Unimolecular free-radical reactions

A summary of the major chemical reactions of free radicals is given in Table 4.3. Broadly speaking these can be classified as unimolecular reactions of dissociations and isomerizations, and bimolecular reactions of additions, disproportionations, substitutions, etc. The complexity of many photochemical reactions stems in fact from these free radical reactions, for a single species formed in a simple primary process can lead to a variety of final products. 

In a separate study of the decomposition of sulfinic acids in the absence of solvent at 200 °C the major products were sulfur dioxide and alkenes . Minor products were water, carbon dioxide, carbon monoxide, carbonyl sulfide and sulfur. The reaction is considered to proceed by a unimolecular free radical mechanism, although kinetic evidence is lacking. Olefin formation results from transfer reactions followed by elimination and one plausible pathway is... 

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)... 

Ionic-polymerization Kinetics. The kinetics of ionic polymerization share some common principles with that of the free-radical reaction. Both are based on the basic steps of initiation, propagation, termination, and chain transfer, and in both the ultimate average molecular weight depends on the ratio of the reaction rates of propagation and termination. There are, however, important differences. In ionic polymerization the termination step appears to be unimolecular, while it is bimolecular in free-radical type polymerization. The dependence of the kinetic scheme of the reaction on the various parameters is therefore different in the two reactions. Likewise, the fact that a cocatalyst has to be brought into the ionic reaction scheme has to be taken into account. 

Pyrolysis is a technique that uses heat to transform macromolecules into a series of lower molecular weight volatile products characteristic of the original sample. The initiation reactions involved in the pyrolysis of most organic materials are the result of free radical reactions initiated by bond breaking or by the unimolecular elimination of simple molecules such as water or carbon dioxide [172-175]. The products of pyrolysis are identified by gas chromatography, often in combination with mass spectrometry. 

Marcus R A 1952 Unimolecular dissociations and free radical recombination reactions J. Chem. Rhys. 20 359-64... 

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. 

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

Describe the principal unimolecular and bimolecular reactions of free radicals and explain the usefulness of electron spin resonance in detecting radical species. 

Marcus, R. A and Rice, O. K., The kinetics of the recombination of methyl radicals and iodine atoms.. /. Phys. Chem 55, 894 (1951). Marcus, R. A., Unimolecular dissociations and free radical recombination reactions. J. Chem. Phys. 20, 359 (1952). 

The kinetic data for the reaction of primary alkyl radicals (RCH2 ) with a variety of silanes are numerous and were obtained by applying the free-radical clock methodology. The term free-radical clock or timing device is used to describe a unimolecular radical reaction in a competitive study [2-4]. Three types of unimolecular reactions are used as clocks for the determination of rate constants for this class of reactions. The neophyl radical rearrangement (Reaction 3.1) has been used for the majority of the kinetic data, but the ring expansion rearrangement (Reaction 3.2) and the cyclization of 5-hexenyl radical (Reaction 3.3) have also been employed. 

The SFRP or NMP has been studied mainly using the stable free radical TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) or its adducts with, e.g., styrene derivatives. It is based on the formation of a labile bond between the growing radical chain end or monomeric radical and the nitroxy radical. Monomer is inserted into this bond when it opens thermally. The free radical necessary to start the reaction can be created by adding a conventional radical initiator in combination with, e.g., TEMPO or by starting the reaction with a preformed adduct of the monomer with the nitroxy radical using so-called unimolecular initiators (Hawker adducts). 

Thus our unimolecular isomerization reaction actually occurs by a sequence of steps and is therefore a multiple-reaction system We need a simplified expression for the overall rate of this rate in terms of Ca alone because zl is an intermediate whose concentration is always very small just as for the free-radical intermediates and dimers in the previous examples. 

Both the collision and activated complex theories predict a mild dependence of Z on T, and the latter also predicts a mild dependence of E on T. In practice, over the limited temp ranges of the usual exptl conditions, these mild dependencies are rarely observed. Both theories also predict that normal values of Z should be 1013 to 1014 sec"1 for unimolecular processes. This agrees with many exptl observations. In many cases, however, because of steric effects, Z can be much smaller than normal . Benson (Ref 12) presents evidence that Z for certain unimolecular gas reactions producing two free radicals, or for reactions involving the opening of a small carbon ring, is larger than normal and is of the order 1016 sec"1... 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

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

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