Reaction mechanisms termination steps

Tertiary peroxyl radicals also produce chemiluminescence although with lower efficiencies. For example, the intensity from cumene autooxidation, where the peroxyl radical is tertiary, is a factor of 10 less than that from ethylbenzene (132). The chemiluminescent mechanism for cumene may be the same as for secondary hydrocarbons because methylperoxy radical combination is involved in the termination step. The primary methylperoxyl radical terminates according to the chemiluminescent reaction just shown for (36), ie, R = H. 

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. 

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. 

Problem 7.19 Oik- of the chain-termination steps that sometimes occurs to interrupt polymerization is the following reaction between two radicals. Propose a mechanism for the reaction, using fishhook arrows to indicate electron flow. 

Chain reactions, 181 branching, 189 initiation step, 182 propagation steps, 182 rate laws for, 188 termination step, 182 well-behaved, 187 Chemical mechanism, 9 Chemical relaxation, 255-260 Coalescence temperature, 262 Col, 170... 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

These remarks represent only the barest outline of at least two aspects of PVC degradation which have been the focus of attention for several years and remain incompletely understood namely the mechanism involved and the related problem of the involvement of HC1. Several excellent reviews give more comprehensive summaries of the earlier work (10, 11, 12). More recent work has made it clear that under appropriate conditions the presence of HC1 can affect the initiation, propagation and termination steps as well as influencing the distribution of polyene sequence lengths. In addition it can undergo photochemical addition reactions with the polyenes, i.e. the reverse of the dehydrochlorination process, as well as forming colored polyene/HCl complexes. These various possibilities will be considered in turn. 

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. 

A reaction rate expression that is proportional to the square root of the reactant concentration results when the dominant termination step is reaction (4c), that is, the termination reaction occurs between two of the radicals that are involved in the unimolecular propagation step. The generalized Rice-Herzfeld mechanism contained in equations 4.2.41 to 4.2.46 may be employed to derive an overall rate expression for this case. 

In order for the overall rate expression to be 3/2 order in reactant for a first-order initiation process, the chain terminating step must involve a second-order reaction between two of the radicals responsible for the second-order propagation reactions. In terms of our generalized Rice-Herzfeld mechanistic equations, this means that reaction (4a) is the dominant chain breaking process. One may proceed as above to show that the mechanism leads to a 3/2 order rate expression. 

It is also worth emphasizing that the initiation and termination steps are not included in the central chain process. For instance, in metal hydride-promoted domino reactions the initial halogen abstraction (or SePh displacement, etc.) and the final hydrogen abstraction from R MH are not classified as part of the domino sequence. More precisely, only the propagation steps within the mechanism of this process will be considered as a strict integral part of the domino reaction. 

A major difference between both kind of syntheses lies in polymerisation kinetics in the former case there is a chain mechanism (initiation, propagation, termination), in the latter case there is no chain reaction and each step is equiprobable. Main comparison points are highlighted in Table 4 [9]. 

The phenomena of nitroxyl radicals regeneration has been discovered in the study of the retarding effect of 2,2,6,6-tetramethyl-4-benzoyloxypiperidine-A-oxyl on PP initiated oxidation [51]. It has been shown that the limiting step of chain termination by the nitroxyl radical is the reaction with the alkyl macroradical of PP. The resulting compound AmOP is fairly reactive with respect to the peroxyl radical and nitroxyl radical is regenerated in this reaction. Thus, the cycle includes the following two reactions (mechanism I) [60-64] ... 

A treatment similar to that for unimolecular reactions is necessary for recombination reactions which result in a single product. An example is the possible termination step for the mechanism for decomposition of C Hg, H + CjH - (Section 6.1.2). The initial formation of ethane in this reaction can be treated as a bimolecular event. However, the newly formed molecule has enough energy to redissociate, and must be stabilized by transfer of some of this energy to another molecule. 

The importance of an energized reaction complex in bimolecular reactions is illustrated by considering in more detail the termination step in the ethane dehydrogenation mechanism of Section 6.1.2 ... 

How many of the reactions in this mechanism might be influenced by the rate of energy transfer One of them is the termination step, which can be thought of as a three-step process (reactions (7) to (9) below). As described in Section 6.4.3, there are possible further complications, since two other product channels are possible (reactions (10) and (11)). 

Termination steps involving two ethyl radicals are also ignored in the original mechanism. Include the following reaction ... 

More complicated mechanisms of the same category are encountered in SrnI reactions (Section 2.5.6) where the electrocatalytic reaction, which corresponds to a zero-electron stoichiometry, is opposed to two-electron consuming side reactions (termination step in the chain process). 

Because of the precise control of the redox steps by means of the electrode potential and the facile measurement of the kinetics through the current, the electrochemical approach to. S rn I reactions is particularly well suited to assessing the validity of the. S rn I mechanism and identifying the side reactions (termination steps of the chain process). It also allows full kinetic characterization of the reaction sequence. The two key steps of the reaction are the cleavage of the initial anion radical, ArX -, and conversely, formation of the product anion radical, ArNu -. Modeling these reactions as concerted intramolecular electron transfer/bond-breaking and bond-forming processes, respectively, allows the establishment of reactivity-structure relationships as shown in Section 3.5. 

In an interesting catalysed conversion of trichloroethene by secondary amines into aminoacetamides, the initial steps are thought to involve the p-elimination of HC1 to produce dichloroethyne (Scheme 9.1), which reacts with the secondary amine under the wet conditions to produce the amide [35] the reaction does not work with N-alkylanilines. Such a mechanism is realistic, as it is well known [36] that trichloroethene is converted into the inflammable and explosive dichloroethyne by bases, and quaternary ammonium salts catalyse the formation of the alkyne when trichloroethene is reacted with oxiranes [37]. Chloroethynes have also been obtained by the catalysed reaction of terminal ethynes with carbon tetrachloride under basic conditions [38]. 

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