Nitrous acid constant

At fairly high nitrous acid concentrations (0.1 m) and at moderate acidities (4 m) the blue color of N203 (Amax = 625 nm) is easily detected by eye. The overall equilibrium of Scheme 3-10 has been determined. A relatively recent determination of the equilibrium constant gave the value K = 3.0 x 10"3 m (Markovits et al., 1981). Accurate determinations of this constant are difficult, as N203 decomposes easily into NO and N02. Pure N203 is stable only as a pale blue solid or as an intensely blue liquid just above its freezing point (-100°C). The liquid starts to boil with decomposition above -40°C. 

For 4-nitrophenol (studied in the range 1.4-10 M nitric acid) the first-order rate coefficients (at constant nitrous acid concentration) decrease approximately 50 % as the nitric acid concentration is increased from 2 M to 5 M but increase considerably as it is further increased to 10 M the increase is greater the lower the fixed concentration of nitrous acid and is attributed to the catalysed reaction. The rate decrease was attributed to superimposition upon the normal catalysis noted above for 4-chloroanisole of the effect of lowering of the concentration of the highly reactive phenoxide ion as the acidity was increased. In 10 M nitric acid the anti-catalysed reaction was again observed. 

Mesitylene was studied using the range 5-7 M nitric acid, and when the nitrous acid concentration is small (< 0.014 M) nitronium ion nitration appears to occur, giving zeroth-order kinetics weakly retarded by nitrous acid. At rather higher nitrous acid concentrations the reaction is catalysed by nitrous acid and the kinetics go over to first-order (at constant nitrous acid concentration). 

The reaction of nitrous acid with hydrogen peroxide1 illustrates the graphical method. Figure 4-3 shows a plot of absorbance against time, reconstructed from the reported parameters. The plot of In (Yt — Too) versus time consists of two portions. The linear segment at long times is characterized by a rate constant of 8.54 x 10"2 s-1. 

Data for the reaction of nitrous acid with hydrogen peroxide1 follow Eq. (4-22), with kf = 3.76 s"1 and ks = 0.0854 s"1. We now show that either kf or ks may be k or k2. That is, an absorbance reading that rapidly rises and slowly declines does not necessarily imply the first step is fast and the second slow. Inspection of the expression for [P]f, Eq. (4-8), and that for Yt, Eq. (4-18), shows their symmetry upon interchange of A i and k2. Absent other information, the two rate constants cannot be assigned. 

TABLE 1. Values of the third-order rate constant for the acid catalysed nitrosation of thiols with nitrous acid in water at 25 °C... 

An important property of the S-nitroso thiourea derivatives is the ability to effect electrophilic nitrosation of any of the conventional nucleophilic centres. This is manifest kinet-ically by the catalysis of nitrous acid nitrosation effected by added thiourea (equation 29). The situation is completely analogous to the catalysis of the same reactions by added halide ion or thiocyanate ion. The catalytic efficiency of thiourea depends on both the equilibrium constant Xxno for the formation of the intermediate and also its rate constant k with typically a secondary amine65. Since Xxno is known (5000 dm6 mol-2), it is easy to obtain... 

When thiocyanate ions are added to nitrous acid in water, a pink colouration develops which is believed to be due to the formation of nitrosyl thiocyanate (equation 34), which is too unstable to be isolated but which can be used as a nitrosating agent in aqueous solution. Because the equilibrium constant for ONSCN formation81 is quite large (30 dm6mol 2) at 25 °C, thiocyanate ion is an excellent catalyst for aqueous electrophilic nitrosation. The well established82 series is Cl- < Br < SCN < (NH2)2CS. Thiocyanate ion is also a sufficiently powerful nucleophile to react in acid solution with nitrosamines in a denitrosation process (equation 35), which can only be driven to the right if the nitrosyl thiocyanate is removed by, e.g., reaction with a nitrite trap such as hydrazoic acid. 

A yellow solution is formed when nitrous acid is added to thiosulphate ion in water84. This is believed to be due to the formation of nitrosyl thiosulphate [O3SSNOI, although this has not been isolated and even in solution decomposition is fairly rapid. The equilibrium constant for its formation Wxno is 1.66 x 107 dm6 mol 2 at 25 °C and the UV-visible absorption spectrum is very similar to that of other S -nitroso compounds85. The rate constant for its formation is very large and is believed to represent a diffusion controlled process. Thiosulphate ion does appear to catalyse nitrosation but, over the range studied... 

Photolytic. Synthetic air containing gaseous nitrous acid and exposed to artificial sunlight (A, = 300-450 nm) photooxidized 2-butanone into peroxyacetyl nitrate and methyl nitrate (Cox et al., 1980). They reported a rate constant of 2.6 x 10 cm /molecule-sec for the reaction of gaseous 2-butane with OH radicals based on a value of 8 x 10 cm /molecule-sec for the reaction of ethylene with OH radicals. 

Photolytic. Dimethylnitramine, nitrous acid, formaldehyde, V.V-dimethylformamide and carbon monoxide were reported as photooxidation products of dimethylamine with NOx. An additional compound was tentatively identified as tetramethylhydrazine (Tuazon et al., 1978). In the atmosphere, dimethylamine reacts with OH radicals forming formaldehyde and/or amides (Atkinson et al, 1978). The rate constant for the reaction of dimethylamine and ozone in the atmosphere is 2.61 x 10 cmVmolecule-sec at 296 K (Atkinson and Carter, 1984). 

Nitrosyidisulfonic acid, reaction mechanisms, 22 129, 130 Nitrous acid, 33 103 decomposition, rate constants, 22 157 as oxidizing agent, 22 133 reaction mechanisms, 22 143-156 electrophilic nitrosations, 22 144-152 with inorganic species, 22 148, 149 nitrite oxidation by metals, 22 152-154 oxidation by halogens, 22 154, 155 in solution, 22 143, 144 reduction by metals, 22 155, 156 Nitrous oxide reductase, 40 368 Nitroxyl, reaction mechanisms, 22 138 Nitrozation, pentaamminecobalt(III) complexes, 34 181... 

Nitrous acid, acetic acid (as well as the other carboxylic acids), and carbonic acid deviate somewhat from the simple rule in the values of their acid constants. The deviation for nitrous acid and the carboxylic acids can be attributed to their electronic structure—the tendency of first-row atoms to form stable double bonds more easily than heavier atoms. For carbonic acid the low value of the first acid constant is due in part to the existence of some of the unionized acid in the form of dissolved C02 molecules rather han the acid H2C0a. It has been found that the ratio of the concentration of dissolved C02 molecules to H COa molecules is about 25, so that the true acid constant for the molecular species H2CO is about 2 X 10" 4. 

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