Free-radical attack

In contrast, perfluoronaphthalene gives a mixture of products with benzoyl peroxide, but they mainly arise from attack by C6H5C(0)0- [173]. 

Pyrroles, furans and thiophenes react preferentially with free radicals at the 2-position. Thus, reaction of pyrrole with benzyl radicals gives 2-benzylpyrrole. With triphenylmethyl radicals, pyrrole behaves like butadiene giving the adduct (163). /V-Methylpyrrole undergoes free radical benzoyloxyla-tion with dibenzoyl peroxide to give the 2-benzoyloxypyrrole (164) and 2,5-dibenzoyloxypyrrole (165). Furan, however, is converted in good yield to a mixture of cis and trans addition products analogous in structure to (163). 

Arylation of TV-substituted pyrroles, thiophenes and furans occurs preferentially in the 2-position, e.g. the o-nitrophenylation of thiophene by phase transfer catalysis yields (166) (77TL1871). 

Thiophene reacts with phenyl radicals approximately three times as fast as benzene. Intramolecular radical attack on furan and thiophene rings occurs when oxime derivatives of type (167) are treated with persulfate (81JCS(P1)984). It has been found that intramolecular homolytic alkylation occurs with equal facility at the 2- and 3-positions of the thiophene nucleus whereas intermolecular homolytic substitution occurs mainly at position 2. 

The homolytic substitution of thiophene by electrophilic carbon radicals provides a good method for the synthesis of (2-thienyl)acetic and (2-thienyl)propionic acids. The electrophilic radical, CH2C02Et, generated from ICH2C02Et, H202 and catalytic Fe2+ in DMSO, reacts with thiophene to form (2-thienyl)acetic ester in 62% yield (92JOC6817). 

Reactivity of Five-membered Rings with One Heteroatom 

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Free-Radical Reactions. Free radicals attack isoprene, and two competing mechanisms, at the double bond or involving C—H bonds, are postulated ... 

Substitution Reactions. Substitution reactions can occur on the methyl group by free-radical attack. The abstraction of an aHybc hydrogen is the most favored reaction, followed by addition to that position. 

Free-Radical Addition. Free-radical attack on a butylene occurs so that the most stable radical carbon stmcture forms. Thus, in peroxide-catalyzed addition of hydrogen haUdes, the addition is anti-Markovnikov. 

Whereas polyisobutylene and butyl mbber exhibit chain cleavage on free-radical attack, halobutyls, particulady bromobutyl and CDB, are capable of being cross-linked with organic peroxides. The best cure rate and optimal properties are achieved using a suitable co-agent, such as y -phenjiene bismaleimide. This cure is used where high temperature and steam resistance is required. 

Electrophilic attack Nucleophilic attack Free radical attack Photochemical reactions Oxidative and reductive reactions... 

The concept and use of free radical attack on pyrimidines has been little developed. However, pyrimidine does react slowly with p-nitrobenzenediazonium chloride to yield some 2- and 4-p-nitrophenylpyrimidines (51JCS2323) in addition, 2,4-and 4,6-dimethyl-pyrimidine are converted by hydroxymethylene radicals (from ammonium peroxydisul-fate/methanol) into 6- and 2-hydroxymethyl derivatives, respectively (77H(6)525). Certain bipyrimidine photoproducts appear to be formed from two similar or dissimilar pyrimidinyl radicals (see Section 2.13.2.1.4). 

Free radical attack at the ring carbon atoms... 

The formation of derivatives of this type by free-radical attack has been mentioned previously (see section E above). The most common route to vinylogous halo ketones is by halogenation of dienol acetates or ethers. Both free halogen and A -halo compounds may be employed, and this approach has frequently been used to obtain 6 (axial) halo compounds ... 

Compounds containing susceptible C—H bonds can be oxidized to alcohols. " Nearly always, the C—H bond involved is tertiary, so the product is a tertiary alcohol. This is partly because tertiary C—H bonds are more susceptible to free-radical attack than primary and secondary bonds and partly because the reagents involved would oxidize primary and secondary alcohols further. In the best method, the reagent is ozone and the substrate is absorbed on silica gel. Yields as high as 99% have been... 

Addition to conjugated systems can also be accomplished by any of the other three mechanisms. In each case, there is competition between 1,2 and 1,4 addition. In the case of nucleophilic or free-radical attack, the intermediates are resonance hybrids and behave like the intermediate from electrophilic attack. Dienes can give 1,4 addition by a cyclic mechanism in this way ... 

The effectiveness of incineration has most commonly been estimated from the heating value of the fuel, a parameter that has little to do with the rate or mechanism of destraction. Alternative ways to assess the effectiveness of incineration destraction of various constituents of a hazardous waste stream have been proposed, such as assessment methods based on the kinetics of thermal decomposition of the constituents or on the susceptibility of individual constituents to free-radical attack. Laboratory studies of waste incineration have demonstrated that no single ranking procedure is appropriate for all incinerator conditions. For example, acceptably low levels of some test compounds, such as methylene chloride, have proved difficult to achieve because these compounds are formed in the flame from other chemical species. 

The polymerization of ethylene starts with the thermal decomposition of an initiator molecule, whose general formula is R — O — O — R. Heating breaks the weak O—O single bond to form a pair of R — O free radicals. Free radicals are highly reactive molecules that contain unpaired electrons. A free radical will attack any bond that has exposed electron density. In this case, a free radical attacks the bond of an ethylene molecule ... 

While free radical attack in step (i) is by no means confined to carbon atom 4, the products obtained in the reactions involving the lower polyisoprenes indicate that this process is the dominant one. Likewise in step (ii) sulfur may frequently add at carbon atom 4 rather than at atom 2. Addition in the manner shown is indicated, however, by infrared spectra, which reveal the formation of —CH=CH— groups during vulcanization. The scheme accounts also for the observed constancy of the C/H ratio during vulcanization and for the relatively low efficiency of utilization of sulfur in the formation of cross-linkages in the absence of accelerators. A preponderance of the sulfur is involved in addition without formation of cross-linkages a considerable fraction of the thus-combined sulfur may occur in five- and six-membered heterocyclic rings formed by the mechanisms indicated. 

Evidence for the Existence of Free 3. Defences against Free-radical Attack 77... 

Human chronic inflammatory diseases are characterized by populations of cells with altered regulation and function. A large body of evidence suggests that many of these cellular abnormalities may be linked to an increase in the production of free radicals and/or deficiencies of antioxidant defence systems. Oxygen free radicals attack cell structures, altering their function, and are cytotoxic. They have therefore been implicated in the pathogenesis of rheumatoid arthritis as well as many other human diseases (HaUiwell, 1991). 

In inflammatory conditions, activated PMNs may pro-teolytically (by release of lysosomal enzymes) and oxidatively (by release of HOCl) inactivate ai-antitrypsin. Studies of synovial fluid samples from patients with RA showed that a i-antitrypsin was both cleaved and oxidized, resulting in inactivation (Chidwick et al., 1991, 1994). Free-radical attack on ai-antitrypsin and its subsequent inactivation may contribute to the destruction of joint tissues in arthritis due to the imbalance between elastase and its inhibitors. 

The hydroxylation of single- and double-stranded DNA are changes characteristic of free-radical attack (reviewed by Winyard et al., 1992). An important mechanism is site-specific OH generation, catalysed by iron bound to... 

Define grafting. How is free radical attack employed during a grafting process ... 

Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

Along with free radical atom abstraction reactions, reactions of radical substitution are known where a free radical attacks the weak Y—Y bond and abstracts radical Y [56] ... 

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