Radical chain reaction initiation steps

A chemical relay race involving radicals is usually termed a chain reaction and the radicals are termed chain carriers. A chain reaction involves the production of a chain carrier that subsequently reacts to produce another chain carrier, namely a chain-propagating reaction. The formation of the first chain carrier in a chemical reaction is termed the initiation step. The chain reaction continues until a reaction involving the formation of a stable species from two chain carriers breaks the chain (chain-terminating step). In our analogy a chain-terminating reaction corresponds to two relay runners that collide with each other and drop their batons instead of passing them on. 

The way to control a radical chain reaction is to control the initiation and termination steps. Radical chain reactions can be favored by adding radical initiators. Likewise, chain reactions can be greatly diminished by adding compounds called inhibitors that react with radicals to increase chain termination. The sensitivity of the radical reaction to radical initiators and inhibitors provides a convenient way to test for this mechanism. 

In metal-catalyzed auto-oxidation the role of the metal ion is to initiate the radical chain. Reactions 8.3.1.6 and 8.3.1.7 show the initiation steps when metal ions are present. The initial hydroperoxide required for metal-catalyzed decomposition, reactions 8.3.1.6 and 8.3.1.7, is normally present in trace quantities in most hydrocarbons. 

Autoca.ta.Iysis. The oxidation rate at the start of aging is usually low and increases with time. Radicals, produced by the homolytic decomposition of hydroperoxides and peroxides (eqs. 2—4) accumulated during the propagation and termination steps, initiate new oxidative chain reactions, thereby increasing the oxidation rate. 

The result of the steady-state condition is that the overall rate of initiation must equal the total rate of termination. The application of the steady-state approximation and the resulting equality of the initiation and termination rates permits formulation of a rate law for the reaction mechanism above. The overall stoichiometry of a free-radical chain reaction is independent of the initiating and termination steps because the reactants are consumed and products formed almost entirely in the propagation steps. 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

Packer and Richardson (1975) and Packer et al. (1980) made use of the fact that electrons can be generated in water by y-radiation from a 60Co source (Scheme 8-29) to induce a free radical chain reaction between diazonium ions and alcohols, aldehydes, or formate ion. It has to be emphasized that the radiolytically formed solvated electron in Scheme 8-29 is only a part of the initiation steps (Scheme 8-30) by which an aryl radical is formed. The aryl radical initiates the propagation steps shown in Scheme 8-31. Here the alcohol, aldehyde, or formate ion (RH2) is the reducing agent (i.e., the electron donor) for the main reaction. The process is a hydro-de-diazoniation. 

The decomposition of tri- and tetrasulfane in CCI4 solution (0.2 mol 1 ) at 70 °C and in the absence of oxygen has been studied by H NMR spectroscopy [64]. Initially, tetrasulfane decomposes to a mixture of tri- and pentasul-fane but slowly and after an induction period hydrogen sulfide and disulfane are formed in addition. These results have been interpreted in terms of a radical-chain reaction. The initial step is assumed to be the homolytic cleavage of the central SS bond which has by far the lowest dissociation enthalpy of the molecule ... 

Ionic polymerisation forms high Molecular weight products and reactions can be easily carried out at room temperature or low temperature. Ionic polymerisations are chain reactions and are analogous to radical chain reactions. They also involve initiation and propagation steps. 

Although the propagation reactions are only shown once, you should be aware that they occur in a sequence a very large number of times before the termination reactions remove the reactive radicals. Thus, free-radical chain reactions are characterised by the formation of a very large number of product molecules initiated by the absorption of a single photon in the initiation step that is, chain reactions act as chemical amplifiers of the initial absorption step. 

The reduction of thiocarbonyl derivatives by EtsSiH can be described as a chain process under forced conditions (Reaction 4.50) [89,90]. Indeed, in Reaction (4.51) for example, the reduction of phenyl thiocarbonate in EtsSiD as the solvent needed 1 equiv of dibenzoyl peroxide as initiator at 110 °C, and afforded the desired product in 91 % yield, where the deuterium incorporation was only 48% [90]. Nevertheless, there are some interesting applications for these less reactive silanes in radical chain reactions. For example, this method was used as an efficient deoxygenation step (Reaction 4.52) in the synthesis of 4,4-difluoroglutamine [91]. 1,2-Diols can also be transformed into olefins using the Barton-McCombie methodology. Reaction (4.53) shows the olefination procedure of a bis-xanthate using EtsSiH [89]. 

In addition to the limitations enumerated above, which are inherent in the photochemistry, there are other side reactions of a free-radical nature which may compete seriously with the desired reaction. It is a simple matter to determine which of the products are derived from these reactions as they can be formed in the dark, using free-radical chain initiators. For example, chain reactions where the propagating steps are of the following type are fairly general. 

Radical chain reactions are comprised of three distinct parts initiation, propagation steps, and termination. The initiation portion involves one or more elementary reactions that produce a radical that can participate in one of the propagation steps. The propagation sequence is where the desired products are formed it consists of two or more reactions in which one product of each elementary reaction is a radical that serves as a reactant in another step of the sequence. Radicals are destroyed in termination steps that give nonradical products by radical-radical couphng and disproportionation reactions. 

Rate laws for radical chain reactions initiated by thermolysis are 1.5 order, first order in the component reacting in the rate-controlling step, and 0.5 order in the initiator. When the initiator is the same component as that reacting in the ratecontrolling step, the reaction will be 1.5 order in this reagent. When chain reactions are initiated by photolysis instead of thermolysis, the rate constant for initiation, the... 

Radical reactions are often called chain reactions. All chain reactions have three steps chain initiation, chain propagation and chain termination. For example, the halogenation of alkane is a free radical chain reaction. 

The photoinduced electron transfer (PET) initialed cyclodimerization was first studied with 9-vinylcarbazole as substrate1 and characterized mechanistically as a cation radical chain reaction.2 The overall reaction sequence3-4 consists of a) excitation of an electron acceptor (A), b) electron transfer from the alkene to the excited acceptor (A ) with formation of a radical ion pair, c) addition of the alkene radical cation to a second alkene molecule with formation of a (dimeric) cation radical, and d) reduction of this dimeric cation radical by a third alkene molecule with formation of the cyclobutanc and a new alkene cation radical. Steps c) and d) of the sequence are the chain propagation steps. The reaction sequence is shown below. 

A general free-radical chain reaction with the thermal or photoinduced dissociation of halogen as the initiation step is operative in radical halogenation [Eqs. (10.12)—(10.14)]. 

Co-oxidation of indene and thiophenol in benzene solution is a free-radical chain reaction involving a three-step propagation cycle. Autocatalysis is associated with decomposition of the primary hydroperoxide product, but the system exhibits extreme sensitivity to catalysis by impurities, particularly iron. The powerful catalytic activity of N,N -di-sec-butyl-p-phenylenediamine is attributed on ESR evidence to the production of radicals, probably >NO-, and replacement of the three-step propagation by a faster four-step cycle involving R-, RCV, >NO, and RS- radicals. Added iron complexes produce various effects depending on their composition. Some cause a fast initial reaction followed by a strong retardation, then re-acceleration and final decay as reactants are consumed. Kinetic schemes that demonstrate this behavior but are not entirely satisfactory in detail are discussed. 

The co-oxidation of indene and thiophenol in benzene proceeds by a three-step cyclic free radical chain reaction. Autocatalysis associated with the hydroperoxide which is the main primary product occurs, but the effect is complicated by other trace components. The reaction is extremely sensitive to catalysts and inhibitors, and its kinetic features are determined by the initiation and termination processes. 

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