theaminyl orazanyl, is the neutral form of theamide ion (NH−2). Aminylradicals are highlyreactive and consequently short-lived, like most radicals; however, they form an important part ofnitrogen chemistry. In sufficiently highconcentration, amino radicalsdimerise to formhydrazine. WhileNH2 as afunctional group is common in nature, forming a part of many compounds (e.g. thephenethylamines), the radical cannot be isolated in its free form.[2]
Amino radicals can be produced by reactingOH radical with ammonia in irradiated aqueous solutions. This reaction is formulated as a hydrogen abstraction reaction.[3]
The rate constant (k1) for this reaction was determined to be1.0×108 M−1 s−1, while the parallel reaction of OH withNH+
4 was found to be much slower. This rate was redetermined by using two-pulseradiolysis competition methods withbenzoate and thiocyanate ions at pH 11.4. A value ofk1 =(9 + 1)×107 M−1 s−1 was obtained from both systems. While in acidic solution, the corresponding reaction of·OH withNH+4 is too slow to be observed by pulse radiolysis.
The amino radical may also be produced by reaction ofe−(aq) withhydroxylamine (NH2OH). Several studies also utilized the redox system ofTiIII−NH2OH for the production of amino radicals using electron paramagnetic resonance (ESR) spectroscopy and polarography.[3]
Reduction of hydroxylamine by e−(aq) has also been suggested to produce the amino radical in the following reaction.[3]
The reactivity of the amino radical in this reaction is expected to be pH dependent and should occur in the region of pH 3–7.
The amino radical has two characteristic electronic states:
The more stable electronic state is2B1, 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).[4][5]
Nitrogen centered compounds, such as amines, arenucleophilic in nature. This character is also seen in amino radicals, which can be considered to be nucleophilic species.[4][5]
The amino radical only exhibits a very low optical absorption in the visible region (λmax = 530 nm,εmax =81 M−1 s−1), while its absorption in the UV (<260 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.
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 oxidizeNH+
4 to produce the amino radical in acid solutions, given that radicals are stronger oxidants than OH. In order to test this,sulfate andphosphate 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.[3]
When the amino radical is reacted withbenzoate 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 11.3 and 12, using 1.5 M NH3 and varying concentrations of phenol between 4 and 10 mM, the formation of the phenoxyl radical absorption was observed with a rate constant of(3 + 0.4)×106 M−1 s−1. This reaction can produce phenoxyl radicals via two possible mechanisms:[3]
While the amino radical is known to be weakly reactive, the recombination process of two amino radicals to formhydrazine appears to be one of the fastest. As a result, it often competes with other NH2 reactions.
At low pressures, this reaction is the fastest and therefore the principal mode of NH2 disappearance.[6]
