Initiation mechanisms, radical reactions photolysis

The excited state of ketones can thus initiate free-radical reactions, and this is probably the mechanism for many examples of enhanced photodecomposition of environmental pollutants sensitized by acetone or other simple carbonyl compounds. A good example of such reactions is the acetone-promoted photooxidation of atrazine (24) and related triazine herbicides described by Burkhard and Guth (1976). In water, atrazine absorbs almost no solar UV and was accordingly quite stable to photolysis, but in the presence of large amounts of acetone (about 0.13 M), its half-life was decreased to about 5 hr. The produets were N-dealkylation products and ring-hydroxylated triazines. Similar products were also identified in riboflavin-sensitized photooxidation of triazines (Rejto et al., 1983). Presumably, a principal mechanism of photodecomposition would be H-abstraction from the N-alkyl substituents of atrazine, perhaps in conjunction with electron transfer from the unshmed pairs of the nitrogen atoms. 

The rate behavior is modeled using kinetic expressions based on elementary reactions of the species involved. Generation of radicals can occur through five different initiation mechanisms. First, species such as DMPA or TED can generate either two carbon radicals or two DTC radicals. If XDT-like initiators are considered, one carbon radical and one DTC radical are generated upon photolysis, and a similar reaction for reinitiation of DTC-terminated polymer chains exists. Lastly, initiation of polymer chains by DTC radicals should be included for completeness. These reactions can be summarized as ... 

The mechanism for the photolysis of diethyl mercury is identical to that for the pyrolysis88, 90. The reaction of C2H5 with diethyl mercury, which apparently was of no importance under the conditions used in the pyrolysis studies, is also insignificant over the temperature range 25-75 °C. The photolysis produces ethyl radicals which initially have about 31 kcal.mole-1 excess energy so that the C4/C2 ratio is dependent on the surface/volume ratio and on the total pressure in the system. At 25 °C with essentially thermalized radicals (200 torr Ne added) CJC2 20 while at 20 torr CJC2 3. 

The mechanism that accounts for the oleofinic oxidation by hydroxyl radicals is the hydrogen abstraction. Moreover Cl radicals may also be an important mechanism for chlorinated organics. When the C-Cl bond is broken by photolysis, a Cl radical is released and can initiate additional oxidation reactions through a chain mechanism as follows ... 

Photolysis of ketones in micelles with simultaneous application of an external magnetic field permits a C isotope enrichment. (Cf. Section 6.1.5.5.) This is the case because C nuclei have a magnetic moment and thus accelerate the spin inversion by the hyperfine interaction mechanism. (Cf. Example 4.9.) Due to the more efficient recombination of radicals containing C, the initial product formed after photolysis in a back reaction is C enriched (TUrro et al., 1980b). 

Comparison with Direct Photolysis Process. The Ti02-mediated photocatalytic oxidation reaction involves a complex free-radical reaction mechanism in which OH radicals are responsible for the oxidation of 4-chlorophenol. The initial reaction step produces 4-chlorocatechol as the main product. In contrast, the direct photolysis of 4-chlorophenol produces a different set of reaction products. Figure 8 shows that the direct photolysis of... 

Atmospheric Reaction Mechanism. In the atmosphere the reaction subsequent to OH radical attack have not been unambiguously determined. It Is presumed that the radicals formed will Initially react with O2 and ultimately produce Cl or Br atoms or CIO or BrO radicals. The photolysis of CFCI3 and CF2CI2 under laboratory conditions leads ultlMtely to the formation of COFCl and C0F , respectively (180, 181) ... 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

From the prevailing NO and HONO levels occurring during this period of the irradiation, the HONO photolysis rate (11,14), and the rate constant for the OH + NO reaction (15), we estimate that steady state OH levels of "2 x 1Q7 molecule cm" were present. From this OH radical concentration and assuming an UDMH + OH rate constant similar to those observed for N2Hi and MMH (, j ) we calculate a UDMH decay rate which is in reasonable agreement with what is observed. Thus, the HONO level measured during the initial period is entirely consistent with our assumed mechanism. 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

The incremental reactivity of a VOC is the product of two fundamental factors, its kinetic reactivity and its mechanistic reactivity. The former reflects its rate of reaction, particularly with the OH radical, which, as we have seen, with some important exceptions (ozonolysis and photolysis of certain VOCs) initiates most atmospheric oxidations. Table 16.8, for example, also shows the rate constants for reaction of CO and the individual VOC with OH at 298 K. For many compounds, e.g., propene vs ethane, the faster the initial attack of OH on the VOC, the greater the IR. However, the second factor, reflecting the oxidation mechanism, can be determining in some cases as, for example, discussed earlier for benzaldehyde. For a detailed discussion of the factors affecting kinetic and mechanistic reactivities, based on environmental chamber measurements combined with modeling, see Carter et al. (1995) and Carter (1995). 

The best evidence for the photolytic decomposition of mercaptans and disulfides into free radicals involves photoinitiation of polymerization of olefins. Thus, photolysis of disulfides initiates the copolymerization of butadiene and styrene,154 as well as the polymerization of styrene207 and of acrylonitrile.19 Thiophenol and other thiols promote polymerization upon ultraviolet irradiation.19 Furthermore, the exchange of RS-groups between disulfides and thiols is greatly accelerated by light. Representative examples are benzothiazolyl disulfide and 2-mercapto-thiazole,90 tolyl disulfide and p-thiocresol, and benzyl disulfide and benzylmercaptan.91 The reaction probably has a free radical mechanism. Similar exchange reactions have been observed of RS-groups of pairs of disulfides have been observed.19... 

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