Initiation, free radical reactions

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

Azobisisobutyronitrile (AIBN) is perhaps the most widely used initiator. It undergoes either thermolytic (A) or photolytic cleavage hv) to give isobutyronitrile radicals which can initiate free-radical reactions. 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

We must not confuse the effects of peroxides, which may have been formed by the action of oxygen, with the effects of oxygen itself. Peroxides initiate free-radical reactions oxygen inhibits free-radical reactions (see Sec. 2.14). 

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. 

In photochemical degradation is necessary to generate an excited state, which may occur due to light incidence on the polymer. When the polymer is irradiated with Kght energy corresponding to the electronic transition of the chromophore existing as part of the chain or as a contaminant, of free radicals form and initiate free radical reactions. ... 

These repetitive excursions into the combustion zone decrease the useful life of oil and result in carbonaceous deposits that cause wear and reduce performance. These may only represent transient temperatures for a small portion of the oil however, it should be noted that, at the molecular level, these higher temperatures are sufficient to break covalent bonds, initiate free-radical reactions, and permit reactions to occur that would have very low rates at lower temperatures. 

Analysis of the pigment profiles in extracts of developing chloroplasts treated with the aryltriazinediones, indicated a build-up of protoporphyrin IX, a recognized potent photosensitizer capable of initiating free radical reactions f21.22L The aryltriazinediones did not have an effect on the ability of the chloroplasts to convert protoporphyrin IX into Mg protoporphyrin IX, but were found to be sensitive inhibitors of protoporphyrinogen oxidase (a PI5Q... 

The formation of nitroxy] radicals testifies to the fact that main-chain decomposition by reaction (24) and side-chain ester group cleavage by reaction (26) take place in the polymer. Thus, the interaction of NO-, with double bonds is able to initiate free-radical reactions of polymer degradation when hydrogen atom abstraction reactions from C-H bonds are inefficient. 

Linseed oil, tung oil, and other highly unsaturated oils are used as the basis for the oil-based paints. They dry (polymerize) by an oxidative mechanism, forming ether bonds between the triglyceride molecules, and through a series of oxidatively initiated free radical reactions attacking the double bonds (47,48). Since multiple points of oxidation are present, a three-dimensional polymeric network results. 

As has already been shown, polyolefins are subjected to photodegradation upon exposure to sunlight. Additives and impurities eatalyze the breaking of the polymer chain by a series of UV-initiated free radical reactions. Some efforts foeus on the addition of photosensitive species, such as carbonyl groups or metal complexes, to accelerate these proeesses. 

A method of four-carbon homologation is described in the oxygen-initiated free-radical reaction of trialkylboranes with butadiene monoxide 4-alkylbut-2-en-l-ols are produced, the tra 5-olefin isomer predominating, as shown in Scheme 129. 

---