Nitrous acid photolysis

The racemic acid 399 was treated with oxalyl chloride to afford an acyl chloride 401, which on reaction with anhydrous hydrazine yielded the hydra-zide 402. The latter was converted to the azide 403 by treatment with nitrous acid. Photolysis of 403 which a high-pressure mercury lamp afforded the lactam 404. This lactam was synthesized earlier by Japanese chemists by an entirely different method (187). 

Figure 5. Comparison of the four dominant atmospheric OH radical sources (photolysis of O3, HONO, HCHO, and ozonolysis of VOC s) for 20" and 21" of July 1998. The difference in the OH production in the early morning hours can be seen easily. After 12 00 on 20 July and 8 00 on 21" July, respectively, the modelled J(HCHO) values were used, because no measmements were available for this time period. The dotted line shows the calculated P(OH) from nitrous acid photolysis during daytime. The data were obtained diuing the BERLIOZ campaign at the Pabstflium site, northwest from Berlin (Platt et al., 2002).

The yields of free radicals from the photolysis of nitrous acid and of aldehydes should be established. 

Cox, R. A., The Photolysis of Gaseous Nitrous Acid—A Technique for Obtaining Kinetic Data on Atmospheric Photooxidation Reactions, Ini. J. Chem. Kinet. Symp., 1, 379-398 (1975). 

It should be noted that only a portion of the O( D) formed generates OH via reaction (2a) the remainder is deactivated to ground-state 0(3P), reaction (2b), which then re-forms O,. For example, at 50% RH and 300 K at the earth s surface, about 10% of the O( D) formed generates OH. As a result, as discussed later in this chapter, the relative importance of (2a) decreases at higher altitudes due to the decrease in water vapor. This is also an important source in polluted areas, where, however, there are additional sources as well. These include the photolysis of gaseous nitrous acid (HONO) and hydrogen peroxide (H202) ... 

In a similar manner, 11/3-nitrites, on photolysis, attack Ci to furnish the corresponding oximes which, upon nitrous acid treatment, gave 18-oxygenated steroids. The most spectacular application of the Barton reaction has been a three-step synthesis of aldosterone.12 Irradiation of corticosterone-11-nitrite (27, R = NO), followed by nitrous acid treatment, gave aldosterone (29) in 15% overall yield. Comparable conversions of 11/3-hydroxyprogesterone to 21-desoxyaldosterone12 and A1-corticosterone to A aldosterone14 have also been achieved. 

At 298 K and atmospheric pressure with 50% relative humidity, about 0.2 HO" are produced per O( D) atom formed. Photolysis of 03 in the presence of water vapor is the major tropospheric source of HO", particularly in the lower troposphere where water vapor mixing ratios are high (for an explanation of the term mixing ratio see below). Other sources of HO" in the troposphere include the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other carbonyls in the presence of NO, and the dark reactions of 03 with alkanes. Note that all these processes involve quite complicated reaction schemes. For a discussion of these reaction schemes we refer to the literature (e.g., Atkinson, 2000). 

The OH (X2n) can be generated from the photolysis of water, hydrogen peroxide, and nitric and nitrous acid. Reactions of OH(A2n) with various hydrocarbons arc important in understanding photochemical smog formation (see Section VIII—2). 

The importance of conformation in selective attack of the 1 l/ -alkoxy radical on C18 and C19 was nicely illustrated by the photolysis of the nitrite 18 derived from the dienone 19. This afforded only C18 attack.7 Treatment of the derived oxime with nitrous acid gave 1-dehydro-aldosterone acetate 20. This is a convenient precursor for labelled aldosterone acetate since selective hydrogenation with tritium gives 1,2-ditritiated aldosterone acetate. 

An enhancement of nitrophenol formation has, for instance, been observed upon irradiation of dissolved Fe(III) and nitrite/nitrous acid, when compared with NO2VHNO2 alone [85]. If the solution pH is sufficiently acidic to allow the existence of the photoactive Fe(III) species FeOH2+, UV photolysis yields hydroxyl [86] that can oxidise nitrite to nitrogen dioxide (reaction 20) ... 

Finally, 4-nitrosophenol has also been detected as a transformation intermediate of phenol upon nitrate photolysis [54,58,79,99,100]. The occurrence of 4-nitrosophenol might be accounted for by the irradiation of photoformed nitrite and by the thermal reactions induced by nitrous acid. Indeed, the formation of 4-nitrosophenol upon nitrate irradiation is favoured at acidic pH [58] and 4-nitrosophenol forms in relevant amount in the presence of both nitrite under irradiation [44,59,110] and nitrous acid in the dark [55, 65-67]. 

The irradiation of nitrophenols in the presence of nitrite and nitrous acid mainly results in hydroxylation processes upon reaction with OH, leading to the corresponding dihydroxynitrobenzenes [109]. The absorption spectra of nitrophenols, nitrite and nitrous acid overlap, thus irradiation also results in the direct excitation of nitrophenols, with possible photolysis. Accordingly, it is very likely that many of the minor intermediates detected in the cited sys-... 

Barton reaction In the Barton reaction " a methyl group in the 7 -position to an OH group is converted into an oxime group. For example, corticosterone acetate (2.45) on reaction with nitrosyl chloride (NOCl) in pyridine forms its 1 l -nitrite 2.46, which on photolysis in toluene gives aldosterone acetate oxime (2.47) in 21% yield. Treatment of 2.47 with nitrous acid (HNO2) gives aldosterone acetate (2.48) (Scheme 2.40). 

Inorganic Reactions. Photooxidation of propylene in the presence of oxides of nitrogen involves numerous inorganic reactions. The role of the NO2 photolysis in initiating O- and Og-reactions has already been discussed. Another inorganic compound of photochemical interest is nitrous acid, HONO, since it provides another source for OH radicals. A reaction scheme for the formation and subsequent photodissociation... 

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