Ionic and radical mechanisms

Ionic and Radical Mechanisms in Olefinic Systems, whith special Reference... 

Ionic and radical mechanisms have been proposed R ... 

The results obtained57 were explained by competition of ionic and radical mechanisms, which lead to Markovnikov and anti-Markovnikov adducts, respectively. At this competition the nature of the unsaturated compound play an important role in determining the preferred mechanism. Thus, the major formation of Markovnikov adducts and therefore the preference of the ionic mechanism in the series of olefins styrene > 1-methylcyclohexene > 2,3-dimethyl-1-butene > isobutene > 1-heptene correlates with the ability of substituents to stabilize the intermediate carbenium ion. 

Fig. 4. Ionic and radical mechanisms in the syntheses of hindered ketones from the copper-catalyzed reaction of Grignard reagents with acid chlorides.

Reductions at anomeric positions, both by ionic and radical mechanisms, deliver hydrogen from the axial direction. Kahne prepared the hemithio orthoester 75 from thiolactone and subjected it to tin hydride reduction, which resulted in predonunant formation of the -glycoside [116] (O Scheme 40). 

Heating dihydroartemisinin (65) for 190°C gives deoxyartemisinin (19) and an epimeric mixture of the aldehydes (66) in yields of 30% and 50% respectively (Equation (5)) <8672181 >. Dehydration accounts for the formation of (19) and loss of formic acid for (66). Ionic and radical mechanisms can possibly explain the different reaction courses. 

It is known that triethyl phosphite reacts with carbon tetrachloride to give ethyl chloride and diethyl (trichloroethyl)phosphonate. Ionic and radical mechanisms have been suggested to account for this remarkable reaction. According to the most reasonable, fundamental one (61), the reaction is initiated by a nucleophilic attack on chlorine (not an Arbusov reaction) followed by formation of the phosphonium salt [71] (Atkinson et al., 1969). 

J.-P. Machon, Structure of polyethylene produced at high pressure by simultaneous ionic and radical mechanisms, Makromol. Chtan. 179, 1581-1591 (1978)... 

Later Dudina and Enikolopyan [4-12] expressed considerations in favor of the possible occurrence of the process of thermal destruction of polyformaldehyde according to ionic and radical mechanisms. Moreover, the authors established that in the thermal decomposition of the polymer, the natural gaseous product is monomeric formaldehyde. The participation of oxygen in the thermal oxidation of polyformaldehyde is manifested only in an initiation of depolymerization, which proceeds according to the "laws of chance" for the polymer with blocked terminal groups, and in the fact that the oxidative direction of the reaction is absent in this case. 

In order to improve their properties, poly(VDF-co-HFP) copolymers or poly(VDF-ter-HFP-ter-TFE) terpolymers can be crossUnked by bisnucle-ophiles, such as diamines or bisphenols, or by irradiation. On the other hand, poly(VDF-ter-HFP-ter-termonomer containing an iodine or bromine atom) terpolymer can be crosslinked by peroxide/coagent systems. Those three main ways of crosslinking exhibit two main crosslinking mechanisms (ionic and radical mechanisms) and different properties. 

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