Occurrence of free radicals

Biacetyl has often been used for sensitizing the pyrolysis of organic compounds . Radicals, resulting from the thermal decomposition of biacetyl, initiate the chain decomposition of such substances. However, NO causes no inhibition of notable significance in such systems . Nevertheless, this cannot be considered as an evidence against the occurrence of chains, since the sensitized decompositions are definitely inhibited by, for instance, propene . 

In an empty vessel, the reaction is practically homogeneous the heterogeneous component is only about 1 % . The pressure increase, accompanying the reaction, is nearly 200 % at high pressures, while at low pressures, it is smaller. 

The decomposition follows first-order kinetics at low conversions, but the rate coefficient begins to decrease above 30 % decomposition. The deviation from the first-order kinetics is probably a result ot the product inhibition. The rate coefficient, determined at low conversions by pressure measurement, can be given as  

Walters reinvestigated the kinetics, following the progress of the reaction by biacetyl analysis. In agreement with the pressure measurement results, the reaction order was found to be unity in the initial stage and to decrease above about 25 % conversion. The temperature dependence of the first-order rate coefficient gave 

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In appearance, the PHIP phenomenon closely resembles those due to CIDNP [16], another phenomenon, which also gives rise to emission and enhanced absorption lines in NMR spectra. However, CIDNP is the consequence of the occurrence of free radicals, and previously has frequently been considered unequivocal proof for free radical reactions. 

Funt and Williams reported that the copolymer compositions of methyl methacrylate and acrylonitrile varied at the same electrode depending upon the salts used in the saturated dimethyl formamide solution (Table 5) (19). They believed in the simultaneous occurrence of free radical and ionic polymerization in the system. Yield of polymers also differed with a variety of salts in the polymerization of methyl methacrylate in dimethyl sulfoxide (<5). Nevertheless no pattern of correlation has been given on the aspects relating to the supporting electrolytes. 

However, it was subsequently shown9 that these observations could be more satisfactorily accounted for by the occurrence of free radical chain reactions. The free radicals might arise from ionic or excited species. The radiation chemistry of liquid water was claimed to be explicable10 in terms of the reactions of the hydrogen atom and the hydroxyl free radical. Ionic mechanisms consequently fell into disrepute, but it is interesting that in recent years ionic processes have been recognised as of increasing importance. The wheel has indeed turned full circle ... 

It has already been pointed out (p. 73) that the postulate of free radical chain reactions provided a reasonable explanation for early results of the radiolyses of gases. It was later suggested10 that the radiation chemistry of aqueous solutions could best be explained by the production of H atoms and OH radicals. Subsequently, the results of a large variety of radiolyses were explained in terms of radical reactions occurring therein. Although such experiments did not provide conclusive evidence for the existence of free radicals in the systems, the results obtained, e.g. product analysis, rate coefficients, were not inconsistent with the occurrence of free radical processes. 

Fig. 6. Occurrence of free radicals and molecules during the CS2-O2 explosion. These curves represent estimated absorption intensities as a function of time, as determined from several hundred experiments with [CS2] - 8 torr, and temperatures of 190-300 C. From Meyerson et with permission of the American Institute of Physics.

The occurrence of free radicals in the pyrolysis of acetaldehyde was proved by Rice etal. as early as in 1932, and later also by others using the Paneth... 

First evidence for the occurrence of free radicals in the thermal decomposition of propionaldehyde was obtained by the para-ortho hydrogen technique . Sworski and Burton investigating the reaction between 850 and 950 °C by the lead-mirror technique, identified methyl and ethyl radicals and also determined their concentration ratio. They showed that the concentration of C2H5 exceeds that of CH3 in the temperature range studied and that the [CH3]/[C2H5] ratio increases with increasing temperature (Table 4). 

First attempts made by the Paneth technique to detect free radicals failed due to experimental diflSculties. However, upon reinvestigation, the occurrence of free radicals was proved in the mercury-sensitized photolysis , as well as in the direct photolysis at several wavelengths. Using the para-ortho hydrogen method, Patat came to a similar conclusion. He found the para-ortho conversion to be negligible below 200 °C, however, at higher temperatures the hydrogen atom and free radical concentrations became measurable. 

The occurrence of free radicals in the photolysis at 2537 A was proved by the Paneth technique. A radical of mass number 41, assumed to be either the allyl-radical or the isomer propenyl radical, was detected in the mercury-sensitized reaction. By adding methyl radicals to the system, the crotonyl radical could also be identified. 

There is a growing interest in F-induced oxidative stress. Studies have indicated that F exposure resulted in lipid peroxidation. In animal experiments, F was shown capable of inducing not only lipid peroxidation in several organs and tissues but also changes in endogenous antioxidant components such as SOD, GPx, and GSH (Sun et al. 1994). Studies of aluminum plant workers exposed to F in the workplace showed that, in addition to marked increases in urinary F concentrations, the levels of serum lipid peroxides and the activity of SOD were increased, compared with those of workers not exposed to F. These results suggest the occurrence of free radical-induced lipid peroxidation in industrial workers chronically exposed to high levels of F. 

BPR = Berry Pseudorotation TR = Turnstile Rotation Scheme 6.15 Topological transformations in the ligand coupling involving phenolic compounds 6.6.3 Occurrence of free radicals... 

The general patterns of the reactivity of aryllead triacetates could be explained by the occurrence of free radicals, generated by a single-electron-transfer mechanism or via a SrnI process. 

Hey, D.H. and Waters, W.A., Some organic reactions involving the occurrence of free radicals in solution, Chem. Rev., 21,169-208, 1937. 

Another mode of dual curing involves the simultaneous occurrence of free radical and cationic radiation-induced cross-linking polymerization of formulations containing appropriate initiators [20, 23, 28]. This method, which is called hybrid curing, leads to coatings with imique properties. A typical hybrid-cure system contains a diacrylate and a diepoxide, the former polymerizing by a free radical and the latter by a cationic mechanism. Exposure of the system to in-... 

The occurrence of free radical chain processes involving electron transfer is, of course, most likely to occur when easily oxidized or reduced substrates are involved, with radical anion formation being favoured by strongly basic or reductive conditions and radical cations by acidic or oxidative environments. 

Airborne Free Radicals The widespread occurrence of free radical intermediates in gas-phase reactions has frequently been the subject of kinetics studies (refer to... 

Liu et al. (1999) observed the rapid photodecomposition of 4-oxo-2-pentenal in sunlight-irradiated synthetic air (ti/2 L5 h 0600-0800 exposure) with the rapid oxidation of added NO to NO2. The formation of free radicals in (II), (III), and /or (IV) is evident. Carbonyl products observed by Liu et al. (1999) in daylight chamber experiments, CH2O, CH2=C(CH3)C(0)H, glycolaldehyde, hydroxyacetone, glyoxal, and methylglyoxal, suggest the possible occurrence of free-radical reactions. Some of these products are unexpected and may have resulted from impurities in the system. 

The possible occurrence of free radicals in biochemical processes has been refuted by several authors because of their socalled high reactivity (Laidler, 1958). If free, the radicals would destroy in no time cellular components, especially the enzyme proteins themselves. We have dealt with this matter in our discussion of turnip peroxidases activity. We would like therefore to examine the chemical properties of the free radicals in a more general way to pro" e that these substances have a graded reactivity. 

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