Peroxide linkages

Many types of peroxides (R-O-O-R) are known. Those in common use as initiators include diacyl peroxides (36), pcroxydicarbonatcs (37), peroxyesters (38), dialkyl peroxides (39), hydroperoxides (40), and inorganic peroxides [e.g. persulfate (41)1, Multifunctional and polymeric initiators with peroxide linkages are discussed in Sections 3.3.3 and 6.3.2.1. 

Observed rates of disappearance for diacyl peroxides show marked dependence on solvent and concentration.146 In part, this is a reflection of their susceptibility to induced decomposition (see 3.3.2.1.4 and 3.3.2.1.5). However, the rate of disappearance is also a function of the viscosity of the reaction medium. This is evidence for cage return (see 3.3.2.1.3).143 The observation144 of slow scrambling of the label in be.x LQy -carbonyl- %0 peroxide between the carbonyl and the peroxidic linkage provides more direct evidence for this process. 

The preceding results on polycarbonate are at variance with the ultrasonic degradation of poly(vinyl pyrollidone) prepared with peroxide linkages where the rate of chain cleavage was determined to be 5000 times faster at the — 0 — 0 — than the — C —C— bonds [164]. 

If the double bond could be reduced without severing the peroxide linkage, such endoperoxides. would serve as convenient precursors to the bicyelie peroxides 7. Unfortunately, the synthesis of dihydroascaridole 2a (Eq. 1) is exceptional, for catalytic hydrogenation of endoperoxides generally leads to cleavage of the peroxide bond with formation of the saturated c -l,4-diol. 

Only one bicyclic peroxide was obtained for each ring system and for 26 and 27 we confirmed that the bromide is cis to the peroxide linkage. In the [2,2.1]-compound this was established from its H n.m.r. spectrum by the absence of long range W-plan coupling for the CHBt proton. The cw-configuration of the [3.2.1] compound was indicated by the lH n.m.r. spectrum of the 2-bromo-3-butoxy-cyclohexanol obtained quantitatively upon reaction with butyllithium (Eq. 24), and has now been confirmed by an X-ray crystal structure determination39). 

The reductive demercuration was marred by the loss of about half of the peroxide due to competing deoxymercuration which afforded 4-cycloocten-l-ol. An additional complication was the formation of a small amount of trans-1,2-epoxy-cw-cyclooct-5-ene. The bicyclic peroxide 50 was readily separated from the unsaturated alcohol by silica chromatography, but complete removal of the epoxide was more difficult. Preservation of the peroxide linkage was markedly higher in the bromodemercuration. The diastereoisomeric dibromoperoxides 51 were separated by HPLC, although only one isomer was fully characterised. 

On the methylene bridge of 9 the proton cis to the peroxide linkage (HA) appears at 8 1.87 (solvent PhH) and shows long range W-plan coupling with the 5-, and 6-endo-protons (HD), while the corresponding trans-proton (HB) exhibits no 4-bond coupling and is at 8 1.35. Compound 9a shows a similar spectrum but offset to lower field by about 0.8 ppm 28). 

Catalytic hydrogenation of PVC causes a significant reduction in the rate of the polymer s subsequent photodegradation (32). Although this result is consistent with the occurrence of alkene sensitization (32), it can also be attributed, per se, to the removal of other possible sensitizers such as carbonyl groups and peroxide linkages. 

The triethylsilane reduction of the peroxy ethyl ether shown in Eq. 307 takes place at the C-0 bond of the methyl ether without reduction of either the iodide or the peroxide functionalities (Eq. 307).499In contrast, a bridged peroxy ether undergoes reduction of both C-0 bonds of the peroxide linkage rather than at the ether bridge (Eq. 308) 499... 

Peroxide cure systems, in rubber compounding, 22 793-794 Peroxide decomposers, 3 111-114 Peroxide decomposition, 24 279-280 Peroxide formation, by VDC, 25 694. See also Hydrogen peroxide Peroxide initiators, 23 379-380 worldwide producers of, 24 303 Peroxide-ketazine process, 23 582-583 flow sheet for, 23 582 versus Raschig process, 23 583 Peroxide linkages, in VDC polymer degradation, 25 713... 

Nitrogen and oxygen can be Incorporated Into the backbone such that they are surrounded by different atom types. For example, organic peroxides contain two covalently bonded oxygen atoms that form the peroxide linkage. These molecules are Inherently unstable. Two covalently bonded nitrogen atoms are also similarly unstable. These unstable structures decompose to form smaller unstable molecules that are used to start the polymerization for some types of monomers. Thus, to be incorporated implies that the molecules are found only singularly in the backbone chain. Sulfur and silicon are considered to be chain formers. They can be found in the backbone in multiple units connected covalently to molecules of the same type or with carbon. Complete molecules with a silicon backbone are possible, and molecules with multiple sulfur links incorporated into the system are common, particularly in sulfur-crosslinked rubber. 

The peroxides discussed here are those that incorporate the peroxide linkage(s) within a cycle or that have multiple peroxide linkages. The enthalpies of formation of these species appear in Table 4. 

I. D. Zaikin, Russ. J. Phys. Chem. (Engl. TransL), 51, 786 (1977). See Reference 32. The title of the work (translated) taken directly from the reference is The Dissociation Energy of the Peroxide Linkage in Peresters . In Reference 3, for this data source, the stmctures of the... 

In a mechanism proposed by Hoganson and Babcock (Fig. 23-35) four successive transfers, each of one H+ + one e, leads to a three-electron oxidation of Mn ions, e.g., from the 2+ and 3+ oxidation states to all Mn4+, and to joining of the two water oxygens to form a manganese peroxide linkage. Oxidation of the peroxide dianion to 02 by the adjoining Mn4+ and Mn3+ ions completes the cycle. This mechanism is hypothetical, and various alternatives have been pre-... 

Peroxygen compounds contain the peroxide linkage (—0—0—) in which one of the oxygen atoms is active. 

This equilibrium is shifted to the right when the basicity of the medium increases. This shows the nucleophilic character of the peroxidic linkage 0-0 due to formation of the HOO- anion. This perhydroxyl anion is considered to be a supemucleophile, as its reactivity is about 200 times higher than that of the OH- anion. The pH effect on the kinetic rate constant for the dismu-tation of superoxide is shown in Figure 4.4. 

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