Nitrous acid, photochemical reactions with

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 irradiation of nitrate in the presence of 2-phenylphenol yields oxidation (phenylhydroquinone, phenylbenzoquinone, 2-hydroxydibenzofuran) and nitration intermediates (2-hydroxy-3-nitrobiphenyl and 2-hydroxy-5-nitrobiphenyl) [124]. 2-Hydroxydibenzofuran is formed by photocyclisation of phenylbenzoquinone. Nitration might be due to reaction with nitrogen dioxide, but an interesting aspect is the effect of pH. The fact that nitration is favoured at low pH might indicate a role of photoformed nitrous acid, due to either photochemical or dark reactivity [124], It is worth noting that the... 

Three membered Rings.—As has been mentioned earlier, oxaziridines are readily formed by photochemical isomerization of nitrones. A purely chemical method has been discovered by some Japanese workers during the course of their work with ethyl 3-hydroxypyrrole-2-carboxylates (240). They have found that the esters produce oxaziridines (241) upon reaction with nitrous acid rather than the expected nitroso derivatives. Ring expansion may occur via a push-pull mechanism and is also seen upon reaction of (240) with aryl diazonium salts, when diaziridines (242) are formed if the pH of the reaction is carefully controlled. 

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