Chain-terminating steps

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... 

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. 

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. 

Chain-terminating steps used up one or both radical intermediates. 

In the chain-terminating steps bonds are formed, but no bonds are borken. 1) All of the chain-terminating steps are highly exothermic. 

From this analysis one concludes that if one radical is formed at a temperature in a prevailing system that could undergo branching and if this branching system includes at least one chain branching step and if no chain terminating steps prevent run away, then the system is prone to run away that is, the system is likely to be explosive. 

To illustrate the conditions under which a system that includes chain propagating, chain branching, and chain terminating steps can generate an explosion, one chooses a simplified generalized kinetic model. The assumption is made that for the state condition just prior to explosion, the kinetic steady-state assumption with respect to the radical concentration is satisfactory. The generalized mechanism is written as follows ... 

Since a radical is consumed and formed in reaction (3.3) and since R represents any radical chain carrier, it is written on both sides of this reaction step. Reaction (3.4) is a gas-phase termination step forming an intermediate stable molecule I, which can react further, much as M does. Reaction (3.5), which is not considered particularly important, is essentially a chain terminating step at high pressures. In step (5), R is generally an H radical and R02 is H02, a radical much less effective in reacting with stable (reactant) molecules. Thus reaction (3.5) is considered to be a third-order chain termination step. Reaction (3.6) is a surface termination step that forms minor intermediates (T) not crucial to the system. For example, tetraethyllead forms lead oxide particles during automotive combustion if these particles act as a surface sink for radicals, reaction (3.6) would represent the effect of tetraethyllead. The automotive cylinder wall would produce an effect similar to that of tetraethyllead. 

Many of the early contributions to the understanding of hydrogen-oxygen oxidation mechanisms developed from the study of explosion limits. Many extensive treatises were written on the subject of the hydrogen-oxygen reaction and, in particular, much attention was given to the effect of walls on radical destruction (a chain termination step) [2], Such effects are not important in the combustion processes of most interest here however, Appendix C details a complex modem mechanism based on earlier thorough reviews [3,4],... 

The important chain termination steps in the static explosion experiments (Fig. 3.1) are... 

In their model, Miller and Bowman [6] consider the important chain-termination steps to be... 

The rate constants for these chain-terminating steps are not well established. However present estimates are probably not greatly in error because radical-radical reactions tend to be fast. 

The main product is thus the result of addition of HBr to the alkene. Minor products detected are consistent with the proposed chain-termination steps. 

In the propagation steps, this radical then reacts with oxygen, producing a peroxyl radical, which then abstracts hydrogen from a further molecule of the substrate. The product is thus the hydroperoxide, reaction having occurred at the allylic position of the alkene. Two possible chain-termination steps might... 

This new radical then abstracts hydrogen from another molecule of HBr, and the chain reaction can continue. You may wish to show typical chain termination steps. 

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,... 

In ethane pyrolysis assume that the chain termination step is the adsorption of ethyl radicals... 

In polyolefins, the chain is propagated by an intermediate free-radical species or by an alkyl species adsorbed onto a solid. Both the free radical and the alkyl have the possibility of termination, and this creates the possibility of growth mistakes by chain transfer and chain-termination steps that create dead polymer before all reactants are consumed. The presence of termination steps produces a broader molecular-weight distribution than does ideal addition polymerization. 

Free-radical polyolefin reactions form polymers with many mistakes in addition to the ideal long-chain alkanes because of chain-branching and chain-termination steps, as discussed. This produces a fairly heterogeneous set of polymer molecules with a broad molecular-weight distribution, and these molecules do not crystallize when cooled but rather form amorphous polymers, which are called low-density polyethylene. 

The first demonstration of in vitro P3HB synthesis in aqueous solution was achieved by Gerngross and Martin [35]. The polymer obtained had significantly higher molecular weight than that synthesized in vivo. This is probably due to a lack of a chain termination step of the PHA polymerization under in vitro conditions, similar to a living polymerization. Many other reports followed [27, 36, 38-44] and were extensively reviewed [2, 6, 7, 12, 26-32]. 

It was proved that in most cases all propagation and chain-transfer (chain-termination) steps take place without isomerization of the chiral center.326... 

A chemical relay race involving radicals is usually termed a chain reaction and the radicals are termed chain carriers. A chain reaction involves the production of a chain carrier that subsequently reacts to produce another chain carrier, namely a chain-propagating reaction. The formation of the first chain carrier in a chemical reaction is termed the initiation step. The chain reaction continues until a reaction involving the formation of a stable species from two chain carriers breaks the chain (chain-terminating step). In our analogy a chain-terminating reaction corresponds to two relay runners that collide with each other and drop their batons instead of passing them on. 

It should be noted that (R23) is a chain-propagating reaction, converting H atoms to HO2 radicals. However, because HO2 is much less reactive than H, O, and OH radicals, reaction (R23) acts in effect as a chain-terminating step. In addition to the gas-phase chainterminating steps, radicals may be deactivated at the walls of the vessel... 

Even though this is a chain-terminating step, the radical pool is rapidly replenished through the H + O2 reaction (Rl). Reactions between formaldehyde and O/H radicals lead to the formyl radical (HCO), which subsequently dissociates thermally (R30) or reacts with O2 to form CO (R31). 

Reaction (R144), which is a chain-terminating step, is the main source of SO3 at higher temperatures. The importance of the SO2 + OH recombination reaction (R145) depends strongly on the temperature. This reaction is rapidly equilibrated under combustion conditions, and HOSO2 is believed to be thermally unstable above 1000 K. Below this temperature HOSO2 reacts rapidly with O2,... 

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