Nitrites photodecomposition

In their pioneering studies of methyl nitrite photodecomposition, Thompson and Purkis (1936) reported the formation of products that suggested the occurrence of process (III),... 

There have been several experimental studies that help our understanding of the mechanism of ethyl nitrite photodecomposition (e.g., see Schuck and Stephens, 1967 McMillan et al., 1971 Christie and Hetherington, 1976/1977 Rose, 1979 Zabarnick and Heicklen, 1985a Morabito and Heicklen, 1985a, b). The following primary processes have been considered in explanation of the results ... 

The major product of ethyl nitrite photodecomposition in the troposphere is expected to be acetaldehyde, formed from the ethoxy radical reaction, C2H5O -I- O2 CH3CHO + HO2, as well as some ill-defined amount of CH2O as well as CH3CHO formed in process (II). 

Table IX-H-4. Estimated quantum yields of the primary processes in iso-propyl nitrite photodecomposition as a function of wavelength (based on the trend lines in figure lX-H-9)...

Data are available only for experiments at 366 nm, but these are useful in defining the processes in -propyl nitrite photodecomposition. Following the procedures employed in the studies of methyl nitrite, ethyl nitrite, and i o-propyl nitrite, the primary quantum yields of photodissociation of n-propyl nitrite should be given by ... 

There are no measurements of primary quantum yields of n-propyl nitrite at wavelengths other than 366 nm of which we are aware at this writing. However, in accord with the observations in iio-propyl nitrite photodecomposition, we will assume that... 

IX-H-5.3. Photolysis Frequencies for n-Propyl Nitrite Photodecomposition within the Lower Troposphere... 

We have used the cross sections of tert-butyl nitrite given in table IX-H-4, an assumed total quantum yield of photodecomposition of unity, independent of wavelength, and the appropriate actinic flux data (cloudless skies, vertical ozone column of 350 DU) to estimate the -values for tert-butyl nitrite photodecomposition within the lower troposphere. These data are summarized in figme IX-H-16. The data of figure IX-H-16 give a photochemical hfetime for tert-butyl nitrite of about 4.3 min. with an overhead Sun. 

Fischer, M., and P. Warneck, Photodecomposition of Nitrite and Undissociated Nitrous Acid in Aqueous Solution, . /. Phys. Chem., 100, 18749-18756(1996). 

To explain the high chemical yield in the transformation A - D, coupled with the fact that the quantum yield for the photodecomposition of nitrites is less than unity, it was suggested that the Barton reaction might take place through a radical "cage mechanism.2 However, recent studies at the Institute on the mechanism of nitrite photolysis have shown this not to be so. Photolysis of an equimolecular mixture of 3/3-acetoxy-androstan-6/8-yl nitrite (102) and 3/8-acetoxy-cholestan-6/8-yf nitrite containing 98% of nitrogen as N16 (103) in iso-octane or toluene... 

Cyclizations arising by intramolecular radical addition have been widely employed in the synthesis of oxygen and sulfur heterocycles. The tetra-hydrofuran derivatives (494) have been prepared in this way via alkoxyl radicals, generated in turn by photolysis of the unsaturated nitrites (495).431 Bridged oxabicyclic compounds have been similarly prepared,432 and the photodecomposition of a nitrite has been employed in the synthesis of the... 

Schrauzer et al. have also investigated the photoreduction of nitrate and nitrite over iron-doped Ti02 under argon and reported that nitrite was reduced to N2, but no reduction of nitrate was observed [59], The photodecomposition of ammonia over iron-doped titania which had been pretreated with hydrogen peroxide was also carried out, and was reported to yield a mixture of nitrate and nitrite [59]. Few details were found for the experiments described in this paragraph. 

Of historic interest is a reaction in which the azide group was synthesized from hydrazine and nitrite in the presence of silver ions [19,98] (see p. 24). Most commonly, silver azide is prepared by mixing aqueous solutions of hydraz-oic acid or sodium azide with silver nitrate. The product is precipitated in fine crystalline form larger crystallites are obtained from more dilute reagents [200]. One author recommended the use of an excess of silver nitrate another believed this would enhance the photodecomposition of the product [202]. Of more significance is the recommendation to make the azide in the dark, or at least under red light, [203,204] and to wash the product completely ion free. 

The photodecomposition pathways of alkyl nitrites have been reviewed by Calvert and Pitts (1966). Earlier a review of the photolysis of complex organic nitrites in solution was given by Akhtar (1964). Since the review of Calvert and Pitts... 

The photodecomposition of the higher nitrites lead to the formation of hydroxynitroso compounds. The overall reaction is... 

Acetone and a stable dimer of nitroso methane are major products of the photodecomposition of butyl nitrite at 25 °C [12]. It was suggested that dimer formation from nitroso methane monomer follows the primary process ... 

See, for example, Jacox, M.E. and Rook, F.L., Photodecomposition of methyl nitrite trapped in soKd state, /. Phys. Chem., 86, 2899, 1982... 

We review in this chapter the nature of the photodecomposition pathways of several of the major products of atmospheric oxidation of the hydrocarbons, namely, the acyclic aldehydes in section IX-B the aldehydes containing additional functional groups in section IX-C the acyclic ketones in section IX-D the cyclic ketones in section IX-E the ketones containing additional functional groups in section IX-F the acyl halides and carbonyl halides in section IX-G nitrous acid, the alkyl nitrites, the nitroalkanes, and the nitroso-compounds in section IX-H nitric acid and the alkyl nitrates in section IX-I the peroxyacyl nitrates in section IX-J the alkyl hydroperoxides in section IX-K and some oxygenates derived from the aromatic hydrocarbons in section IX-L. 

The near ultraviolet absorption bands of nitrous acid, like all nitrites, involves an n n transition, and the absorption bands overlap well the envelope of actinic flux present in the troposphere see figure IX-H-2. The photodecomposition of HONO is efficient in generating OH radicals in the troposphere. Two primary processes have been proposed to rationalize the observed photodecomposition of HONO ... 

There have been both experimental and theoretical studies of the photochemistry of the nitrites that have elucidated the mechanism of photodecomposition. Several primary processes have been considered in the previous studies of the photodecomposition of methyl nitrite ... 

If process (I) occurs in the photodecomposition of pure CH3ONO, the products CH3O and NO are expected to react largely to reform CH3ONO, so the simple measurement of the quantum yields of methyl nitrite disappearance or product formation alone cannot provide estimates of fi or fu. 

There have been several useful studies that have allowed estimates of the quantum yields of photodecomposition of methyl nitrite. Wiebe et al. (1973) provided the first meaningful estimates of primary process (I) in their studies at wavelength 366 nm. They photolyzed mixtures of CH3ONO and and measured the quantum yields of CH30 N0 and formation. From the mechanism outlined in the introduction... 

Figure IX-H-4. Photolysis frequencies for methyl nitrite versus solar zenith angle as calculated for a clear day in the lower troposphere with a vertical ozone column of 350 DU. The total quantum yield of photodecomposition was taken as either 1.0 or 0.78, independent of wavelength. 

McMillan et al. (1971) reported that the quantum yields of primary photodecomposition of isopropyl nitrite did not vary with wavelength over the banded region see section IX-H. Perhaps this invariance is also present in the other small alkyl nitrites. At this point in our limited understanding of the mechanism, the current authors have assumed that within the banded region, the photodecomposition of ethyl nitrite photolysis is wavelength independent. 

In the calculation given here, we have assumed that the quantum yield of photodecomposition of ethyl nitrite in the lower atmosphere is 0.31, independent of wavelength. Using this assumption and the approximate cross sections given in table IX-H-3, we have calculated the approximate y-values as a function of solar zenith angle for ethyl nitrite on a cloudless day in the lower troposphere and an overhead ozone column of 350 DU. The data, given in figure IX-H-7, point to a photochemical lifetime of ethyl nitrite of only about 29 min. with an overhead Sun. 

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