Chain termination, oxidation

Both chain-terminating oxidation inhibitors, e.g. hindered phenols and amines, and peroxide-destroying inhibitors, e.g. dithiophosphate and dithiocarbamates, can be included in marine formulations. Mixtures of phenols and amines are often used for synergy but they must have good high-temperature performance. The sulphur-containing oxidation inhibitors also have extremely useful anti-wear properties. Oxidation inhibitors can be used advantageously in some base oils refined from low sulphur crudes and in synthetic basestocks. They compensate for the lack of natural antioxidant species. 

All the individual steps are catalyzed by enzymes NAD" (Section 15 11) is required as an oxidizing agent and coenzyme A (Figure 26 16) is the acetyl group acceptor Coen zyme A is a thiol its chain terminates m a sulfhydryl (—SH) group Acetylation of the sulfhydryl group of coenzyme A gives acetyl coenzyme A... 

Poly(ethylene oxide)s [25372-68-3] are made by condensation of ethylene oxide with a basic catalyst. In order to achieve a very high molecular weight, water and other compounds that can act as chain terminators must be rigorously excluded. Polymers up to a molecular weight of 8 million are available commercially in the form of dry powders (27). These must be dissolved carefliUy using similar techniques to those used for dry polyacrylamides. Poly(ethylene oxide)s precipitate from water solutions just below the boiling point (see Polyethers, ethylene oxide polymers). 

One characteristic of chain reactions is that frequentiy some initiating process is required. In hydrocarbon oxidations radicals must be introduced and to be self-sustained, some source of radicals must be produced in a chain-branching step. Moreover, new radicals must be suppHed at a rate sufficient to replace those lost by chain termination. In hydrocarbon oxidation, this usually involves the hydroperoxide cycle (eqs. 1—5). 

The major use of 4-cumylphenol is as a chain terminator for polycarbonates. Its use in place of phenol gives a polycarbonate with superior properties (33). Eor a low molecular weight polycarbonate used for injection-molding appHcations, the use of 4-cumylphenol as a chain terminator significantly lowers the volatiHty of the resin. Other uses of 4-cumylphenol include the production of phenoHc resins, some of which have appHcations in the electronics industry (34). Another appHcation of 4-cumylphenol involves its reaction with ethylene oxide to form a specialty surfactant. 

Polyall lene Oxide Block Copolymers. The higher alkylene oxides derived from propjiene, butylene, styrene (qv), and cyclohexene react with active oxygens in a manner analogous to the reaction of ethylene oxide. Because the hydrophilic oxygen constitutes a smaller proportion of these molecules, the net effect is that the oxides, unlike ethylene oxide, are hydrophobic. The higher oxides are not used commercially as surfactant raw materials except for minor quantities that are employed as chain terminators in polyoxyethylene surfactants to lower the foaming tendency. The hydrophobic nature of propylene oxide units, —CH(CH2)CH20—, has been utilized in several ways in the manufacture of surfactants. Manufacture, properties, and uses of poly(oxyethylene- (9-oxypropylene) have been reviewed (98). 

A related technique involves incorporation of monofunctional poly(etliylene oxide) chains as nonionic, internal emulsifier groups. Even PMDI can be dispersed in water using this nonionic method (Scheme 4.24). High-molecular-weight (ca. 2000 g/m) monols are usually used which act as chain terminators and long, hydrophilic tails which function as an emulsifying agent. 

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

Blattmann has also suggested an alternative pathway which involves a terminal oxidation of the chain after 3-hydroxylation and has presented data for the metabolism of 2-hydroxybutylbutylnitros-amine indicating that this does occur. ... 

The reduction of ground state O2 with organic substances is fairly slow in aqueous or nonaqueous solutions [57] in spite of the high redox potential of O2. The O2 is utilized effectively, however, as the terminal oxidant in the respiratory chain in a biomembrane with redox enzymes composed of membrane proteins such as heme proteins containing cytochrome c oxidase [58-60] or quinol oxidase [61,62]. 

The autoxidation of aldehydes, and of other organic compounds, may be lessened considerably by very careful purification—removal of existing peroxides, trace metal ions, etc.—but much more readily and effectively by the addition of suitable radical inhibitors, referred to in this context as anti-oxidants. The best of these are phenols and aromatic amines which have a readily abstractable H atom, the resultant radical is of relatively low reactivity, being able to act as a good chain terminator (by reaction with another radical) but only as a poor initiator (by reaction with a new substrate molecule). 

Ionomer membranes are used in fuel cells in order to separate the anode and cathode compartment and to allow the transport of protons from the anode to the cathode. The typical membrane is Nation , which consists of a perfluorinated backbone and side chains terminated by sulfonic groups. In the oxidizing environment of fuel cells, Nation , as well as other membranes, is attacked by reactive oxygen radicals, which reduce the membrane stability. Direct ESR was used recently in our laboratory to detect and identify oxygen radicals as well as radical intermediates formed in perfluorinated membranes upon exposure to oxygen radicals [73,74]. The three methods used to produce oxygen radicals in the laboratory and the corresponding main reactions are shown below. 

Cyclic Chain Termination in Oxidation of Organic Compounds... 

Therefore, v diminishes with the increasing initiation rate. At Vi > /cp2(2/ct)-1[RH]2, hydrocarbon oxidation mainly proceeds as a nonchain radical process, with the predominance of chain termination products. Parameter kp(2kt) 1/2 increases with temperature, so that for each hydrocarbon the temperature T j, exists, below which oxidation occurs as a nonchain process at vj = const. The values of calculated for several hydrocarbons oxidized with Vi= 10 7 mol L-1 s-1 are given below [2],... 

Chain termination during the oxidation of hydrocarbons usually is a result of the interaction of two peroxyl radicals by a multistep mechanism. The mechanism of dispropotionation is... 

When R is a tertiary alkyl radical, the formed tetroxide decomposes with the formation of two RO and 02. The chain termination includes the following stages in hydrocarbon oxidation by tertiary the C—H bond [12,13,15,165,166] ... 

The last reaction occurs much rapidly than the disproportionation of two cumylperoxyl radicals and accelerates chain termination in oxidized cumene [15]. The addition of cumene hydroperoxide helps to avoid the influence of the cross termination reaction Me2PhCOO + CH302 on the oxidation of cumene and to measure the pure disproportionation of cumylperoxyl radicals [15]. 

The decay of the biradical produces ketone molecule in the triplet state, which is an emitter of light [222], The CL intensity was proved to be propotional to the rate of chain initiation, which is equal to the rate of chain termination. The observed luminescence spectra were found to be identical with the spectra of the subsequent ketone in the triplet state. The intensity of CL (/chi) produced by oxidized hydrocarbon is the following ... 

Rate Constants of Chain Termination in Oxidized Hydrocarbons by the Reaction of Alkyl Radicals with Peroxyl Radicals at 293 K in RH Solution... 

The formed alkyl radical reacts with dioxygen in the oxidized hydrocarbon. Participating in chain termination, the newly formed peroxyl radical accelerates it ... 

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