Fission, homolytic

Inorg anic Compounds. Hydrogen chloride reacts with inorganic compounds by either heterolytic or homolytic fission of the H—Cl bond. However, anhydrous HCl has high kinetic barriers to either type of fission and hence, this material is relatively inert. 

Photochemical Reactions. The photochemistry of chlorine dioxide is complex and has been extensively studied (29—32). In the gas phase, the primary photochemical reaction is the homolytic fission of the chlorine—oxygen bond to form CIO and O. These products then generate secondary products such as chlorine peroxide, ClOO, chlorine, CI2, oxygen, O2, chlorine trioxide [17496-59-2] CI2O2, chlorine hexoxide [12442-63-6] and... 

The low bond-strength of the O—0 bond renders peroxides susceptible to homolytic fission to give oxy radicals on heating. Diacyl peroxides give rise to acyloxy radicals which then decompose to aryl radicals and carbon dioxide, Eq. (5). For example, dibenzoyl... 

Platinum removes a halogen atom from the halide, causing homolytic fission of the C-halogen bond. The resulting Pt -XR radical pair can either react to form Ptn(R)X or separate, with subsequent reaction with RX leading to either PtX2 or PtRX species or reaction with solvent molecules. 

The material is arranged as follows. Photochemical reactions are discussed first (Section VI,A) as they represent the most thoroughly studied and only definitely established examples of the simplest type of reaction, viz., the homolytic fission of the Co—C bond. Thermal (i.e., nonphotochemical)... 

The primary step in the photolysis of methylcobalamin is homolytic fission to give the Co(II) cobalamin and methyl radicals. Recombination can occur, i.e., the reaction is reversed, unless the radicals and/or Co(II) are removed by further reactions ... 

It is possible that the general instability of secondary alkyl ligands such as isopropyl and cyclohexyl (see Section B,2) may also be due to homolytic fission at room temperature, but here also other mechanisms, such as the elimination of Co—H, are possible. 

Probably homolytic fission (see Section B,l,a) isom. Isomerization of / - to a-substituted ethyl complex (see text) Other products stated, where reported + Reaction observed, but products not reported — No reaction observed... 

One would expect that homolytic fission, and perhaps even the elimination of CoH, would occur in both acid and alkaline solution, but there is little information on this point. 

The decomposition of 4-pyridylmethyl- and a-(2-pyridyl)ethylpenta-cyanide, which probably involves homolytic fission (see Section B,l,a), occurs only after the loss of one cyanide to give the (presumably trans) organotetracyanoaquo complexes 100,102), i.e., in this case we observe the order H2O > CN . The decomposition of corrinoids possessing secondary alkyl ligands is accelerated by the addition or presence of bases and cyanide. Isopropylcobalamin is more unstable than the cobinamide 61) cyclohexyl-... 

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

Reductions by metal ions are covered in Section 6 in terms of (/) electron-acceptance and ii) electron-acceptance concerted with homolytic fission. One group of reactions, which includes oxidations and reductions by metal ions, is that between a metal ion and a neutral free radical. These form a self-contained class which is treated separately in Section 7. 

In broad terms, the following types of reactions are mediated by the homolytic fission products of water (formally, hydrogen, and hydroxyl radicals), and by molecular oxygen including its excited states—hydrolysis, elimination, oxidation, reduction, and cyclization. 

But homolytic fission can also take place, thus generating species possessing an unpaired electron—radicals, e.g. (1) and (2) ... 

Homolytic fission of an R3C—X bond is, in the gas phase, always less energy-demanding than heterolytic fission. This energetic advantage is, however, often reversed in polar solvents, because of the energy then developed—in heterolytic fission—from solvation of the developing ions. 

This reflects the relative ease with which the C—H bond in the alkane precursor will undergo homolytic fission, and more particularly, decreasing stabilisation, by hyperconjugation or other means, as the series is traversed. There will also be decreasing relief of strain (when R is large) on going from sp3 hybridised precursor to essentially sp2 hybridised radical, as the series is traversed. The relative difference in stability is, however, very much less than with the corresponding carbocations. 

Another dinuclear carbonyl which presents interesting problems is ](ri C5H5)Fe(CO) ] 2 Does the photochemistry proceed exclusively through homolytic fission to produce two (13 -05 )Fe-(C0)2 radicals or by other possible routes The discussion of this reaction has involved mechanistic and synthetic studies (77), flash photolysis (78) and low-temperature photolysis (29) - the latter work, in THF or ethyl chloride at -78°C, invokes an intermediate in which the Fe-Fe direct bond is broken but the two halves of the molecule are held together by a CO bridge. Clearly such an intriguing problem merits more detailed investigations. 

When a covalent bond breaks to produce radicals, i.e. one electron of the bond pair goes to each atom, homolytic fission has occurred. These highly reactive chlorine radicals attack the methane molecules. 

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