Unimolecular decomposition of free

Two important elementary step reactions were described in these radical chains of Rice. One was the ubiquity of the metathetical reaction of H atom abstraction by free radicals from hydrocarbons. The second was the rapid unimolecular decomposition of free radicals into olefins and secondary atoms or radicals, as a chain step competitive with metathesis. 

Three main categories of unimolecular decompositions of free radicals exist. 

Unimolecular decomposition of free radicals by beta-scission (bs)... 

Unimolecular decompositions of free radicals a) Beta-scissions of hydrocarbon radicals ... 

REACTIONS OF FREE RADICAL GENERATION BY HYDROPEROXIDES 4.3.1 Unimolecular Decomposition of Hydroperoxides... 

Figure 4.4 An energy profile for the unimolecular decomposition of AB+, showing a reverse activation barrier (free) and a kinetic shift (fkjn)- Adapted from [65],...

The decomposition of free radicals is another type of unimolecular process of importance to this system. This may include reactions such as... 

Such reactions are, of course, unimolecular decompositions, not free radical chain processes. This fact made perfluorodiacyl peroxides ideal precursors for the laser flash photolysis studies which will be described in Sect. 4. Kinetics for the thermal decomposition of a number of perfluorodiacyl peroxides have been measured and their AH+ values were approximately 24 kcal/mol, about 5 kcal/mol... 

It was shown around 25 years ago that alkyl cations may exist as distinct moieties in intermediates during unimolecular decomposition of organic ions [56, 57]. This was an important discovery since alkyl ions complexed in this way may serve as better models for intermediates in elimination and substitution reactions in solution than free alkyl ions. Complex formation may be enforced upon protonation. 

If the particular reaction studied is the unimolecular decomposition of a free radical, such as (3), then the use of a trap will enable the effective concentration of the radical to be measured. A radical trap will indicate the presence or absence of a free radical reaction and may sometimes provide evidence for a partly or entirely molecular reaction. Rate data for free radical reactions are derived assuming the occurrence of a steady state concentration of radicals. The time required to produce a steady state concentration of methyl radicals in the pyrolysis of AcH is shown for various temperatures in Fig. 1. Realistic values for rate coeflBcients may be obtained only if the time of product formation is long compared to the time to achieve the steady state concentrations of the radicals concerned. Thus deductions from the results from the bromination of isobutane , neopentane , and toluene have been criticised on the grounds that a steady state concentra-... 

M is Br2 or any other gas that is present. By the principle of microscopic reversibility , the reverse processes are also pressure-dependent. A related pressure effect occurs in unimolecular decompositions which are in their pressure-dependent regions (including unimolecular initiation processes in free radical reactions). According to the simple Lindemann theory the mechanism for the unimolecular decomposition of a species A is given by the following scheme (for more detailed theories see ref. 47b, p.283)... 

Figure Bl.7.5. (a) MIKE spectrum of the unimolecular decomposition of 1-butene ions (m/z 56). This spectrum was obtained in the second field-free region of a reverse geometry magnetic sector mass...

Experiments in which free-radical scavengers are added indicate that a chain reaction is involved, because the reaction is greatly retarded in the presence of the scavengers. The mechanism shown below indicates that one of the steps in the chain is an electron-transfer process and that none of the steps involves atom abstractions. The elimination of nitrite occurs as a unimolecular decomposition of a radical anion. ... 

The kinetic isotope effect observed on the overall rates of hydrogenolytic demethylation of propylene in the presence of deuterium was successfully interpreted in terms of a free radical chain mechanism. Little differences were inferred to exist between the rates of addition of H or D- to propylene and also between those of unimolecular decomposition of the produced hot n-propyl radicals, and thus the kinetic isotope effect was ascribed mainly to the difference between the steady state concentrations of [H-] in the presence of hydrogen and the concentrations of [D ] + [H ] in the presence of deuterium. In more detail, conversion of rather inactive allyl radical by metathesis with deuterium into an active D is relatively slow. This was concluded to be the main cause of the observed kinetic isotope effect, which agrees well with the calculated... 

The kinetic isotope effect observed on the overall rates, ranging from 2.0 at 700 C to 1.5 at 800 C, was interpreted in terms of a free radical chain mechanism where a unimolecular decomposition of a hot n-propyl radical, produced by addition of H or D to propylene, plays a key role. The difference in the observed overall rates was mainly ascribed to that in steady state Concentrations of H and D which are produced, in part, through the reaction between allyl radical and H2 or D2. No appreciable difference between the decomposition rates of C3H7 and C3H6D was infered by RRKM theory. 

Correlated or geminate radical pairs are produced in unimolecular decomposition processes (e.g. peroxide decomposition) or bimolecular reactions of reactive precursors (e.g., carbene abstraction reactions). Radical pairs formed by the random encounter of freely diffusing radicals are referred to as uncorrelated or encounter (P) pairs. Once formed, the radical pairs can either collapse, to give combination or disproportionation products, or diffuse apart into free radicals (doublet states). The free radicals escaping may then either form new radical pairs with other radicals or react with some diamagnetic scavenger... 

The decompositions of hydroperoxides (reactions 4 and 5) that occur as a uni-or bimolecular process are the most important reactions leading to the oxidative degradation (reactions 4 and 5). The bimolecular reaction (reaction 5) takes place some time after the unimolecular initiation (reaction 4) provided that a sufficiently high concentration of hydroperoxides accumulates. In the case of oxidation in a condensed system of a solid polymer with restricted diffusional mobility of respective segments, where hydroperoxides are spread around the initial initiation site, the predominating mode of initiation of free radical oxidation is bimolecular decomposition of hydroperoxides. 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

Free valence also persists in unimolecular reactions of radicals, such as decomposition and isomerization. 

Fig. 11 Unimolecular decomposition pathways of 2-oxepinoxy radical (1). The relative free energies (298 K, kcal/mol) at the B3LYP/ 6-311 + G(d,p)//B3LYP/6-31G(d) level are shown for each intemiediate relative to 1, and each free energy of activation is relative to the reactant for that specific step, [courtesy of Michael Fadden (J Phys Chem A 2000 104 8121-8130) Reprinted with permission of J Phys Chem A.]...

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