Chain carriers, propagation cycles

Reactions involving intermediates are classified as non-chain or chain. A chain reaction is a special type of complex reaction where the distinguishing feature is the presence of propagation steps. Here one step removes an intermediate or chain carrier to form a second intermediate, also a chain carrier. This second chain carrier reacts to regenerate the first chain carrier and the characteristic cycle of a chain is set up, and continues until all the reactant is used up (see Section 6.9). 

A chain reaction involves at least two highly reactive intermediates, called chain carriers, which take part in a cycle of reactions called propagation. In the first propagation step one intermediate is removed and the second formed. This intermediate then goes on to regenerate the first intermediate in the second propagation step. At least one step removes reactant, and at least one forms product. The cycle... 

The initiation step(s) first produces a chain carrier enabling propagation to be set up. Since one step of initiation can set up a long cycle of propagation events, very little reactant is used up in initiation. 

The intermediates are CH3O, CH and CH3OCH. The best way to identify a chain reaction is to look for a cycle of steps where intermediates are regenerated, and identify them. CH and CH3OCH are recycled in steps 2 and 3 and are therefore chain carriers, and steps 2 and 3 are propagation steps. The reaction is a chain. 

Once a chain has been started, the propagation cycle typically runs through many repetitions before being terminated. This makes chain reactions very sensitive A minute amount of an initiator that generates chain carriers can set off a massive and fast reaction. 

Characteristic of most chain reactions is that the propagation cycle is repeated many times between initiation and termination. If so, the initiation and termination terms (first and last in eqn 9.7) are small compared with the propagation terms. The so-called long-chain approximation [4] disregards them. Equation 9.7 can then be used to express the concentration of one chain carrier as a function of that of the other ... 

This examination also illustrates another facet of chain reactions Granted quasi-stationary conditions and long chains, the concentrations of the chain carriers are coupled, in reactions like 9.5 through the requirement that the rates of the two propagation steps must be equal. Therefore, only one of the two chain carriers can be in dissociation equilibrium with its source, the other gets boosted by the propagation cycle to a higher than thermal concentration. 

Most of the useful radical reactions in synthetic chemistry involve a chain mechanism, in which radical species are continually regenerated and trapped. Such propagation steps are illustrated for reduction of a substrate RX (4.4). The feasibility of this sequence depends on the relative reaction rates which themselves are determined by the structures of the radicals (including that used to initiate the reaction). In reactions such as this, the trialkyltin radical is sometimes referred to as the chain carrier as it is continuously regenerated to propagate the cycle. 

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) ... 

When this feature exists, and the cycle is on the average repeated more than once (as it is in this reaction under usual conditions), the reaction is called a chain reaction, and the active intermediates (here H and Br) are referred to as chain carriers. The preceeding two reactions are referred to as chain-propagating steps. 

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