Scission homolytic

The photochemical and thermal decomposition of CIO2 both begin by homolytic scission of a C1 0 bond ... 

Homolytic scission of the 0-0 bond of hydrogen peroxide may be effected by heat or UV irradiation.245 The thermal reaction requires relatively high temperatures (>90 Photolytic initiation generally employs 254 nm light. Reactions in organic media require a polar cosolvent (e.g. an alcohol). 

The phosphazene backbone has a particularly high resistance to thermal treatment and to homolytic scission of the -P=N- bonds, possibly due to the combination of the high strength of the phosphazene bond and its remarkable ionic character [456]. As a consequence, the onset of thermal decomposition phenomena (as detected, for instance, by TGA) are observed at considerably high temperatures for poly[bis(trifluoroethoxy)phosphazene], [NP(OCH2CF3)2]n [391, 399, 457], for phosphazene copolymers substituted with fluorinated alcohols of different length [391, 399, 457], for polyspirophosphazenes substituted with 2,2 -dihydroxybiphenyl groups [458], and for poly(alkyl/aryl)-phosphazenes [332]. 

The one-equivalent, homolytic scission of peroxides may be either reductive... 

A less common reactive species is the Fe peroxo anion expected from two-electron reduction of O2 at a hemoprotein iron atom (Fig. 14, structure A). Protonation of this intermediate would yield the Fe —OOH precursor (Fig. 14, structure B) of the ferryl species. However, it is now clear that the Fe peroxo anion can directly react as a nucleophile with highly electrophilic substrates such as aldehydes. Addition of the peroxo anion to the aldehyde, followed by homolytic scission of the dioxygen bond, is now accepted as the mechanism for the carbon-carbon bond cleavage reactions catalyzed by several cytochrome P450 enzymes, including aromatase, lanosterol 14-demethylase, and sterol 17-lyase (133). A similar nucleophilic addition of the Fe peroxo anion to a carbon-nitrogen double bond has been invoked in the mechanism of the nitric oxide synthases (133). 

Chlorine dioxide gas is unstable and can rapidly decompose at high concentrations. It also decomposes rapidly to chlorine and oxygen with exposure to mild heat. Chlorine dioxide will decompose upon exposure to sunlight (Vogt et al. 1986). The gas-phase absorption spectrum for chlorine dioxide is the same as in aqueous solution (Kaczur and Cawfield 1993). The primary photochemical reaction of CIO2 in the gas phase corresponds to homolytic scission of one of the chlorine-oxygen bonds (i.e., C102 CIO +... 

SCHEME 15. Thermolysis of poly(methacrylonitrile peroxide) by homolytic scission of the per-oxy links... 

In view of the present calculated results, the SET mechanism would be described as follows. Basically, the polar four-center reaction in Scheme 14 leads to C—C bond formation. However, when the alkyl group is bulky, only the two-center (Mg—O) reaction takes place. The aUcyl-Mg bond is cleaved homolytically owing to the persistent Mg tetravalency and the stability of the resultant radical species. Hence, biradical intermediates are formed not by a single electron transfer but by the C—Mg homolytic scission. 

Nesmeyanova and Perevalova in related studies involving diferrocenylmercury and palladium black obtained similar result (58). Their yields of biferrocenyl amounted to only 1 to 6%, however. It is postulated that the reactions proceed via homolytic scission of the carbon-mercury bond with the formation of ferrocenyl radicals. 

The question as to whether disulfides dissociate in solution into free radicals has been extensively studied. The sulfur-sulfur bonds in dialkyl and diaryl disulfides,13 154161 229 2 I0 in dialkyl tri- and tetrasulfidcs, 9 as well as in S8 undergo homolytic scission to free radicals under irradiation. 

Williams83 has recently considered the possibility of ion molecule reactions in alcohols, ketones, ethers, and esters. He postulates that the primary reactions of the parent ion are the inter- or intra-molecular abstraction of a hydrogen atom and the formation of a x-bond between oxygen and the adjacent carbon atom after homolytic scission of a bond to that adjacent carbon atom. After examining some of the liquid phase data on the radiolysis of these oxygen compounds it was concluded that such ion molecule reactions may be of importance in these systems also. 

The chain scission of PIB consists in a homolytic scission of the backbone with subsequent disproportionation or backbiting / -rearrangement, as shown in Figure 6.6. 

The effectiveness of nitric acid as a cocatalyst is probably caused by two factors. One of these is the homolytic scission of the nitric acid molecule (Reaction 7) to produce free radicals which act as initiators (5) (Reaction 1). 

An initial addition of a ruthenium-oxygen double bond to a a-C—H bond leads to an intermediate containing a carbon-ruthenium bond. This bond suffers a homolytic scission leading to a carbon radical, which is oxidized to a carbocation that provides a carbonyl group by deprotonation. 

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