Nitrous acid and nitrites

Nitrous acid, HNO2, has not been isolated as a pure compound but it is a well known and important reagent in aqueous solutions and has also been studied as a component in gas-phase equilibria. Solutions of the free acid can readily be obtained by acidification of cooled aqueous nitrite solutions but even at room temperature disproportionation is noticeable  

It is a fairly weak acid with pX 3.35 at 18°C, i.e. intermediate in strength between acetic (4.75) and chloroacetic (2.85) acids at 25°, and very similar to formic (3.75) and sulfanilic (3.23) acids. Salt-free aqueous solutions can be made by choosing combinations of reagents which give insoluble salts, e.g.  

When the presence of salts in solution is unimportant, the more usual procedure is simply to acidify NaN02 with hydrochloric acid below 0°. 

In the gas phase, an equilibrium reaction producing HNO2 can be established by mixing equimolar amounts of H2O, NO and NO2  

Microwave spectroscopy shows that the gaseous compound is predominantly in the rram-planar (Cj) configuration with the dimensions shown. The differences between the two N-O distances is notable. Despite the formal single-bond character of the central bond the barrier to rotation is 45.2kJmol- . Infrared data suggest that the trans-form is 2.3kJmoI more stable (AG°) than the cis- form at room temperature. 

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The aqueous solution chemistiy of nitrous acid and nitrites has been extensively studied. Some reduction potentials involving these species are given in Table 11.4 (p. 434) and these form a useful summaiy of their redox reactions. Nitrites are quantitatively oxidized to nitrate by permanganate and this reaction is used in titrimetric analysis. Nitrites (and HNO2) are readily reduced to NO and N2O with SO2, to H2N2O2 with Sn(II), and to NH3 with H2S. Hydrazinium salts yield azides (p. 432) which can then react with further HNO2 ... 

The occurrence of nitrous acid and nitrites in atm. air has already been discussed. 

Half of the nitrite ion will be converted to nitrous acid and half will remain. Thus there will be present nitrous acid and nitrite ion, forming a buffer solution. 

The N2O3 hydrolyzes rapidly in basic solutions to form nitrous acid and nitrite salts. 

The general mechanism of nitrosation of secondary amines is shown in Figure 12.26. Nitrosation reagents are formed in acidic solutions from nitrous acid and nitrites, respectively, by a sequence of reactions indicated in Figure 12.27. Nitrites are present in foods as additives or contaminants. In foods and beverages obtained by fermentation, nitrites may arise by reduction of nitrates by microbial reductases. In beer, for example, wild yeasts partly assimilate nitrates to give ammonia, and the activity of nitrate reductase... 

Fm (nitrous acid and nitrites), and - -V (nitrates) of nitrogen, the half-redox equilibria to take into account are... 

Nitrous acid and nitrites oxidize iodide ions, as can be predicted by examining the... 

The more powerful anticatalysis of nitration which is found with high concentrations of nitrous acid, and with all concentrations when water is present, is attributed to the formation of dinitrogen trioxide. Heterolysis of dinitrogen trioxide could give nitrosonium and nitrite ions 2N2O4 + HjO N2O3 + 2HNO3. 

Qualitative Analysis. Nitric acid may be detected by the classical brown-ring test, the copper-turnings test, the reduction of nitrate to ammonia by active metal or alloy, or the nitrogen precipitation test. Nitrous acid or nitrites interfere with most of these tests, but such interference may be eliminated by acidifying with sulfuric acid, adding ammonium sulfate crystals, and evaporating to alow volume. 

Studies of the chemical reactivity of N2O3 are complicated by its extensive dissociation into NO and NO2 which are themselves reactive species. With water N2O3 acts as the formal anhydride of nitrous acid and in alkaline solution it is converted essentially quantitatively to nitrite ... 

Nitrite is an extremely reactive chemical and is soluble in the aqueous phase of meat. It is usually used for curing in the form of the sodium salt. The nitrite ion is the conjugate base of nitrous acid (a weak acid) and has a PK of 3.36. The usually mild acid conditions found in meat give formation of only a small quantity of nitrous acid when nitrite is added to the meat ( ) ... 

Buffer solutions HN02, nitrous acid, and NaN02, sodium nitrite... 

In a variation of this procedure [19], a small amount of acid is added to a suspension of a sulphonated amine in sodium nitrite solution the sulphonic acid group on the amine generates nitrous acid and diazotisation proceeds to completion. 

The pKa appears in the problem (3.35), HA is nitrous acid, and A is the nitrite ion. We can now enter the appropriate values into this equation ... 

Aldosterone Aldosterone, llj3,21-dihydroxypregn-4-en-2,18,20-trione (27.2.4), is synthesized from 21-0-acetylcorticosterone, which when reacted with nitrosyl chloride in pyridine gives the nitrite 27.2.1. When photochemically irradiated, this compound is transformed to the oxime 27.2.2, which is hydrolyzed by nitrous acid and forms the semiacetal 27.2.3, which is an acetate of the desired aldosterone. Alkaline hydrolysis of the acetyl group of this compound leads to the desired aldosterone (27.2.4) [33]. 

Four routes to form peroxynitrite from nitric oxide. The reaction of nitric oxide with superoxide is only one mechanism leading to the formation of peroxynitrite. Supetoxide could also reduce the nitrosyidioxyl radical. If nitric oxide is directly reduced to nitroxyl anion, it will react with molecular oxygen to form peroxynitrite. At acidic pH, nitrite may form nitrous acid and nitrosonium ion, which reacts with hydrogen peroxide to form peroxynitrite. 

The products obtained are picric acid (in the form of the corresponding picrate), methylamine, and nitrous acid (as nitrite). 

DNA also may be damaged by reactive chemicals introduced into the environment as products of industrial activity. Such products may not be injurious per se but may be metabolized by cells into forms that are. Two prominent classes of such agents (Fig. 8-35) are (1) deaminating agents, particularly nitrous acid (HN02) or compounds that can be metabolized to nitrous acid or nitrites, and (2) alkylating agents. 

According to Fanner [38] the first stage of hydrolysis proceeds in accordance with the eqn. (1) (p. 7). Afterwards, the products react with one another to form nitrous acid or nitrites and the series of compounds ennumerated above. Fanner s hypothesis seems unsatisfactory when hydrolysis of nitric esters is carried out in a neutral medium since, for example, if ethyl alcohol is treated with an aqueous solution of potassium nitrate no oxidation of the ethyl alcohol occurs even if the mixture of compounds is boiled. 

Hydrazoic acid is formed (I) by reaction of sodium nitrate with molten sodamide, (2) by reaction of nitrous oxide with molten sodamide, (3) by reaction of nitrous acid and hydrazinium ion (N2H51), (4) by oxidation of hydraz.mium sails. (5) by reaction of ethyl nitrite with NaOH solution and acidifying. See also Azides... 

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