Unimolecular homolysis

Decomposition of Thiols. Thiols decompose by two principal paths (i43— i45). These are the carbon—sulfur bond homolysis and the unimolecular decomposition to alkene and hydrogen sulfide. For methanethiol, the only available route is homolysis, as in reaction 29. For ethanethiol, the favored route is formation of ethylene and hydrogen sulfide via the unimolecular process, as in reaction 30. 

The amount of induced decomposition that occurs depends on the concentration and reactivity of the radical intermediates and the susceptibility of the substrate to radical attack. The radical X- may be formed from the peroxide, but it can also be derived from subsequent reactions with the solvent. For this reason, both the structure of the peroxide and the nature of the reaction medium are important in determining the extent of induced decomposition, relative to unimolecular homolysis. 

Ideally all reactions should result from unimolecular homolysis of the relatively weak 0-0 bond. However, unimolecular rearrangement and various forms of induced and non-radical decomposition complicate the kinetics of radical generation and reduce the initiator efficiency.46 Peroxide decomposition induced by radicals and redox chemistry is covered in Sections 3.3.2.1.4 and 3.3.2.1.5 respectively. 

The kinetic form of the decomposition in various solvents indicates competing unimolecular homolysis of the peroxide link (a) and radical induced decomposition (b). Other diacyl peroxides behave similarly, except that, in the case of acetyl peroxide, induced dceomposition is much less important. More highly branched aliphatic or a-phenyl-substituted diacyl peroxides decompose more readily, partly because induced decomposition is more important again and partly because of the occurrence of decomposition involving cleavage of more than one bond (for a mechanistic discussion of these cases, see Walling et al., 1970). 

R = Et, Pr), prepared independently, undergo unimolecular homolysis (effectively the reverse process) and the activation enthalpy for these processes were determined.838... 

Chromium(III) organocations (see previous section) have attracted a good deal of attention. The nature of the R group in Cr(H20)jR controls the reactivity. When R is a primary group, the complex is stable in Oj. A chain mechanism holds for Oj reaction with a complex containing a seondary or tertiary alkyl R group while reaction is indirect and via unimolecular homolysis with benzylchromium(lll) (Sec. 2.1.6). 

The unimolecular decompositions are nevertheless not without their complications. Consider, for example, the unimolecular decomposition of benzoyl peroxide in benzene or carbon tetrachloride, where good first-order kinetics indicate that the contribution of induced decomposition is small.58 Initial homolysis of the O—O bond leads to two benzoyl radicals (Equation 9.21), which can fragment according to Equation 9.22 to yield phenyl radicals and carbon... 

If initiation involves simple unimolecular homolysis of the alkyl hydroperoxide,... 

The most common initiation or homolysis reaction is the breaking of a covalent C-C bond with the formation of two radicals. This initiation process is highly sensitive to the stability of the formed radicals. Its activation energy is equal to the bond dissociation enthalpy because the reverse, radical-radical recombination reaction is so exothermic that it does not require activation energy. C-C bonds are usually weaker than the C-H bonds. Thus, the initial formation of H radicals can be ignored. The total radical concentration in the reacting system is controlled both by these radical initiation reactions and by the termination or radical recombination reactions. In accordance with Benson (1960), the rate constant expressions of these unimolecular decompositions are calculated from the reverse reaction, the recombination of two radical species to form the stable parent compound, and microscopic reversibility (Curran et al., 1998). The reference kinetic parameters for the unimolecular decomposition reactions of K-alkanes for each single fission of a C-C bond between secondary... 

Elementary uni- and bimolecular reactions will necessarily show first- and second-order kinetic behaviour, but the reverse is not necessarily true a first-order reaction may not be unimolecular and a second-order reaction may not be bimolecular. For example, we considered the decomposition of dibenzylmercury in Chapter 1, in which the mechanism could either be elementary, giving a mercury atom and a 1,2-diphenylethane molecule directly (reaction 2.13a), or the reaction could be complex, with a slow initial homolysis of a carbon-mercury bond, followed by rapid further reactions to give the products (reaction 2.13b). Similarly for the Cope rearrangement of diene 2 to diene 4, the reaction could be elementary, with a concerted cyclic movement of electrons (reaction 2.14a), or might involve a di-radical intermediate 3 which rapidly reacted further to give the observed product 4 (reaction 2.14b). Both these mechanisms would lead to first-order kinetics, so the establishment of first-order kinetic behaviour for both these reaction schemes does not establish the... 

We also made molecular-dynamic simulation of thermal decomposition of some individual energetic materials, including RDX, at extremely high temperatures [93,94]. It turned out that the primary fragmentation mechanism at these conditions is entirely different from the low-temperature variant. In the case of the RDX unimolecular decomposition, it can be mentioned that elimination of NO2 group by homolysis of one N-N bond is observed for all reaction conditions whereas perhydrotriazine ring fission (depolymerization to... 

Thermal Rearrangement of Benzyl Silylmethyl Ethers A Case for Anchimerically Accelerated Unimolecular Bond Homolysis. ... 

The diphenyl and phenyl-1-naphthyl systems react solely via path B, since the concerted attack of the allyl group at the backside of the carbon atom is sterically inhibited. This induces 100% allyl radical formation, in line with the observed deuterium scrambling in the allyl group. Such a process represents a unique case of anchimerically accelerated unimolecular bond homolysis and will be treated in detail in Section IV. 

THERMAL REARRANGEMENT OF BENZYL SILYLMETHYL ETHERS A CASE FOR ANCHIMERICALLY ACCELERATED UNIMOLECULAR BOND HOMOLYSIS... 

Thus, under steady-state radiolysis the rate of the dismutation (reaction 9) is reduced substantially, while relatively slow unimolecular decomposition processes might take place, e.g., homolysis, internal disproportionation. 

Kinetic data indicate that catalysis by [VO(dpm)2] is complicated by the formation of a complex, [V0(dpmX -Bu02H)] (Kassn= 1-9 x 10 M" at 25 °C) which may undergo unimolecular homolysis forming r-BuO, or, alternatively may react with a second hydroperoxide again forming t-BuO equation (144). Cyclization of vanadium occurs between the 4+ and the 5+ states. 

The Half-Life for Homolysis of Ethane at Room Temperature The —90 kcal / mol C-C BDE of ethane sets a lower limit to the activation energy for the thermally induced homolysis of the molecule. In Chapter 7 we will introduce the Arrhenius equation, which can be used to calculate rate constants from activation energies If we assume an Arrhenius pre-exponential factor (A) of 10 (a common value for a unimolecular process), the half-life for homolysis of ethane at 25 °C would be approximately 10 years. Our universe is postulated to have been around for at most only 10 ° years. Thus, hydrocarbons are thermally very stable ... 

An accelerated homolysis is defined as one that proceeds at an accelerated rate over that expected for a simple unimolecular homolysis. If a compound A-B undergoes unimolecular homolysis, eq 5, the rate constant can be predicted from the Arrhenius equation, eq 6, where BDE(A-B) is the bond dissociation energy of the A-B bond (18). However, if AB undergoes an assisted homolysis... 

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