Radical chain reactions homolysis

The easy homolysis of C-Br bond in CBr4 allowed us to conduct the radical chain reaction of CBr4 with 3,3,3-trifluoropropene under common conditions (benzoyl peroxide), although in this case the strong electrophiles are used as reagents (an addend and a monomer), i.e. a very unfavorable combination of polar factors for proceeding the process takes place (ref. 6). 

Initiation normally requires molecules with weak bonds to undergo homolytic cleavage to produce free radicals. Since bond homolysis even of weak bonds is endothermic, energy in the form of heat (A) or light (hv) is usually required in die initiation phase. However, some type of initiation is required to get any free-radical reaction to proceed. That is, you must first produce free radicals from closed-shell molecules in order to get free-radical reactions to occur. Benzoyl peroxide contains a weak 0-0 bond that undergoes thermal cleavage and decarboxylation (probably a concerted process) to produce phenyl radicals which can initiate free-radical chain reactions. 

Simple homolysis of the C-I bond by heating or by light is the most straightforward approach and was the first used for adding perfluoroalkyl iodides to olefins. One presumes that both the thermal and the photochemically induced addition reactions of perfluoroalkyl radicals proceed via free radical chain reactions as depicted in the Scheme below. However, the conditions of these reactions are rarely ideal for preparative purposes because high temperatures are required for the thermolytic process and long photolysis times are required for the photolytic method [60]. 

The ability of radicals to propagate by abstraction is a key feature of radical chain reactions, which we shall come to later. There is an important difference between homolysis and abstraction as a way of making radicals homolysis is a reaction of a spin-paired molecule that produces two radicals abstraction is a reaction of a radical with a spin-paired molecule that produces one new radical and a new spin-paired molecule. Radical abstractions like this are therefore examples of your first radical reaction mechanism they are in fact substitution reactions at H and can be compared with proton removal or even with an Sfj2 reaction. 

The diphenylantimony radical (16), which is prepared from the homolysis of tetraphenyldistibine by irradiation, reacts with alkyl iodide to form alkyl(diphenyl)stibine (17) by a radical chain reaction in high yield (equation 20) . 

What does the peroxide do Why does its presence change the mechanism The peroxide undergoes homolysis of the weak 0-0 bond extremely easily, and because of this it initiates a radical chain reaction. We said that H-Cl in the gas phase undergoes homolysis in preference to heterolysis other types of bond are even more susceptible to homolysis. You can see this for yourself by looking at this table of bond dissociation energies (AGfor X-Y —> X + Y ). 

In the initiation part of free-radical chain reactions, a small amount of one of the stoichiometric starting materials (i.e., a starting material that is required to balance the equation) is converted into a free radical in one or more steps. An initiator is sometimes added to the reaction mixture to promote radical formation. In the example, though, no initiator is necessary light suffices to convert Br2, one of the stoichiometric starting materials, into a free radical by cr-bond homolysis. 

Scheme 28, by application of the known thiol ester-thiopyrone phototransformation to selenium-containing systems. 5/f-[l]Benzoselenino[2,3-6]-pyridine, 4.ff-selenolo[2,3-6][l]benzoselenine, and 9/f-seleno[3,2-6][l]benzo-selenine have similarly been obtained by the corresponding selenol ester-seleninone conversion. Phenyl areneselenosulphonates undergo facile photo-induced homolysis of the selenium-sulphur bond in the presence of alkenes, a free-radical chain reaction leads to the formation of -phenylselenosulphones. ... 

Ultraviolet light causes the addition of cyclohexane to perchlorophenylace-tylene (Ballester et al., 1986b). Presumably, this process is initiated by homolysis of the latter to the acetylenic radical [91]. This addition has been interpreted as a radical chain reaction (98). 

Chlorine atoms may be generated from molecular chlorine under mild conditions using a catalytic amount of an initiator, In-ln. Thus, homolysis of a molecule of initiator occurs upon irradiation or gentle heat to give free radicals, In (Eq. 9.3). These free radicals may then react with molecular chlorine to produce In-Cl and a chlorine atom (Eq. 9.4) to initiate the free-radical chain reaction. For safety and convenience, sulfuryl chloride, SO2CI2, rather than molecular chlorine is used in this experiment as the source of chlorine radicals. 

The first step in our procedure for initiating the free-radical chain reaction is the homolysis of l,r-azobis[cyclohexanenitrile] (1), abbreviated as ABCN, to form nitrogen and the free radical 2 (Eq. 9.5). The rate of this reaction is sufficiently fast at 80-100 °C to generate enough chlorine atoms to initiate the chain process. The radical 2 then attacks sulfuryl chloride to generate chlorine atoms and SO2 according to Equations 9.6 and 9.7. The series of reactions depicted in Equations 9.S-9.7 comprise the initiation steps of the reaction. 

In chain methods, it is important to avoid radical-radical reactions. However, radicals that are generated in a stoichiometric quantity by bond homolysis can be productively removed by radical-radical coupling. Despite the inherent problems in controlling reactions that occur at rates near the diffusion-controlled limit, radical-radical coupling reactions can be selective and preparatively useful. 

The silver(I) salts of carboxylic acids react with halogens to give unstable intermediates which readily decarboxylate thermally to yield alkyl halides. The reaction is believed to involve homolysis of the C-C bond and a radical chain mechanism. 

When (/3-hydroxyalkyl)mercury(II) acetates are treated with NaBH4 and no additional reagent, they first form (( - h y d roxy a Iky 1) me rcu ry (II) hydrides. These decompose via the chain reaction shown in Figure 1.12 to give a mercury-free alcohol. Overall, a substitution reaction R—Hg(OAc) — R—H takes place. The initiation step for the chain reaction participating in this transformation is a homolysis of the C—Hg bond. This takes place rapidly at room temperature and produces the radical Hg—H and a /3-hydroxylated alkyl radical. As the initiating radical, it starts the first of the two... 

Radical 3-Bond Scission. Model studies clearly show that certain bonds, such as the (-CH2-CH2-) bond in 1,3-diphenylpropane (15), are rapidly cleaved even though these bonds are too strong to undergo substantial bond homolysis under reaction conditions (Table III). An alternative route for bond breaking is through a chain reaction sequence involving 3-bond scission, as follows ... 

As noted above, dioxygen reacts with organic molecules, e.g. hydrocarbons, via a free radical pathway. The corresponding hydroperoxide is formed in a free radical chain process (Fig. 4.3). The reaction is autocatalytic, i.e. the alkyl hydroperoxide accelerates the reaction by undergoing homolysis to chain initiating radicals, and such processes are referred to as autoxidations [1]. 

Mayo [16] proposed an alternative mechanism that is currently widely supported. Figure 7.7 shows a schematic of the Mayo mechanism. A Diels-Alder reaction between two styrene molecules produces an intermediate dimer (DH), also referred to as Mayo dimer . DH is highly reactive and has never been isolated. To complete the auto-initiation, DH reacts with a third styrene molecule via molecular assisted homolysis [17] to form a phenyltetraline radical (D ) and a phenethyl radical (SH ). A second reaction involving DH is to undergo chain transfer with a growing radical chain to produce a dead polymer chain (PS-H) and a new growing radical. The chain transfer constant (A ct) of DH has been estimated at 10, which is the highest Kcl ever reported for a molecule that contains no heteroatoms [18,19]. 

Alkylcobalt complexes provide an easy bridge between two-electron ionic chemistry and one-electron radical chemistry. This has made them popular and useful radical precursors ionic reactions provide alkylcobalt complexes which then provide alkyl radicals via C-Co bond homolysis. In the mid 1970s and early 1980s, radical chain Sh2 reactions of allylcobaloximes were studied (eq 1). These reactions were not applied to specific synthetic problems. ... 

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