HOONO

Peroxonitrous acid, HOONO. Isomer of nitric acid (HNO2 plus H2O2). 

HOONO peroxonitrous acid H2Se04 selenic acid... 

HOONO Peroxonitrous acid Unstable, isomeric with nitric acid some salts are more stable ... 

Similarly, recent experiments" have been interpreted to mean that about 10% of the reaction of hydroperoxy radical with nitric oxide gives per-nitrous add, HOONO, instead of nitrogen dioxide and hydroxyl radical. Because this reaction is of major importance, even 10% of a second channel would be important, although it has been argued that such compounds would not be sufFidently stable to accumulate in the atmosphere." Whether such peroxynitrogen compounds are stable in the gas phase and whether they can be found in the atmosphere must await further experiments. 

Peroxonitrous Acid and Its Salts. Pcroxonitrous acid HOONO, is an isomer of nitric acid. HNOj, to which it rapidly converts. The half-life of peroxonitrous acid at 0 C is 10 s at 27°C, 0.23 s. It has been known since 1904 that the yellow solution made by mixing nitrous acid and hydrogen peroxide at low temperature contains a stronger oxidant than either ingredient alone, but the chemistry involved was not put on a sound basis until 1994. Additional preparatory methods are also available. 

The oxidation of primary and secondary alcohols by stable organic nitroxyl radicals has been reviewed.111 The kinetics of reactions of alkanes and arenes with peroxynitrous acid suggest the participation of the same active oxidizing species in both gas and aqueous phase HOONO or its decomposition product OONO. 112 The oxidation of the alkaloids reserpine and rescinnamine by nitric acid has been studied.113... 

Oxidation/hydroxylation of aromatic compounds by OH and HOONO is expected to enhance their degradation rate and hence decrease their lifetime on particulate matter, which in the case of pollutants is beneficial from the point of view of human health. Oxidation of PAHs could also lead to the production of photosensitizers such as quinones and aromatic carbonyls [10, 40, 41]. These compounds, if present in the gas phase, are also able to form aggregates and are therefore involved in the formation of secondary organic aerosol [42]. In contrast, nitration induced by OH + N02 or HOONO could lead to highly mutagenic nitro-PAHs [43] or phytotoxic nitrophenols [44, 45], in which case the health and environmental impact of the reaction intermediates is not negligible and is sometimes higher than that of the parent molecules. 

The upper wavelength range in which reaction 9 has been studied is relatively near the lower end of solar UVB radiation, but at present very little information is available on the real environmental role of such a process, if it has any. However, since the occurrence of reaction 9 under UVB irradiation cannot be excluded, Mack and Bolton discussed the need to revisit the reaction pathways proposed for the transformation of organic molecules in the presence of irradiated nitrate and nitrite in the light of the findings concerning the possible formation of HOONO/ONOCT [24], Since new data concerning the transformation reactions of phenol in the presence of N03 + hv, N02 + hv and HOONO are now available, the possible role of peroxynitrous acid in that particular case will be discussed later (Sect. 3.2.3). 

Differently from HNO3, HOONO is a weak add (see reaction 10). The anion, peroxynitrite, was shown to react with carbon dioxide to yield the nitrating agent 0N00C02 [73,74]. In contrast, peroxynitrous acid is too short-lived to undergo such a reaction [75]. 

Dark processes have also been studied in the presence of benzene and naphthalene as substrates [62]. Nitration by HNO3 requires stronger acidic conditions when compared with phenol (HNO3 > 1 M, pH < 0). Furthermore, no transformation of benzene and naphthalene was observed in the presence of HN02. This finding is consistent with the hypothesis that phenol nitration in the presence of HN02 is initiated by attack on the hydroxyl function [65]. Finally, both benzene and naphthalene can be hydroxylated and nitrated in the presence of HOONO [62]. 

Generation of Fe2+ also takes place upon irradiation of dissolved Fe(III) in acidic solution (photo-Fenton reaction [96]). It is, however, difficult to study phenol nitration in the Fe(III)/H202/HN02/UV system due to the very fast thermal reaction between H2O2 and HNO2 to yield peroxynitrous acid, HOONO, that also nitrates phenol [57]. 

The pH trend of nitrophenol formation is interesting also because with this datum, it is possible to discuss the role of peroxynitrous acid in a transformation process induced by nitrate photolysis, thus giving a first answer to the observation made by Mack and Bolton [24], HOONO forms upon irradiation of nitrate (reactions 9 and 10), although its formation at k > 280 nm is uncertain. In the presence of HOONO phenol undergoes various transformation reactions, among which nitration [57]. The pH trend of nitrophenol formation in the presence of HOONO is of the kind Rate a [H+], as discussed in... 

Sect. 3.1, and thus not compatible with the trend observed in acidic nitrate solutions under irradiation (Rate oc [H+] JCa + [H+] 1, with JCa 10 33 [56]). Accordingly, the formation of nitrophenols upon UV irradiation of nitrate in acidic solution is due to the generation of HNO2 and not of HOONO. 

Furthermore, dark processes leading to nitro derivatives can take place when nitrite-containing water is acidified and/or added with oxidants in general and hydrogen peroxide in particular (in the latter case HOONO would form in acidic solution) [55,57,97,152]. 

Rearrangement of the peroxynitrite to nitrate has been a long outstanding issue. A previous theoretical study suggested that this rearrangement proceeds via a three-centered transition state, which for HOONO corresponds to an activation energy of about 60 kcalmol [89-92]. However, the three-centered transition state is likely hindered by the substitute of a carbon chain to the hydrogen atom, and... 

Peroxonitrous acid is an acid of medium strength (p/fa = 6.8), which rapidly decomposes in neutral conditions (ti/2 = Is at pH = 7.4). Peroxonitrite can isomerize to nitrate or decompose to nitrite and dioxygen. Isomerization to nitrate is a major pathway in acidic media. The mechanism for the decomposition is still in doubt, but it is believed that HOONO homolyzes to give the NO2/OH radical pair. Peroxonitrites are stable in basic solution. Peroxonitrous acid is formed as a yellow intermediate in the reaction of nitrite solutions with hydrogen peroxide that leads to nitrates. 

The reaction HO2 + NO is of course a key question in the atmosphere. Are there similar problems there with water vapor adducts And also the possibility that the HOONO molecule which you formed by OH + NO2 may be formed there. 

Yes. So far as I know, no one has investigated the effect of water vapor on the HO2 -f NO reaction. What you are suggesting is that in the presence of water vapor HO2 -h NO could lead to HOONO ... 

Yes, this would make the possibility greater that you end up with the HOONO molecule because the water vapor could carry off the excess energy, which otherwise would make it difficult for HOONO to form. 

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