Propagation step in chain reaction

Like the bimolecular reactions, unimolecular reactions are often found as individual steps in complex reactions. These include the unimolecular breakdown of molecules into radicals often found as first initiation steps and propagation steps in chain reactions, e.g. 

In most instances, phenoxy radicals do not engage in propagation steps in chain reactions, consistent with their use as free radical inhibitors. They are, however, reactive enough to oxidize ascorbic acid or to reduce some quinones (Neta and Steenken, 1981). 

By about 1947 many of the major themes of free radical chemistry had emerged from this war-initiated polymer research data on structure and reactivity and establishment of the utility of competitive kinetics for determining relative rates of chain propagation steps in chain reactions by product analyses as I ve just described th first good rate constants for elementary steps in chain processes - here the first really reliable results were on vinyl acetate polymerization from Paul Bartlett s laboratory ( ) degradative chain transfer, first demonstrated by Bartlett and Altschul and showing how the length of kinetic... 

These reactions together with the primary process (61) lead to a quantum yield of 2. Reaction (66) has been well established as a chain propagating step in the reaction between H202 and 03 (82), while reaction (67) has frequently been used for reactions involving H atoms and 02 (88). 

Cp2Fe2(CO)4] promotes photosubstitution of CO by RNC in [CpFe (CO)2I].120 The propagation step in these reactions is halogen atom abstraction from reactant by the 17-electron radicals [CpMo(CO)2PR3] and [CpFe (CO)2(CNR)], respectively. A radical chain pathway has also been proposed for the [Cp2Fe2(CO)4]-promoted photosubstitution in Eq. (27).121 In... 

Many reactions start slowly at first and then speed up, as reagents are consumed and products are made. This is particularly true of chain reactions, in which products are made, and some reactive intermediate is regenerated to "keep the chain going." Polymerizations, explosions, and nuclear bombs are examples of chain reactions. These chain reactions have precise components that must be identified in a successful reaction mechanism (1) chain initiation, (2) chain propagation, (3) chain termination. The propagation step in chemical reactions usually involves the formation of very reactive free radicals (odd-electron species, while the chain termination steps may involve radical-radical reactions, which shut off the supply of reactive intermediates. We return to the gaseous hydrogen-bromine reaction discussed above ... 

A transfer agent thus competes with the monomer for the growing chains. It does not use up the initiator radicals but lowers the average molecular weight of the polymer by reducing the number of propagation steps in the reaction sequence. Certain industrial transfer agents are used specifically for this purpose. 

This step results in the formation of a molecule of allyl chloride (2-chloro-l-propene) and a chlorine atom. The chlorine atom then brings about a repetition of the first chain-propagating step. The chain reaction continues until the usual chain-terminating steps (see Section 10.4) consume the radicals. 

In practice side reactions intervene to reduce the efficiency of the propagation steps The chain sequence is interrupted whenever two odd electron species combine to give an even electron product Reactions of this type are called chain terminating steps Some commonly observed chain terminating steps m the chlorination of methane are shown m the following equations... 

Recall from Section 5.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a radical, the halogen, and the alkane yield alkyl halide product plus more radical to carry on the chain. The chain is occasionally terminated by the combination of two radicals. 

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. 

Transfer reaction to the monomer, leading to the insertion of an unsaturated end group, is an important reaction in cationic chain polymerisation. As the activation energies of both termination and transfer reactions are higher than that of the propagation step, cationic chain polymerisation can only lead to high molecular masses when undertaken at low temperatures, typically — 100°C. 

Bartlett and Nozaki [65] discovered the chain reaction of benzoyl peroxide decomposition in the presence of alcohols where the chain propagating step is the reaction of the ketyl radical with peroxide. 

Reactions of phenoxyl and aminyl radicals with RH and ROOH are chain propagation steps in oxidation inhibited by phenols and amines (see Chapter 14). Both reactions become important when their rates are close to the initiation rate (see Chapter 14). Mahoney and DaRooge [57] studied the oxidation of 9,10-dihydroanthracene inhibited by different phenols. He went on to estimate the values of rate constants ratio of the reaction of ArO with RH and the reaction In + In (reactions (9) and (10), see Chapter 14) by the kinetic study. The values of kw for the reaction... 

In chain reactions, the inhibitor 7n interacts with free radical P and makes it unavailable for the propagation of chain, thus providing an extra termination step... 

Reaction (3.1) is the initiation step, where M is a reactant molecule forming a radical R. Reaction (3.2) is a particular representation of a collection of propagation steps and chain branching to the extent that the overall chain branching ratio can be represented as a. M is another reactant molecule and a has any value greater than 1. Reaction (3.3) is a particular chain propagating step forming a product R It will be shown in later discussions of the hydrocarbon-air... 

Radical chain hydrosilylation of alkenes using RsSiH, where R = alkyl and/ or phenyl, is not very useful in synthesis because the hydrogen abstraction step is slow under standard experimental conditions (cf. Section 3.1). These reactions proceed under drastic conditions (Reaction 5.9) [3], but can be promoted under milder conditions by the presence of catalytic amounts of a thiol [27]. Thus, the propagation steps in Scheme 5.1 are replaced by those reported in Scheme 5.4, where the thiol acts as the catalyst and the H transfer agent (see Iso Section 4.5). 

Chain Reactions. In chain reactions the intermediate is formed in a first reaction, called the chain initiation step. It then combines with reactant to form product and more intermediate in the chain propagation step. Occasionally the intermediate is destroyed in the chain termination step. Thus,... 

The essential feature of the chain reaction is the propagation step. In this step the intermediate is not consumed but acts simply as a catalyst for the conversion of material. Thus, each molecule of intermediate can catalyze a long chain of reactions, even thousands, before being finally destroyed. 

This is a major chain propagation step in the overall reaction mechanism for ozone formation in photochemical air pollution. Because H02 is intimately tied to OH through reaction (17) and cycles such as that in Fig. 1.4, when NO is present the sources and sinks of H02 are, in effect, sources or sinks of the OH radical. 

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