Subsequent photolysis reactions

Photolysis of chromium alkoxycarbenes with azoarenes produced 1,2- and 1,3-diazetidinones, along with imidates from formal azo metathesis (Eq. 21) [85, 86]. Elegant mechanistic studies [87-89] indicated the primary photoprocess was trans-to-cis isomerization of the azoarene followed by subsequent thermal reaction with the carbene complex. Because of the low yields and mixtures obtained the process is of little synthetic use. 

Thiophenols may also be synthesized via the photochemical decomposition route 156). Thus, treatment of arylthallium ditrifluoroacetates with an aqueous solution of potassium Ar,AT-dimethyldithiocarbamate led in quantitative yields to the formation of the corresponding aryl AT,A -dimethyl-dithiocarbamates. Subsequent photolysis in aqueous acetone then led to disulfides which were reduced to the thiophenols. A small amount of aryldithiocarbamate formed as a by-product in the photolysis was converted to the same thiophenol by hydrolysis. The overall reaction sequence is illustrated in Eq. (18). 

Photolysis of the nitrosimines, such as 35a, gives rise to a variety of products that appear to have come from loss of NO and subsequent radical reactions (Scheme 3.22) [196]. Similar products, indicative of radical reactions, are also observed in the thermolysis of sterically hindered nitrosimines (e.g., 40, with Rj = tert-Bu and R2 = 2-MePh) [197]. Steric constraints were proposed to disfavor the cyclic pathway... 

The end result of these studies showed very clearly that two major processes were important, i.e. photolysis and photo-oxidation. Photolysis reactions were posited to be the result of the well-known Norrish Type 1 and Norrish Type 2 cleavage reactions. As we shall see, the Type 1 cleavage followed by several subsequent reactions can account for many of the observed degradation products. 

Several subsequent dark reactions involving Ph—NO2H and R- have been discussed elsewhere (and are not repeated here), to account for both the formation of phenylhydroxylamine and acetone 8,ii,i6,i8) and to explain the nature of radicals detected in ESR-Studies upon photolysis of nitrobenzenes in hydrogen donor solvents 21-24) ... 

The photoelectrochemical kinetic scheme involves a photochemical reaction that is followed by an electrochemical reaction. The photochemical reaction is used to produce or deactivate the reducing agent. Catalytic metallic nuclei are formed in the subsequent electrochemical reaction. For example, the Fe reducing agent (Red) needed for reaction (8.24) is generated in the photochemical reduction of complexed Fe ions. This redox photolysis is the ligand-to-metal charge transfer with the overall reaction of oxidation of [ 204] ... 

A slightly modified approach has also been used in a number of studies in which the sum of (03 + N02) is plotted against NOz (e.g., St. John et al., 1998). This minimizes the effects of short-term variations in 03 caused by its rapid reaction with NO. Thus, when 03 is titrated by the NO reaction, the measured 03 concentrations will be small however, the N02 generated is a source of 03 through its subsequent photolysis. Flence the sum of (03 + N02) is sometimes chosen as a measure of the ultimate formation of ozone. The ozone production efficiency determined from slopes of plots of (03 + N02) against NOz in the Nashville, Tennessee, area was measured to be typically 5-6 if it was assumed that NOr is not removed by other processes. The production efficiency appeared to be about the same for the general urban plume and for an air mass in which a plume from a power plant was also embedded. Including other losses for NOy such as deposition lowers the estimated production efficiency by about a factor of two (St. John et al., 1998 Nunnermacker et al., 1998). 

This relationship between 03 and NOz = (NO,. -NOx) in areas downwind of urban centers can be anticipated, based on the oxidation of NO to N02 by H02 and R02 radicals and the subsequent photolysis of N02 to form 03. As discussed in Chapter 6.J, at lower NOx concentrations, reactions of H02 and R02 with themselves and each other compete with their reactions with NO. However, the oxidation of NO to N02 leads to 03 formation since photolysis of N02, generating 03, is a major fate for N02. This then gives rise to the observed relationship between NOz and 03. Under these low-NOx conditions, the formation of H202 and other peroxides is important and deter-... 

Photolysis of 2-oxetanones gives decarboxylative cleavage to alkenes, similar to pyrolysis, but subsequent photoaddition reactions of the alkene product may lead to complex reaction mixtures. A very useful example of 2-oxetanone photolysis is that of 5-oxabicyclo[2.2.0]oct-2-en-6-one, the photoisomer of a-pyrone when it was irradiated in a argon matrix at 80 K, carbon dioxide and cyclobutadiene were formed (equation 7) (73JA1337). 

Postcolumn photochemical reactions are another approach to the detection problem. High-intensity UV light, generally provided by a Hg or Zn lamp, photolyzes the HPLC effluent, which passes through a Teflon (47) or quartz tube. The photolysis reaction determines the nature of the subsequent detection. If the compound has a UV chromophore, such as an aromatic ring, and an ionizable heteroatom, such as chlorine, then the products of the reaction can be detected conductometrically. Busch et al. (48) have examined more than 40 environmental pollutants for applicability to detection with photolysis and conductance detection. Haeberer and Scott (49) found the photoconductivity approach superior to precolumn derivatization for the determination of nitrosoamines in water and waste water. The primary limitation of this detection approach results from the inability to use mobile phases that contain ionic modifiers, that is, buffers and... 

Reaction Type. It is often difficult to study thermal reactions in simple molecular crystals, because so often the sample melts at the necessary reaction temperature. This problem is exacerbated when small amounts of product lower the melting point of a crystal substantially. For this reason photolysis, especially at low temperature, has been a preferred method of initiation. As the sample is warmed from cryogenic temperature, subsequent thermal reactions of the photolytic intermediates can be studied with less danger of melting. 

Some molecules in this group (HONO, NC j 0, HONC ) have been extensively studied because the photofragments OH and NO can be probed by tunable lasers. These molecules are important minor constituents in the earth atmosphere and their photochemistry plays a major role in air pollution. Atmospheric pollutants N0X (NO, NO2, NO3) are formed from combustion of fuel and subsequent chemical reactions in the atmosphere. Photolysis of alkyl oxides produces NO and NO2 that can be probed by LIF the internal energy distribution provides an important clue to the mechanism of photodissociation. 

The 1,1-cycloaddition process also occurs in nonphotolytic reactions involving azomethine ylides. Thermolysis of oxazolinone (147) led to a 3,5-fused bicyclic dihydropyrrole in 80% yield.72 The alkene stereochemistry was maintained in the product, although subsequent photolysis scrambled the methyl and trideuteromethyl groups. Nondeuterated oxazolinone gave the cyclization product which was converted to a dihydropyridine on warming with acid.74... 

Figure 189 ABH reacts with aldehyde-containing compounds through its hydrazide end to form hydrazone linkages. Glycoconjugates may be labeled by this reaction after oxidation with sodium periodate to form aldehyde groups. Subsequent photolysis with UV light causes photoactivation of the phenyl azide to a nitrene. The nitrene undergoes rapid ring expansion to a dehydroazepine, which can couple to nucleophiles, such as amines.

Ethylene oxide (C2H40) and acetaldehyde (CH3CHO) were found as main products. CO was also detected. Figure 2 shows the time dependence of these three products. The yield of C2H40 increases linearly with time, which indicates that C2H40 does not react to any appreciable extent through secondary reactions or subsequent photolysis. The yield of... 

Dissolved metals affect the concentration of atmospheric trace gases, such as ozone, organics and sulfur compounds. Ozone is formed in the troposphere through a complex series of homogeneous reactions as shown schematically in Fig. 4. The chain represented in the figure by thick arrows involves the cooperative oxidation of organic molecules and NO with the intermediacy of HO, formed by subsequent photolysis of NO2 and O3, and HO2 radicals. 

As both reactions (2.37) and (2.19) lead to the oxidation of NO to NO2 the subsequent photolysis leads to the formation of ozone (see reactions (2.2) and (2.20)). The individual reaction mechanism depends on the identity of the organic compounds and the level of complexity of the mechanism. Although OH is the main tropospheric oxidation initiator, reaction with NO3, O3, 0( P) or photolysis may be an important loss route for some NMHCs or the partially oxygenated products produced as intermediates in the oxidation (see reaction (2.38)). 

The approved platinum complexes, with antineoplastic activity, differ in their photochemical stability. Among these, cisplatin, in particular, undergoes a photolysis reaction. Early studies proposed that this degradation was due to the photoaquation of the chloride ligands, and its subsequent photon-induced isomerization to trans-platin (32,33). Later studies found that the main photolysis-induced degradation product, was trichloroammineplatinate II (TCAP) (34,35). This decomposition product is limited to 1% in cisplatin for injection by the USP. 

Stutz et al. (1999) used a quasi-steady-state box model to explain the measurements of iodine oxides at Mace Head. Their model did not include the aerosol phase. They reproduced the measured 10 mixing ratios of 6 pmol mol and found that for these conditions, cycles involving the production of HOI by reaction of lO with HO2 and subsequent photolysis of HOI would lead to an O3 destruction rate of —12.5 nmol mol d , whereas the lO self-reaction would lead to an O3 destruction rate of —3.8 nmol mol d . ... 

Although complex chemical transformations — mainly photochemical — take place in the atmosphere, many chemically stable compounds may be transported intact via the atmosphere and subsequently enter the aquatic and terrestrial environments in the form of precipitation. Although the whole issue of chemical reactions in the troposphere lies outside the scope of this account, some comments are given in Chapter 4, Section 4.1.2, and reference should be made to the comprehensive account of principles given by Finlay-son-Pitts and Pitts (1986). The persistence in the troposphere of xenobiotics — even those of moderate or low volatility — is determined by the rates of transformation processes. These involve reactions with hydroxyl radicals, nitrate radicals, and ozone, or direct photolysis. Reactions with hydroxyl radicals are generally the most important. Illustrative values are given for the rates of reaction (cm3 s 1 molecule1) with hydroxyl radicals, nitrate radicals, and ozone (Atkinson 1990). 

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