Radical chain reaction termination steps

The way to control a radical chain reaction is to control the initiation and termination steps. Radical chain reactions can be favored by adding radical initiators. Likewise, chain reactions can be greatly diminished by adding compounds called inhibitors that react with radicals to increase chain termination. The sensitivity of the radical reaction to radical initiators and inhibitors provides a convenient way to test for this mechanism. 

Termination steps (Section 4 17) Reactions that halt a chain reaction In a free radical chain reaction termination steps consume free radicals without generating new radicals to continue the chain... 

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. 

Although the propagation reactions are only shown once, you should be aware that they occur in a sequence a very large number of times before the termination reactions remove the reactive radicals. Thus, free-radical chain reactions are characterised by the formation of a very large number of product molecules initiated by the absorption of a single photon in the initiation step that is, chain reactions act as chemical amplifiers of the initial absorption step. 

Radical chain reactions are comprised of three distinct parts initiation, propagation steps, and termination. The initiation portion involves one or more elementary reactions that produce a radical that can participate in one of the propagation steps. The propagation sequence is where the desired products are formed it consists of two or more reactions in which one product of each elementary reaction is a radical that serves as a reactant in another step of the sequence. Radicals are destroyed in termination steps that give nonradical products by radical-radical couphng and disproportionation reactions. 

Radical reactions are often called chain reactions. All chain reactions have three steps chain initiation, chain propagation and chain termination. For example, the halogenation of alkane is a free radical chain reaction. 

Termination Any two radicals present in the reaction mixture can combine in a termination step, and end the radical chain reaction. Thus, radical reactions produce a mixture of products. 

The co-oxidation of indene and thiophenol in benzene proceeds by a three-step cyclic free radical chain reaction. Autocatalysis associated with the hydroperoxide which is the main primary product occurs, but the effect is complicated by other trace components. The reaction is extremely sensitive to catalysts and inhibitors, and its kinetic features are determined by the initiation and termination processes. 

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. 

A free radical chain reaction proceeds through a succession of free radicals. In the photochemical chlorination of an alkane, the initiating step is the homolytic lission of chlorine molecules to produce chloroalkanc molecules and chlorine free radicals. These two reactions constitute the propagating step. However, the chlorine free radicals may also combine to form chlorine molecules or react with the alkane free radicals to form chloroalkane molecules. Both of these reactions constitute terminating steps of the chain reaction. Il should be noted, however, that the foregoing sequence cannot take place in the dark. Exposure to light allows the series of reactions then to proceed rather violently. 

Write initiation, propagation, and termination steps for this radical-chain reaction. Estimate a AH0 for the overall reaction using the bond-dissociation energies of Table 4-6. Would you expect bromotrichloromethane to be a selective or nonselective brominating agent Explain. 

The first example of a free-radical chain reaction successfully conducted in sc C02, which demonstrated the potential of this solvent for preparative scale chemistry, was a report from the McHugh group (Suppes et al., 1989) dealing with the oxidation of cumene (eq. 4.4). The propagation steps for this reaction are depicted in Scheme 4.11. Pressure (and thus viscosity) had little effect on the initiation, propagation, or termination rate constants. No unusual kinetic behavior was observed near the critical point. 

Write all steps in the free-radical chain reaction between a halogen and an alkane, and identify the initiation, propagation, and termination steps. 

In a free-radical chain reaction, initiation steps generally create new free radicals. Propagation steps usually combine a free radical and a reactant to give a product and another free radical.Termination steps generally decrease the number of free radicals. 

In a free-radical chain reaction, every propagation step must occur quickly, or the free radicals will undergo unproductive collisions and participate in termination steps. We can predict how quickly the various halogen atoms react with methane given relative rates based on the measured activation energies of the slowest steps ... 

This type of reaction is important industrially since it is one of the few that allows compounds containing functional groups to be made from alkanes. As you might guess, since it needs light for initiation, the process is another example of a radical chain reaction. As with the radical addition of HBr to alkenes, we can identify initiation, propagation, and termination steps in the mechanism. 

El and Es are known (see above) and must be very small, so 12 = 23.5 kcal. mole- (the value 28.6 kcal given by Kalb and Allen appears to be incorrect). Hence reaction (84) is quite slow, and the steady-state concentration of sulphate radical-ions relatively high. This would account for reaction (66) being favoured as the chain-termination step, rather than reaction (77), the dominant termination step in the silver ion-catalysed reaction. 

The important features of autoxidation are auto-catalytic and free radical chain reactions. The rate of oxidation is initially slow and increases as the reaction progresses. However, once autoxidation is initiated, the reaction continues until the reaction substrate or catalytic factor becomes extinct. In short, unsaturated lipids undergo three reaction phases initiation, propagation, and termination. The participation of reactive oxygen radicals in autoxidation reactions is summarized in the following reaction steps. 

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