Free radicals redox reactions

Arenesulfonyl chlorides77 as well as alkenesulfonyl chlorides78 react with vinylarenes in the presence of RuCl2(PPh3)3 and 1 molar equiv. of Et3N to form a,/ -unsaturated sulfones in 70-90% yields. The reaction mechanism for the ruthenium(II) catalyzed reaction involves a free-radical redox-transfer chain process as outlined below77 ... 

II) Mx+1 can destroy free radicals, being reduced to Mx in the process. The reduction of a redox metal by a free radical can be illustrated as follows Mx + 1 + A- — Mx + A+1, where A- is =Si and =SiCH2. As long as sufficient Mx+1 is available in the system in a well dispersed state, free radical chain reactions are quickly terminated by the destruction of these radicals thus, the rate of oxidative damage is... 

Especially important are the radical-redox reactions by which transition metals promote free radical generation by catalyzing the decomposition of the peroxides formed during bleaching. This faster free radical generation results in higher oxidation rates and a more extensive depolymerization of the cellulose. 

Moore DE, Chen BC. Reactions of non-steroidal anti-inflammatory drugs with polyacrylamide free radicals. Redox Report 1997 3 41 7. 

Thermal oxidation is also autocatalytic and considered as metal-catalyzed because it is very difficult to eliminate trace metals (from fats and oils or food) that act as catalysts and may occur as proposed in Equation 4. Redox metals of variable valency may also catalyze decomposition of hydroperoxides (Scheme 2, Equations [6] and [7]). Direct photooxidation is caused by free radicals produced by ultraviolet radiation that catalyzes the decomposition of hydroperoxides and peroxides. This oxidation proceeds as a free radical chain reaction. Although there should be direct irradiation from ultraviolet light for the hpid substrate, which is usually uncommon under normal practices, the presence of metals and metal complexes of oxygen can become activated and generate free radicals or singlet oxygen. 

Atom transfer radical polymerization (ATRP) reactions mediated by transition metals have also garnered interest.Essentially, ATRP reactions are similar to traditional free-radical polymerization reactions in that they can be described by initiation, propagation, and chain-transfer steps involving carbon radicals. Transition metals mediate this process via redox processes (M => and promoting chain transfer by donation of a... 

However, a commercially feasible process for bulk polymerization in a continuous stirred tank reactor has been developed by Montedison Fibre [103,104]. The heat of reaction is controlled by operating at relatively low-conversion levels and supplementing the normal jacket cooling with reflux condensation of unreacted monomer. Operational problems with thermal stability are controlled by using a free radical redox initiator with an extremely high decomposition rate constant. Since the initiator decomposes almost completely in the reactor. 

Redox initiators produce polymerization-inducing free radicals by reaction of a reducing agent with an oxidizing agent. The required thermal activation energy is quite low, so that polymerizations can be induced at much lower temperatures than is the case for purely thermal decomposition of peroxides or peresters. Five kinds of redox systems can be distinguished ... 

Free radical polymerization reactions are conducted under an inert atmosphere. Initiation with thermal or redox free-radical initiators is common [9,14], Industrial processes are designed to reduce residual monomer levels to below regulated limits [24, 25], This is achieved by pushing the reaction to completion by increasing the reaction temperature or increasing the initiator levels toward the end of the reaction. Also postpolymerization strategies have been employed, such as an enzymatic treatment with amidase [26]. 

The agreement between the changes in the observed rate laws of the MB oxidations of thiols with changes in the reactant concentrations, pH and ionic strength and the changes predicted from the steady-state derived rate laws based on the proposed mechanism for disulfide formation support this mechanism. Of particular interest to us is that it is possible to extract information concerning the behavior of a free radical chain reaction by kinetic analysis of the reaction. Such kinetic analysis of other redox reactions may be expected to be of value in not only establishing the possible intermediacy of free radicals but also the details of steps in the chain sequence in which free radicals are reaction intermediates. 

Hydrogen peroxide may react directiy or after it has first ionized or dissociated into free radicals. Often, the reaction mechanism is extremely complex and may involve catalysis or be dependent on the environment. Enhancement of the relatively mild oxidizing action of hydrogen peroxide is accompHshed in the presence of certain metal catalysts (4). The redox system Fe(II)—Fe(III) is the most widely used catalyst, which, in combination with hydrogen peroxide, is known as Fenton s reagent (5). 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

The reaction rate of fumarate polyester polymers with styrene is 20 times that of similar maleate polymers. Commercial phthaHc and isophthaHc resins usually have fumarate levels in excess of 95% and demonstrate full hardness and property development when catalyzed and cured. The addition polymerization reaction between the fumarate polyester polymer and styrene monomer is initiated by free-radical catalysts, commercially usually benzoyl peroxide (BPO) and methyl ethyl ketone peroxide (MEKP), which can be dissociated by heat or redox metal activators into peroxy and hydroperoxy free radicals. 

Catalyst Selection. The low resin viscosity and ambient temperature cure systems developed from peroxides have faciUtated the expansion of polyester resins on a commercial scale, using relatively simple fabrication techniques in open molds at ambient temperatures. The dominant catalyst systems used for ambient fabrication processes are based on metal (redox) promoters used in combination with hydroperoxides and peroxides commonly found in commercial MEKP and related perketones (13). Promoters such as styrene-soluble cobalt octoate undergo controlled reduction—oxidation (redox) reactions with MEKP that generate peroxy free radicals to initiate a controlled cross-linking reaction. 

Polymerization Initiator. Some unsaturated monomers can be polymerized through the aid of free radicals generated, as transient intermediates, in the course of a redox reaction. The electron-transfer step during the redox process causes the scission of an intermediate to produce an active free radical. The ceric ion, Ce" ", is a strong one-electron oxidizing agent that can readily initiate the redox polymerization of, for example, vinyl monomers in aqueous media at near ambient temperatures (40). The reaction scheme is... 

Ascorbic acid is a reasonably strong reducing agent. The biochemical and physiological functions of ascorbic acid most likely derive from its reducing properties—it functions as an electron carrier. Loss of one electron due to interactions with oxygen or metal ions leads to semidehydro-L-ascorbate, a reactive free radical (Figure 18.30) that can be reduced back to L-ascorbic acid by various enzymes in animals and plants. A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-aseorbie add. Ascorbic acid and dehydroascor-bic acid form an effective redox system. 

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