The steps in a chain reaction

In a chain reaction we are led to distinguish four types of sequences  

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The steps in a chain reaction that are repeated over and over to form the product. The sum of the propagation steps should give the net reaction. 

The steps in a chain reaction in which reactive intermediates are destroyed or rendered inactive, thus ending the chain. 

Propagation steps (Section 4 17) Elementary steps that repeat over and over again in a chain reaction Almost all of the products in a chain reaction arise from the propagation steps... 

The step or steps in a chain reaction in which reactive intermediates are stabilized and rendered inactive, thus ending the chain reaction. For example, a common termination reaction is the combination of two radicals Le., R- + R - R—R. Another type of termination reaction is disproportionation e.g., 2CH3—CH2- ethane + ethene. 2. The quenching of a reaction. 3. The halting of any process. 

Radical chain processes break down whenever the velocity of a termination reaction is comparable to the velocity of the rate-controlling step in a chain reaction. This situation would occur, for example, if one attempted to use EtsSiH as the hydrogen atom donor in the alkyl halide reduction sequence in Figure 4.6 and employed typical tin-hydride reaction conditions because the rate constant for reaction of the silane with an alkyl radical is 4 orders of magnitude smaller than that for reaction of Bu3SnH. Such a slow reaction would not lead to a synthetically useful nonchain sequence, however, because no radical is persistent in this case. In fact, a silane-based radical chain reduction of an alkyl halide could be accomplished successfully if the velocity of the initiation reaction was reduced enough such that it (and, hence, also the velocity of alkyl radical termination... 

The preliminary step in a chain reaction, where the reactive intermediate is first formed. 

By definition, a propagation step in a chain reaction is one in which products are formed, and the site of the reactive center changes but the number of active centers is not changed. (This statement is qualified in [2].) There are two major propagation reactions under the conditions of most free-radical polymerizations. These are addition and atom transfer reactions. 

Chain initiation step The first step in a chain reaction produces reactive species (such as radicals) that then propagate the reaction. 

Light induced formation of a diarylmethyl cation from the dithienylethanol (418) has been proposed as the initiation step in a chain reaction leading to the dimer (419). ... 

Chain Reactions Chain reactions where reactive intermediate substances chain carriers such as atoms, free radicals, or ions) are responsible for the permanent repetition of steps are consecutive reactions of a special type. We distinguish the following elementary steps in a chain reaction ... 

Radicals play a key role in chain reactions, in which one or more reactive reaction intermediates (frequently radicals) are continuously regenerated, usually through a repetitive cycle of elementary steps (the propagation step ). The propagating reaction is an elementary step in a chain reaction in which one chain carrier is converted into another. The chain carriers are almost radicals. Termination occurs when the radical carrier reacts otherwise. An example of one of the possible ozone destructions is shown below (R-Cl - chloro-organic compound) ... 

A chain reaction in which there is a net gain in the number of active centers as the closed sequence rc )eats itself. A step in a chain reaction in which there is a net gain in the number of active centers. 

The first step in a chain reaction is the initiation, which is the formation of the propagating chain from a reagent. Afterwards, propagation occurs, where the propagator reacts with one molecule of the reactant to form other propagator. Radicals combine themselves, and the chain ends in a process called termination. 

A step in a chain reaction characterized by the formation of reactive intermediates (radicals, anions, or cations) from nonradical or noncharged molecules. 

A very common mistake is to confuse the propagation step in a chain reaction with the termination step. In the photobromina-tion of an alkane such as methane, the product is formed in a chain reaction in which bromine and methyl radicals carry the chain (Fig. 11.45). It is all too tempting to imagine that the product, methyl bromide, is formed by the combination of one... 

If we wish to find the point of hormone action in terms of a pacemaker step in a chain reaction sequence, it is imperative that we be able to define rather precisely the nature of the successive steps involved which terminate in the response otherwise we should soon be in a position where we would not be able to interpret the significance of isolated effects studied without relation to the total picture. Indeed, to the extent that the underlying sequence of reactions is understood, it has been possible to approach the problem of hormone mechanisms. From this statement, it is therefore not surprising that insulin action, which involves carbohydrate metabolism, is best understood, whereas the mechanism of estrogen action to produce growth and differentiation in uterus and vagina (where the biochemical sequences involved are almost completely unknown) is still relatively obscure (c/. the review of Roberts and Szego, 1953). 

James Chadwick A British scientist, Chadwick confirmed the existence of a neutral particle in the atom known as the neutron. Although neutrons do not carry electrical charges, through the process of fission neutrons can be made to leave their host atoms and form new nuclei. Chadwick s experiments showed how the fission process would create a transfer of energy, a vital step in a chain reaction that would result in an atomic blast. 

A chain-propagating reaction, or more simply a propagating reaction, is an elementary step in a chain reaction in which one chain carrier is converted into another. The conversion can be a unimolecular reaction or a bimolecular reaction with a reactant molecule. 

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

FIGURE 13.19 In a chain reaction, the product of one step in a reaction is a highly reactive reactant in a subsequent step, which in turn produces reactive species that can take part in other reaction steps. 

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