Free radicals complex, mechanism

The rate law obtained from a chain-reaction mechanism is not necessarily of the power-law form obtained in Example 7-2. The following example for the reaction of H2 and Br2 illustrates how a more complex form (with respect to concentrations of reactants and products) can result. This reaction is of historical importance because it helped to establish the reality of the free-radical chain mechanism. Following the experimental determination of the rate law by Bodenstein and Lind (1907), the task was to construct a mechanism consistent with their results. This was solved independently by Christiansen, Herzfeld, and Polanyi in 1919-1920, as indicated in the example. 

Mo containing Y zeolites were also tested for cyclohexene oxidation with oxygen as oxidant and t-butyl hydroperoxide as initiator [86]. In this case the selectivity for cyclohexene oxide was maximum 50%, 2-cyclohexene-l-ol and 2-cyclohexene-l-one being the main side products. The proposed reaction scheme involves a free radical chain mechanism with intermediate formation of cyclohexenyl hydroperoxide. Coordination of the hydroperoxide to Mo + in the zeolite and oxygen transfer from the resulting complex to cyclohexene is believed to be the major step for formation of cyclohexene oxide under these conditions. 

The catalysis of the selective oxidation of alkanes is a commercially important process that utilizes cobalt carboxylate catalysts at elevated (165°C, 10 atm air) temperatures and pressures (98). Recently, it has been demonstrated that [Co(NCCH3)4][(PF6)2], prepared in situ from CoCl2 and AgPF6 in acetonitrile, was active in the selective oxidation of alkanes (adamantane and cyclohexane) under somewhat milder conditions (75°C, 3 atm air) (99). Further, under these milder conditions, the commercial catalyst system exhibited no measurable activity. Experiments were reported that indicated that the mechanism of the reaction involves a free radical chain mechanism in which the cobalt complex acts both as a chain initiator and as a hydroperoxide decomposition catalyst. 

While undergoing these reactions, hydrogen peroxide may react as a molecule, or it may first ionize or be dissociated into free radicals. The mechanism is very complex in many cases and may depend on the types of catalyst and reaction conditions. 

Bodenstein and Lind have studied the thermal reaction between H2 and Br2 to form HBr. Their careful study showed this reaction to be of apparently much greater complexity than the H2-I2 reaction. As was pointed out later in the interpretations of this data by Christiansen, Herzfeld, and Polanyi the reaction proceeds by a free radical chain mechanism involving bromine and hydrogen atoms. Assuming the bromine atom concentration to be governed by the Br2 = 2Br thermal equilibrium, the mechanism of HBr formation is... 

The rate law is more complex. Two results give evidence for a free radical chain mechanism. The more compelling is the non-integral order of the reaction under some conditions—i.e., 3/2-order at [NH3]/... 

Deduced initiation kinetics from the complex free radical chain mechanism of decomposition. Small revisions in mechanism of ref. 93 led to a revised. 4-factor. 

Thermal decomposition of uniform repetitive polymers was extensively studied in literature [17-19] in relation to the thermal stability of synthetic polymers. A kinetics equation has been developed based on the study of the steps occurring during pyrolysis involving a free radical chain mechanism [17]. For some natural polymers such as rubber, this theory is directly applicable. However, for non-repetitive polymers, or for polymers with more complex decomposition pathways, the theory does not provide appropriate kinetics equations. 

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... 

Transition-metal ions such as Fe(III), Cu(II), Co(II), Co(III), and Mn(II) have been shown to be effective homogeneous catalysts for the autoxidation of sulfur dioxide in aqueous solution. Hoffmann and coworkers have shown that Fe(III) and Mn(II) are the most effective catalysts at ambient concentrations for the catalytic autoxidation of S(IV) to S(VI) in cloudwater and fogwatet (Jacob and Hoffmann, 1983 Hoffmann and Jacob, 1984 Hoffmann and Calvert, 1985). Mechanisms for the homogeneous catalysis by Fe(lII) and Mn(II) that have been proposed include a free-radical chain mechanism, a polar mechanism involving inner-sphere complexation followed by a two-electron transfer from S(IV) to bound dioxygen, and photoassisted electron transfer. 

It can, however, be questioned if the oxidation of cyclohexene in the pre.sence of several transition metals is a free-radical chain mechanism. Arzoumanian et al. have reported that the catalytic decomposition of cyclohexene hydroperoxide by a rhodium complex in benzene is not a free-radical chain reaction. They found that the decomposition of each cyclohexene hydroperoxide gives rise to only one free radical [9]. In this ca.se the decomposition products were found to be 18 % l-ol (2), 33 % 1-one (1) and 35 % polymers together with small amounts of water and oxygen. They were also able to identify two free-radical species in solution, namely the cyclohexenylperoxy radical (6) and the cyclohexenyloxy radical (7). 

One-electron oxidation or reduction of saturated molecules frequently results in the generation of free radicals . The catalysis of certain free-radical reactions by ions or complexes of transition metals, such as (Tu, Co, and Mn, which exhibit variable oxidation states, is a consequence of this. Among such reactions are the autoxidations of hydrocarbons and other organic molecules (initially to hydroperoxides), which proceed by free-radical chain mechanisms in which the important propogation steps are ... 

Whereas homolytic ligand dissociation is not commonly observed for inorganic complexes, it has been identified as an important process in organometallic chemistry where it is favored by the characteristic weakness of transition metal-alkyl tr-bonds. Recent determinations yield metal alkyl band dissociation energies for CH3— Mn(CO)s (ca. 120 kJ/mol) and for several alkylcobalt complexes (ca. 80-100 kJ/mol) . Homolytic dissociation of such complexes results in the formation of free radicals and in the opening up of free radical catalytic pathways, e.g., for hydrogenation". Important biochemical examples of free radical catalytic mechanisms, initiated by the homolytic dissociation of a transition metal-carbon bond (i.e., the 5 -deoxyadenosy 1-cobalt bond of coenzyme 8,2) are provided by the coenzyme B,2-promoted rearrangements (see Section... 

Certain oxidative addition reactions of binuclear complexes also proceed through free radical chain mechanisms, e.g. ... 

This operation is normally conducted in the liquid phase, by a chain free radical reaction mechanism, whose basic step is the formation of hydroperoxides, which then react and split to produce a wide variety of oxygenated compounds. The more complex the feed employed, the wider the variety of products obtained. 

The reactions of [W(N2)2(diphos)2] with RBr are clearly catalyzed by visible light. The homolytic fission of the R— Br bond that takes place at the metal center is preceded by the loss of one N2 molecule. The resulting C—bond is formed by an alkyl radical attack on the remaining coordinated dinitrogen. Product distribution in these photocatalyzed reactions depends on the solvent and the stability of the free radical. This mechanism is strongly supported by flash photolysis experiments. When gem-dibromides are used in the photo-catalyzed reaction, diazoalkanes are produced. With CH2Br2 for example, the diazomethane complex [WBr(diphos)2(N2CH2)]Br is obtained. More recently it has been shown that some of the diazoalkanes do not react with protonic acids, but that the unique carbon atom is attacked by nucleophiles such as MeLi to yield diazenido complexes. ... 

What do we mean by oxygen activation A reasonable working definition is catalysis of the oxidation of a hydrocarbon substrate by 2 not involving a classical free radical autoxidation mechanism or direct oxidation by a metal salt. The latter stipulation is needed to exclude Wacker-type oxidation processes in which the oxygen in the product is, initially at least, derived from water. On the other hand, it should not matter whether dioxygen complex formation precedes or follows the oxidation of the substrate by an oxometal complex (see Figure 4). The former pertains to liquid and the latter to gas phase processes. 

A complex sequence of chemical reactions has been postulated for the oxidation of hydrocarbon polymers. Most of the evidence supports a free-radical chain mechanism with typical steps of initiation, propagation, chain branching, termination and inhibition, parallel to the similar mechanisms postulated for volatile hydrocarbons [9-11]. 

However, polypropylene has no functional groups present for the silane to react with. Therefore, vinyl silanes are used. In this case, the vinyl group must react with the polymer molecule via a free radical reaction mechanism. Free radicals can be generated through heat, shear, or with a catalyst, such as an organic peroxide. There are various examples of two-component silane-peroxide systems. In 1986, Union Carbide introduced a product called UCARSIL PC-1 A/ PC-IB. This system was a complex mixture of organosilane and peroxide. The... 

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