Electron reactions, chain propagating

Restoration of electrons and chain propagation is provided by ion-molecular reactions and associative detachment, producing additional sulfuric acid ... 

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

During this, the electrons of the partial X—Z multiple bond are used. Experiments show that the ester can be further active in the polymerization. Its reactivity, however, is reduced in comparison with ion pairs. From a mechanistical point of view, the chain propagation should proceed in the manner of a SN2 reaction, that is with the monomer as nucleophile and the ester as substrate. With the assistance of quantum chemical calculations using the CNDO/2 method, the differences between covalent species and free ions should be examined. The following contains the three types of anions used ... 

A careful analysis based on these experimental results excluded a chain-propagation process [33a]. On account of the 3-position of the methylthio or methoxy substituent in the thiophene or pyrrole rings, three isomeric dimers may be formed. The main reaction path can be deduced from the mesomeric forms of the radical cations (2)". The two most important mesomeric structures are those with the unpaired electron in... 

One important feature of ion-radical organic reactions consists of a possibility to nudge them by the introduction of active reactants. Thus, in the reaction of an electron acceptor with electron donors (nucleophiles), the addition of a tiny amount of a nucleophile, which is more active at initiation of the one-electron transfer allows the less reactive nucleophile to start its own chain propagation. A method called entrainment is widely used in chemical practice as a recent example (see Schmidt et al. 2007). 

The photoinduced electron transfer (PET) initialed cyclodimerization was first studied with 9-vinylcarbazole as substrate1 and characterized mechanistically as a cation radical chain reaction.2 The overall reaction sequence3-4 consists of a) excitation of an electron acceptor (A), b) electron transfer from the alkene to the excited acceptor (A ) with formation of a radical ion pair, c) addition of the alkene radical cation to a second alkene molecule with formation of a (dimeric) cation radical, and d) reduction of this dimeric cation radical by a third alkene molecule with formation of the cyclobutanc and a new alkene cation radical. Steps c) and d) of the sequence are the chain propagation steps. The reaction sequence is shown below. 

Mechanistic information from these reactions points to the initial formation of a radical anion of the aromatic compound, followed by loss of halide ion (3.15) subsequent attack by a second enolateanion and electron transfer to a second molecule of aryl halide provides the substitution product, and the reaction is propagated. The operation of a chain mechanism is indicated by the observation that quantum... 

Reaction 8 may, therefore, be the major chain-propagating reaction of H02 between 250° and 400°C. The radicals produced will, of course, undergo the same fates as those produced in Reaction 4, regenerating (eventually) alkyl radicals. The main difference between the alkene-H02 addition route and the alkylperoxy radical isomerization route is that in the former case the hydroperoxyalkyl radicals formed are necessarily a-radicals—i.e., radicals in which the unpaired electron is borne by a carbon atom adjacent to that bearing the hydroperoxy group, such as... 

Here Q denotes an alkyl radical with two unpaired electrons (in QOOH and QO) which may rearrange to form a stable alkene. The compound QO is a cyclic ether3 (which may break down to form an aldehyde4 and a smaller alkene). The sequence of reactions (R64) to (R67) is chain propagating, in that the initial alkyl radical has produced one HO2 or OH radical in addition to one or more stable components. However, it is also possible that a second oxygen molecule may add to QOOH to form a peroxy alkyl hydroperoxy radical,... 

The associative step (equation 3) determines the nature of the product, since in this step the synthetically important bond formation between the aromatic moiety and the nucleophile takes place. The rates for the association of a number of nucleophiles with a variety of aryl and heteroaryl radicals has been measured in electrochemical studies29-31 and competitive product studies.32-35 The range of nucleophiles, classified according to the atom which becomes directly bonded to the aromatic ring, and, where the nucleophile is ambident, the regiochemistry of the association reaction shown in equation (3) are detailed in Section 2.2.3. The final propagating step (equation 4), that returns the chain-propagating electron from product radical anion to another molecule of substrate, is essential if a chain reaction is to continue. 

Gersmann et al.74 suggested another mode of initiation, which proceeds by deprotonation of RH to a carbanion that transfers one electron to an oxygen molecule capture of oxygen produces a peroxy radical that oxidizes another molecule of the initial carbanion, the last two steps constituting a chain-propagation process. In hydrocarbon oxidations, Russell73 also showed that reactions of carbanions with 02 proceed via a two-step one-electron transfer pathway [Eqs. (29)-(32)]. 

The simplest way to catalyze the polymerization reaction that leads to an addition polymer is to add a source of a free radical to the monomer. The term free radical is used to describe a family of very reactive, short-lived components of a reaction that contain one or more unpaired electrons. In the presence of a free radical, addition polymers form by a chain-reaction mechanism that contains chain-initiation, chain-propagation, and chain- termination steps. 

Irradiation of NADH model compounds in the presence of benzyl bromide or p-cyanobenzyl bromide in acetonitrile brings about reduction of the benzyl halides to the corresponding toluene compounds114. Like the S l substitution reaction, this photoreduction also occurs via an electron-transfer chain mechanism. Unlike in that case, though, here an electron transfer from the excited state of the NADH compound is solely responsible for the initiation step. In the propagation, the benzyl radical produced by C—Br bond cleavage in the radical anion abstracts hydrogen from the NADH compound. This yields a radical intermediate, from which electron transfer to benzyl bromide occurs readily (equations 39-42). 

The fragmentation of the radical anion (RNu)- along the chain propagation cycle of the process has been taken as evidence of the proposed mechanism. For example, in the case of dihalobenzenes YArX, the radical anion formed upon the first substitution YArNu- may transfer the extra electron to the C—Y bond (intramolecular ET) or to YArX (inter-molecular ET). The ratio between monosubstituted and disubstituted products formed will depend on the relative rate constants for both types of competing ET reactions. 

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 ... 

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