Unsaturated oxidation rates

The unsaturated fatty acid oxidation proceeds at a rate higher over that for saturated acids. For example, if the oxidation rate for saturated stearic acid is taken as a reference value, the oxidation rate for oleic acid is i 1 times, linolic acid, 114 times, linolenic acid, 170 times, and arachidonic acid, nearly 200 times as high as that for stearic acid. 

The oxidation rates for bromoform were slower than the oxidation rates of unsaturated chlorinated aliphatic compounds, including the TCE. Because the hydroxylation rate constant of TCE is 109 Mr1 s 1 and the hydrogen abstraction of bromoform is 1.1 x 108 M 1 s aromatics and alkenes react more rapidly by hydroxyl addition to double bonds than does the more kinetically difficult hydrogen atom abstraction. No oxidative destruction of chloroform by Fenton s reagent was experimentally observed an explanation for this is that both H202 and Fe2+ have rate constants about one magnitude higher with respect to hydroxyl radicals than chloroform. 

It is important to note that the molecular structure of organic compounds has a determined effect on the oxidation rate constants. For example, if a compound is "saturated" with four chlorine atoms per carbon atom such as tetrachloroalkane, its reactivity rate with hydroxyl radicals is expected to be significantly lower than that of an "unsaturated" compound with only two or three chlorine atoms such as di- and trichloroalkanes. 

Formation of an enamine radical cation 45 was proposed as the chain initiation step in the autooxidation of enamines and SchifFs bases of a,/ -unsaturated ketones to give unsaturated 1,4-diones37. Pyrrolidine enamine of 10-methyl-A1(9)-octal-2-one (44) reacts with oxygen at room temperature to produce, after acid hydrolysis, 10-methyl-A1 (9)-octalin-2,8-dione (47) in 20% yield. Addition of a catalytic amount of FeCl3, Cu(OAc)2 or CuCl2 causes a pronounced enhancement in the oxidation rate and increases the yield to 80-85% after 1 h. 

The formation of keto-phosphonate structure within macromolecule leads to the removal of internal unsaturation. Triallyl cyanurate and ionizing irradiations [210] made a E-P block copolymer-PE blend thermally stable. Triallyl cyanurate increases the crosslinking density probably due to addition reactions between polymeric and allyl radicals produced by ionizing radiation. The addition of 2,2,4-trimethyl-l,2-di-hydroquinoline and bis[4(l-methyl-1-phenylethyl)pheny 1]-amine stabilized a PE-EPDM blend against heat [211]. Popov et al. [212] studied the ozone effect on PE-iPP blend. The oxidation rate was detected in relation to... 

Anisidine Value. Anisidine value is a measure of secondary oxidation or the past history of an oil. It is useful in determining the quahty of crude oils and the efficiency of processing procedures, but it is not suitable for the detection of oil oxidation or the evaluation of an oil that has been hydrogenated. AOCS Method Cd 18-90 has been standardized for anisidine value analysis (103). The analysis is based on the color reaction of anisidine and unsaturated aldehydes. An anisidine value of less than ten has been recommended for oils upon receipt and after processing (94). Inherent Oxidative Stability. The unsaturated fatty acids in all fats and oils are subject to oxidation, a chemical reaction that occurs with exposure to air. The eventual result is the development of an objectionable flavor and odor. The double bonds contained in the unsaturated fatty acids are the sites of this chemical activity. An oil s oxidation rate is roughly proportional to the degree of unsaturation for example, linolenic fatty acid (C18 3), with three double bonds, is more susceptible to oxidation than linoleic (C18 2), with only two double bonds, but it is ten times as susceptible as oleic (C18 l), with only one double bond. The relative reaction rates with oxygen for the three most prevelent unsaturated fatty acids in edible oils are ... 

Caicuiated Inherent Oxidative Stability = Sum of the decimal fraction of each unsaturated fatty acid times its relative oxidation rate. 

With cuprous oxide catalyst, product oxidation rates are quite high. This fact, combined with the need of maintaining the cuprous oxide state, leads to best results with a rather high olefin/oxygen ratio, for example, CgHg/O = 5. Then the conversion per pass of the olefin is about 10-20%, and selectivity to unsaturated aldehyde is often of the order of 60-85%. The reaction of propylene is... 

Wada, S. and Koizumi, C. 1983. Influence of the position of unsaturated fatty acid esterified glycerol on the oxidation rate of triglyceride, JAOCS, 60, 1105. 

In general, the oxidation rate of pure dry oils was approximately doubled by a 10 C. (18° F.) rise in temperature in the absence of catalysts. In the presence of light or metal catalysts the coefficient was much smaller. There is no published information on the temperature coefficient of the coupled reaction between hemoglobin and unsaturated fat. Numerous more recent studies on frozen meats and poultry have emphasized the importance of low storage temperatures in retarding rancidity (Cook and White, 1939, 1941 Ramsbottom, 1947 Atkinson et al., 1947 Hall et al., 1949 Klose et al., 1950 Palmer et al., 1953). 

Transition metal-promoted hydroperoxide deconposition is inqiortant to the oxidative stability and quality of foods for several reasons. First, the abstraction of hydrogen from an unsaturated fatty acid results in the formation of a single alkyl radical. Followii hydrogen abstraction, oxygen adds to the alkyl radical to form a peroxyl radical and subsequent abstraction of a hydrogen from another fatty acid or antioxidant to form a lipid hydroperoxide (Figure 1). These reactions by themselves do not result in an increase in free radical numbers. If these reactions were tiie only steps in tiie lipid oxidation reactions, the rapid exponential increase in oxidation that is commonly observed in lipids would not occur. Transition metal-promoted decomposition of lipid hydroperoxides results in the formation of additional radicals (e.g. alkoxyl and peroxyl) which exponentially increase oxidation rates as they start to attack otiier unsaturated fatty acids. 

The chemical resistance of vulcanizates of styrene-butadiene and natural rubbers is similar. Although the copolymers are less unsaturated, their rate of oxidation is slightly greater however, they are more effectively stabilized by antioxidants and properly compounded styrene-butadiene vulcanizates are actually better than those of natural rubber. 

In the electrochemical oxidation of acetylene [200] reaction (31) evidently becomes slow, since the heat of adsorption of acetylene on platinum is higher than the heat of adsorption of unsaturated hydrocarbons with a double bond. Moreover, the heat of dissociation of the CH bonds is also greater for acetylene than for ethylene. On the basis of these suxjpositions Bockris and co-workers [200] wrote the equation for the oxidation rate of acetylene in the following form ... 

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

---