Reactivity radical-type

These results, as well as rate studies " and kinetic isotope effects ", support a concerted, 5ptra-structured oxenoid-type transition state for the CH oxidations". The original oxygen-rebound mechanism has been discounted (see the computational work in Section I.D). Recently, however, the stepwise radical mechanism was revived in terms of the so-called molecule-induced homolysis , but such radical-type reactivity was severely criticized on the basis of experimental" and theoretical grounds. 

Strategy IV. Cooperative substrate-centered radical-type reactivity based on redox non-innocent substrates... 

Although some of the species desribed in this section are stable in certain solvents or under certain conditions, they reveal a remarkably high reactivity toward a variety of substrates. This section focuses on the diverse radical-type reactivity of open-shell organometallic species of the groups 9(V111B) and lO(VIII) transition metals. 

Figure 56. Radical-type reactivity of [Rh°(por)] complexes toward isocyanides and...

While many important details of the iron- and manganese-catalyzed reactions are yet to be explored, the common features of the corresponding mechanisms are well established and also applicable in the presence of other catalysts. Thus, the formation of the SO5, SO4 and HSO5 intermediates was reported in all of the free-radical type reactions. These species are very reactive oxidants and this explains the apparent... 

HO2, was considered as a reactive intermediate in both cases. The addition of radical scavengers strongly retarded the oxidation of the phosphinate ion confirming the radical type mechanism. It was also demonstrated that the reaction ceased when the catalyst was masked with EDTA. 

Interaction between the radical-cations of primary alkyl halides and acetonitrile is on the verge of Sivj2 versus S l type reactivity. Some product is formed by direct substitutiorr, but rearrangement also occurs to give the most stable carbonium ion [20, 21], Thus 1-bromopentane yields a mixture of three N-pentylacetamides on... 

We synthesized polystyrene with benzyl ester in the middle of the skeleton by using the following initiator that dissociates into stable tetramethylpyperidinoxyl radical and reactive benzyl-type radical. 

Reactions of this type proceed by radical-chain mechanisms in which the bonds are broken and formed by atoms or radicals as reactive intermediates. This... 

In recent years, direct, time-resolved methods have been extensively employed to obtain absolute kinetic data for a wide variety of alkyl radical reactions in the liquid phase, and there is presently a considerable body of data available for alkene addition reactions of a wide variety of radical types [104]. For example, rates of alkene addition reactions of the nucleophilic ferf-butyl radical (with its high-lying SOMO) have been found to correlate with alkene electron affinities (EAs), which provide a measure of the alkene s LUMO energies [105,106]. The data indicate that the reactivity of such nucleophilic radicals is best understood as deriving from a dominant SOMO-LUMO interaction, leading to charge transfer interactions which stabilize the early transition state and lower both the enthalpic and entropic barriers to reaction, with consequent rate increase. A similar recent study of the methyl radical indicated that it also had nucleophilic character, but its nucleophilic behavior is weaker than that expressed by other alkyl radicals [107]. 

For full control of macromolecular syntheses, it is necessary to master the art of the design and synthesis of active centres, as well as of their modification during polymerization, and not only from the quantitative point of view, concerning their number and reactivity. The qualitative aspect is also of importance, the change of the basic character of the centres, for example from the anionic to the radical type. Modification of centres is at present the... 

On the one hand, part of the ozone (O3) dissolved in water reacts directly with the solutes M. Such direct reactions are highly selective and often rather slow (minutes). On the other hand, part of the ozone added decomposes before it reacts with solutes this leads to free radicals. Among these, the OH radicals belong to the most reactive oxidants known to occur in water. OH can easily oxidize all types of organic contaminants and many inorganic solutes (radical-type reactions). They are therefore consumed in fast reactions (microseconds) and exhibit little substrate selectivity. Only a few of their reactions are of specific interest in water treatment processes. Measured oxidations in model solutions indicate up to 0.5 mol OH formed per mole of ozone decomposed. The higher the pH, the faster the decomposition of ozone, which is catalyzed by hydroxide ions (OH ). The decomposition is additionally accelerated by an autocatalyzed sequence of reactions in which radicals formed from decomposed ozone act as chain carriers. Some types of solutes react with OH radicals and form secondary radicals (R ), which still act as chain carriers. Others, for instance, bicarbonate ions, transform primary radicals to inefficient species () and thereby act as inhibitors of the chain reaction. Therefore the rate of the decomposition of ozone depends on the pH of the water as well as on the solutes present. The overall effect is a superposition of the direct reaction and the radical-type reaction. For a review, see Hoigne (1988). 

With these particular monomers, copolymerization is in fact random. Now, toward either free radical type, it happens, butadiene is about 1.4 times as reactive as styrene, so that, if monomer concentrations were equal, butadiene units would lend to predominate in the product. Furthermore, since butadiene is consumed faster, the relative concentrations of monomers would change as reaction goes on, and so would the composition of the polymer being produced. These effects can be compensated for by adjusting the ratio of monomers fed into the reaction vessel indeed, by control of the feed ratio, copolymers of any desired composition can be made. 

Thus, research efforts in different industrial laboratories have been directed toward the preparation of 1-bro-moalkyl alkyl carbonates assumed to be more stable than the 1-iodo derivatives, and more reactive than the parent chloro compounds. For example, 1-bromoethyl ethyl carbonate was made by the halide exchange of 1-chloroethyl ethyl carbonate with LiBr or NaBr, or by a radical type bro-mination of diethyl carbonate (Ref. 82). However, in the case of halide exchange, the conversion is low and a mixture results. Even with a large excess of bromide salt, this problem remains. Radical bromination was found to give unsatisfactory results for the same reasons than the chlorination, and failed in the case of unsymmetrical dialkyl carbonates because of its non-regioselectivity. 

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