Hydroperoxide and peroxidation chain

Autoca.ta.Iysis. The oxidation rate at the start of aging is usually low and increases with time. Radicals, produced by the homolytic decomposition of hydroperoxides and peroxides (eqs. 2—4) accumulated during the propagation and termination steps, initiate new oxidative chain reactions, thereby increasing the oxidation rate. 

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. 

The type of initiator utilized for a solution polymerization depends on several factors, including the solubiUty of the initiator, the rate of decomposition of the initiator, and the intended use of the polymeric product. The amount of initiator used may vary from a few hundredths to several percent of the monomer weight. As the amount of initiator is decreased, the molecular weight of the polymer is increased as a result of initiating fewer polymer chains per unit weight of monomer, and thus the initiator concentration is often used to control molecular weight. Organic peroxides, hydroperoxides, and azo compounds are the initiators of choice for the preparations of most acryUc solution polymers and copolymers. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

In this reaction scheme, the steady-state concentration of peroxyl radicals will be a direa function of the concentration of the transition metal and lipid peroxide content of the LDL particle, and will increase as the reaction proceeds. Scheme 2.2 is a diagrammatic representation of the redox interactions between copper, lipid hydroperoxides and lipid in the presence of a chain-breaking antioxidant. For the sake of clarity, the reaction involving the regeneration of the oxidized form of copper (Reaction 2.9) has been omitted. The first step is the independent decomposition of the Upid hydroperoxide to form the peroxyl radical. This may be terminated by reaction with an antioxidant, AH, but the lipid peroxide formed will contribute to the peroxide pool. It is evident from this scheme that the efficacy of a chain-breaking antioxidant in this scheme will be highly dependent on the initial size of the peroxide pool. In the section describing the copper-dependent oxidation of LDL (Section 2.6.1), the implications of this idea will be pursued further. 

Moreover, redox cycling of free or weekly chelated iron may decompose a lipid hydroperoxide and thus initiate a chain of lipid peroxidation (Halliwell and Gutteridge, 2000). 

A molecule of linear alkyl ether possesses a very convenient geometry for intramolecular hydrogen atom abstraction by the peroxyl radical. Therefore, chain propagation is performed by two ways in oxidized ethers intermolecular and intramolecular. As a result, two peroxides as primary intermediates are formed from ether due to oxidation, namely, hydroperoxide and dihydroperoxide [62],... 

Oxidation to CO of biodiesel results in the formation of hydroperoxides. The formation of a hydroperoxide follows a well-known peroxidation chain mechanism. Oxidative lipid modifications occur through lipid peroxidation mechanisms in which free radicals and reactive oxygen species abstract a methylene hydrogen atom from polyunsaturated fatty acids, producing a carbon-centered lipid radical. Spontaneous rearrangement of the 1,4-pentadiene yields a conjugated diene, which reacts with molecular oxygen to form a lipid peroxyl radical. 

Fig. 3-5 Dependence of the degree of polymerization of styrene on the polymerization rate. The effect of chain transfer to initiator is shown for t-butyl hydroperoxide (o), cumyl hydroperoxide ( ). benzoyl peroxide ( ), and azobisisobutyronitrile ( ) at 60°C. After Baysal and Tobolsky [1952] (by permission of Wiley-Interscience, New York).

Hydrogen atoms in allylic position are favorite sites for hydroperoxidation of chains. So, this mechanism proceeds in the formation of lateral hydroperoxides, and not like for other polymers, in intramolecular peroxides. Rearrangement of chemical structures coming from ozonides are rapidly observed (Scheme 33). 

Calvert and Hanst88 using infrared analysis, have also re-investigated the photooxidation of acetaldehyde at 20°C. using 3130 A. radiation. Acetaldehyde pressures were chiefly about 42.5 nun., but the oxygen pressure was varied from 0 to 745 mm. Analyses were made for carbon monoxide, carbon dioxide, formic acid, methanol, acetic acid, peracetic acid, acetyl peroxide, methyl hydroperoxide, and unreacted acetaldehyde (Table X). Chains were short. Although they do not detect methyl hydroperoxide or diacetyl peroxide, the non-observance of a peroxide does not necessarily mean it is not formed. The decomposition of hydroperoxides on the smallest particle of catalyst is remarkably fast. 

There was no indication of a peroxide chain, the yields being below unity and independent of pressure of a fixed hydrocarbon-oxygen mixture. It is therefore likely that hydroperoxide was formed by a reaction such as... 

---