Chain reaction mechanism

Atoms and free radicals are highly reactive intermediates in the reaction mechanism and therefore play active roles. They are highly reactive because of their incomplete electron shells and are often able to react with stable molecules at ordinary temperatures. They produce new atoms and radicals that result in other reactions. As a consequence of their high reactivity, atoms and free radicals are present in reaction systems only at very low concentrations. They are often involved in reactions known as chain reactions. The reaction mechanisms involving the conversion of reactants to products can be a sequence of elementary steps. The intermediate steps disappear and only stable product molecules remain once these sequences are completed. These types of reactions are refeiTcd to as open sequence reactions because an active center is not reproduced in any other step of the sequence. There are no closed reaction cycles where a product of one elementary reaction is fed back to react with another species. Reversible reactions of the type A -i- B C -i- D are known as open sequence mechanisms. The chain reactions are classified as a closed sequence in which an active center is reproduced so that a cyclic reaction pattern is set up. In chain reaction mechanisms, one of the reaction intermediates is regenerated during one step of the reaction. This is then fed back to an earlier stage to react with other species so that a closed loop or... 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

Addition polymerizations of unsaturated monomers leading to the formation of products of high molecular weight invariable proceed by chain reaction mechanisms. Primary activation of a monomer M (or a pair of monomers) is followed by the addition of other monomers in rapid succession... 

Thus, the enhancements in chlorine removal from W diads compared to EV diads and from m-W diads compared to r-W diads observed in the (n-Bu)3SnH reduction of DCP, TCH, and PVC are consistent with the free-radical chain reaction mechanism. Inductive effects produced by neighboring 7-Cl s tend to favor the reduction of W diads relative to EV diads and steric interactions resulting from different preferred conformations in each isomer favor the removal of Cl from m-W diads relative to r-W diads. 

Recognition that the kinetics of this reaction could be explained on the basis of the chain reaction mechanism presented above was one... 

The essential characteristic of a chain reaction mechanism is the existence of a closed cycle of reactions in which unstable or highly reactive intermediates react in propagation steps with stable reactant molecules or other intermediates and are regenerated by the sequence of reactions... 

The elementary reactions comprising the chain reaction mechanism are generally classified as initiation, propagation, or termination reactions. In the initiation reaction an active center or chain carrier is formed. Often these are atoms or free radicals, but ionic species or other intermediates can also serve as chain carriers. In the propagation steps the chain carriers interact with the reactant molecules to form product molecules and regenerate themselves so that the chain may continue. The termination steps consist of the various methods by which the chain can be broken. 

Complex rate expressions or fractional reaction orders with respect to individual reactants are often indicative of chain reaction mechanisms. However, other mechanisms composed of many elementary reactions may also give rise to these types of rate expressions, so this criterion should be applied with caution. 

Because of the variety of chain reaction mechanisms that exist, it is difficult to develop completely general rate equations for these processes. However, some generalizations with regard to the overall rate expression can be made for certain classes of reactions. 

Some of these are considered in Section 4.2.3. Illustration 4.3 indicates the techniques used in the derivation of a reaction rate expression from a chain reaction mechanism. 

ILLUSTRATION 4.3 USE OF THE BODENSTEIN STEADY-STATE APPROXIMATION TO DERIVE A RATE EXPRESSION FROM A CHAIN REACTION MECHANISM... 

The following chain reaction mechanism has been proposed for this reaction. 

Havel Sb. Ved. Praci, Vysoka Chem. Tech-nol, Pardubice 1 (83), 1965] has suggested the following chain reaction mechanism for the Co+ + + catalyzed oxidation of aldehyde to peracetic acid. 

The primary radical yields are often 3. A much higher value (>10) indicates chain reaction. In fact, the chain reaction mechanism for the formation of HC1 from a gaseous mixture of hydrogen and chlorine exposed to radium irradiation is one of the earliest example of this kind, although the detailed chemistry was later shown to involve dissociated atoms rather than electrons and ions, as was originally proposed (see Bansal and Freeman, 1971). 

The rate law obtained from a chain-reaction mechanism is not necessarily of the power-law form obtained in Example 7-2. The following example for the reaction of H2 and Br2 illustrates how a more complex form (with respect to concentrations of reactants and products) can result. This reaction is of historical importance because it helped to establish the reality of the free-radical chain mechanism. Following the experimental determination of the rate law by Bodenstein and Lind (1907), the task was to construct a mechanism consistent with their results. This was solved independently by Christiansen, Herzfeld, and Polanyi in 1919-1920, as indicated in the example. 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

Suppose the chain-reaction mechanism for radical-addition polymerization of a monomer M (e.g., CH2CHC1), which involves an initiator I (e.g., benzoyl peroxide), at low concentration, is as follows (Hill, 1977, p. 124) ... 

It is assumed that all similar fluorination reactions proceed via an intricate radical chain-reaction mechanism. The overall reactions for the substitution of hydrogen by fluorine (RH + F2 - RF + HF, AH298 -430 kJ/mol per carbon atom) are more exothermic than the reactions for adding fluorine to the double bonds... 

On the other hand, the molecular weight for the electro-oxidative polymerization remains constant throughout the reaction course. That is, the electro-oxidative polymerization proceeds seemingly through a chain reaction mechanism. (In practice the polymerization proceeds heterogeneously only in the diffusion layer of the electrode. The details are described below.)... 

In the early days of alkene chemistry, some researchers found that the hydrohalogenation of alkenes followed Markovnikov s rule, while others found that the same reaction did not. For example, when freshly distilled but-l-ene was exposed to hydrogen bromide, the major product was 2-bromopropane, as expected by Markovnikov s rule. However, when the same reaction was carried out with a sample of but-l-ene that had been exposed to air, the major product was 1-bromopropane formed by antl-Markovnikov addition. This caused considerable confusion, but the mystery was solved by the American chemist, Morris Kharasch, in the 1930s. He realised that the samples of alkenes that had been stored in the presence of air had formed peroxide radicals. The hydrohalogenation thus proceeded by a radical chain reaction mechanism and not via the mechanism involving carbocation intermediates as when pure alkenes were used. 

The reaction temperatures and some of the activation energies cited above seem to be too low to support a radical-chain reaction mechanism. Guryanova found that exchange of radioactive elemental sulfur with the p sulfur atoms of bis-p-tolyl tetrasulfide proceeds at 80-130 °C with an activation energy of only 50 kJ/mol in the case of the corresponding trisulfide the activation energy was determined as 60 kJ/mol. These data sharply contrast with the observation that liquid sulfur has to be heated to more than 170 °C to detect free radicals by electron spin resonance spectroscopy and the activation energy for homolytic SS bond scission has been determined as 150 kJ/mol (see above). 

Free radicals chain reaction mechanism. The reaction... 

In this article, the features and mechanism of the crystal-to-crystal reactions of 1,3-diene compounds are described on the basis of the molecular packing structure and intermolecular interactions in the crystals for starting materials and products. The dimerization and isomerization of unsaturated compounds as well as addition polymerization via a chain reaction mechanism are ideal sohd-state reactions, because they produce no leaving group during the reac-... 

The polymerization proceeds via a radical chain-reaction mechanism, judging from some features of the polymerization initiation by irradiation or upon heating, no formation of oligomers, and polymer formation irrespective of the medium or atmosphere. The propagating radicals are readily detected by ESR spectroscopy during polymerization in the crystalline state (Fig. 2), because termination between the propagating radicals occurs less frequently in the solid state [50]. 

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