Limits autoxidations

Althou various supercritical fluids have been found useful as solvents for fatty acids and their esters, carbon dioxide is thus far the most commonly used extractant because of its inherent advantages. Extraction with carbon dioxide is effective at moderately low tenperatures, vhich limits autoxidation, decomposition and polymerization of the hi ily xmsaturated fatty... 

It has been extensively shown that pecific feeding regimes can increase the concentrations of boh polyunsaturated lipids and pro-oxidative metal pedes, which boh make he milk more prone to autoxidation (6 7 8 9 10). Thus several studies have been perfomted to limit autoxidation in milk through optimized feeding (II 12). Moreover, numerous studies have shown that antioxidants in he feed are transferred to he milk and hereby improve he oxidative stability of milk. It has been reported hat increasing concentrations of dietary vitamin E can effectively reduce he intensity of oxidized flavor in milk (13 14). Recent studies confirm hat a-tocopherol protects milk fat from oxidation (IS). In contrast, earlier studies have not been able to show hat... 

It is obvious that although of great importance, primary antioxidants are not the complete answer to limiting autoxidation in polyethylene. In practice, they are commonly used in combination with other types of antioxidants, such as hydroperoxide decomposers, which inhibit other reactions of the autoxidation cycle. 

Distillation to small volume of a small sample of a 4-year-old mixture of the alcohol with 0.5% of the ketone led to a violent explosion, and the presence of peroxides was subsequently confirmed [1]. Pure alcohols which can form stable radicals (secondary branched structures) may slowly peroxidise to a limited extent under normal storage conditions (isopropanol to 0.0015 M in brown bottle, subdued light during 6 months to 0.0009 M in dark during 5 years) [2], The presence of ketones markedly increases the possibility of peroxidation by sensitising photochemical oxidation of the alcohol. Acetone (produced during autoxidation of isopropanol) is not a good sensitiser, but the presence of even traces of 2-butanone in isopropanol would be expected to accelerate markedly peroxidation of the latter. Treatment of any mixture or old sample of a secondary alcohol with tin(II) chloride and then lime before distillation is recommended [3], The product of photosensitised oxidation is 2-hydroperoxy-2-propanol [4]. 

It should be emphasized that clear-cut situations described in Schemes 1-3 are uncommon and typically the combination of these models needs to be considered for kinetic and mechanistic description of a real system. However, even when one of the limiting cases prevails, each of these models may predict very different formal kinetic patterns depending on where the rate determining step is located. For the same reason, different schemes may be consistent with the same experimental rate law, i.e. thorough formal kinetic description of a reaction and the analysis of the rate law may not be conclusive with respect to the mechanism of the autoxidation process. 

Earlier studies demonstrated a rich variety of oxidation states, geometries and compositions of the intermediates and products formed in the autoxidation reactions of cysteine (RSH). Owing to the complexity of these systems, only a limited number of detailed kinetic papers were published on this subject and, not surprisingly, some of the results are... 

The complex and incompletely understood phenomena of cool flames and then-close relationship with autoignition processes is discussed in considerable detail. As the temperature of mixtures of organic vapours with air is raised, the rate of autoxidation (hydroperoxide formation) will increase, and some substances under some circumstances of heating rate, concentration and pressure will generate cool flames at up to 200° C or more below their normally determined AIT. Cool flames (peroxide decomposition processes) are normally only visible in the dark, are of low temperature and not in themselves hazardous. However, quite small changes in thermal flux, pressure, or composition may cause transition to hot flame conditions, usually after some delay, and normal ignition will then occur if the composition of the mixture is within the flammable limits. 

Several explosions or violent decompositions dining distillation of aldoximes may be attributable to presence of peroxides arising from autoxidation. The peroxides may form on the -C=NOH system (both aldehydes and hydroxylamines perox-idise [1]) or perhaps arise from unreacted aldehyde. Attention has been drawn to an explosion hazard inherent to ketoximes and many of their derivatives (and not limited to them). The hazard is attributed to inadvertent occurence of acidic conditions leading to highly exothermic Beckmann rearrangement reactions accompanied by potentially catastrophic gas evolution. Presence of acidic salts (iron(III)... 

This group covers polymeric peroxides of indeterminate stmcture rather than polyfunctional macromolecules of known stmcture. These usually arise from autoxidation of susceptible monomers and are of very limited stability or... 

The partial pressure of oxygen was varied between 12 and 476 mm. of Hg at 25°C. all values of K in this pressure range lay between 1.54 and 1.75 X 10 2 (mole % )1/2/min., so that at the lower limit the rate of autoxidation was still substantially independent of the partial pressure of oxygen. 

When a slow steady-state autoxidation of a suitable hydrocarbon is disturbed by adding either a small amount of inhibitor or initiatory a new stationary state is established in a short time. The change in velocity during the non-steady state can be followed with sensitive manometric apparatus. With the aid of integrated equations describing the nonsteady state the individual rate constants of the autoxidation reaction can be derived from the results. Scope and limitations of this method are discussed. Results obtained for cumene, cyclohexene, and Tetralin agree with literature data. 

However, the rate of oxidation in the presence of bromide ion (Figure 2) is exactly first order with respect to cobalt. The autoxidation of hydrocarbons catalyzed by cobalt and bromide ion is characterized by the fact that the rate increases with increasing cobalt concentration, while the rate at high cobalt concentrations reaches a limiting value in the absence of bromide ion. 

Comparison of the two different types of detector, UV and ELSD, showed that the chromatographic profiles of the autoxidized samples are similar except for TO. Compounds without conjugated dienes in their structure cannot be detected with a UV detector at 235 nm thus, the autoxidation products of TO are not detected. The detection limit of ELSD was 150 ng, quantified by 1,3-diolein and 2-monolinolein, and its sensitivity in detecting autoxidation products of TLn and TL approached that of the UV detector used in the study. The two detectors could therefore be used in series. 

We shall not continue any further into the labyrinth of autoxidation, but shall merely point out that the complexities we have described are multiplied manyfold when one considers the situations that will arise in oxidation of an olefin that reacts by a combination of the addition-polymerization and the abstraction routes, or when the temperature is high enough to homolyze the peroxide products and the reaction is thus producing its own initiator, or when there are several nonequivalent hydrogens in the substrate. Furthermore, the products will themselves be subject to oxidation. Clearly the possibilities are almost without limit. 

This group covers polymeric peroxides of indeterminate structure rather than polyfunctional macromolecules of known structure. These usually arise from autoxidation of susceptible monomers and are of very limited stability or explosive. Polymeric peroxide species described as hazardous include those derived from butadiene (highly explosive) isoprene, dimethylbutadiene (both strongly explosive) 1,5-p-menthadiene, 1,3-cyclohexadiene (both explode at 110°C) methyl methacrylate, vinyl acetate, styrene (all explode above 40°C) diethyl ether (extremely explosive even below 100°C ) and 1,1-diphenylethylene, cyclo-pentadiene (both explode on heating). 

Another important kinetic feature of autoxidation is autocatalysis. One of the oxidation products is the hydroperoxide, while peroxides can also be produced in termination. Peroxides and hydroperoxides are stable within a limited temperature range, but at higher temperatures they decompose homolytically giving alkoxy radicals ... 

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