Peroxides weak 0-0 bond

Homolytic cleavage of most a bonds may be achieved if the compound is subjected to a sufficiently high temperature, typically about 200 °C. However, some weak bonds will undergo homolysis at temperatures little above room temperature. Bonds of peroxy and azo compounds fall in this category, and such compounds may be used to initiate a radical process. Di-tert-butyl peroxide, dibenzoyl peroxide... 

The photochemical oxidation reactions of unsaturated organic molecules lead to the formation of peroxides which are characterized by the weak bond O-O. In terms of bond energies this is one of the weakest covalent bonds weak enough to be split in thermal processes. When a cyclic peroxide dissociates into carbonyl compounds, one of these may be formed in an electronically excited state (Figure 4.83). The energy difference results from the loss of the weak 0-0 bond and the formation of two C=0 tt bonds over 60kcalmol-1 is then available as free energy. There are also reasons of... 

The reaction has a radical-chain mechanism and the chains can be initiated by light or by chemicals, usually peroxides, ROOR. Chemical initiation requires an initiator with a weak bond that dissociates at temperatures between 40-80°. Peroxides are good examples. The 0-0 bond is very weak (30-50 kcal) and on heating dissociates to alkoxyl radicals, RO-, which are reactive enough to generate the chain-propagating radicals from the reactants. The exact sequence... 

There are many reagents that add to alkenes only by radical-chain mechanisms. A number of these are listed in Table 10-3. They have in common a relatively weak bond, X—Y, that can be cleaved homolytically either by light or by chemical initiators such as peroxides. In the propagation steps, the radical that attacks the double bond does so to produce the more stable carbon radical. For addition to simple alkenes and alkynes, the more stable carbon radical is the one with the fewest hydrogens or the most alkyl groups at the radical center. 

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. 

Overview articles on the formation and decomposition of polymer peroxides [35, 36, 37] confirm the formation of weak bonds during the polymer synthesis following from various experimental data. 

When a compound that has an especially weak bond is heated, the weak bond is selectively cleaved to produce radicals. Because the bond energy of the oxygen-oxygen bond is small, only about 30 keal/mol (126 kJ/mol), peroxides readily undergo bond homolysis when they are heated to relatively low temperatures (80°-100°C). Commercially available peroxides, such as benzoyl peroxide and fert-butyl peroxide, are commonly used as sources of radicals. 

Notice that we didn t put HBr on the list of molecules that form radicals by homolysis relative to the weak bonds we have been talking about, the H-Br bond is quite strong (just about as strong as a C-C bond). Yet we said that Br radicals were involved in the addition reaction we talked about on p. 1020. These radicals are formed by the action of the alkoxy radicals (generated by homolysis of the peroxide) on HBr—a process known as radical ... 

The choice of reaction conditions has much less effect on the behavior of azo initiators. The activation energies for decomposition of azo compounds are similar to those of peroxides although the azo initiators do not contain a weak bond like... 

Radicals are formed from covalent bonds by adding energy in the form of heat (A) or light (hv). Some radical reactions are carried out in the presence of a radical initiator, a compound that contains an especially weak bond that serves as a source of radicals. Peroxides, compounds with the general structure RO OR, are the most commonly used radical initiators. Heating a peroxide readily causes homolysis of the weak 0-0 bond, forming two RO- radicals. 

It was mentioned at the beginning of this section that many radical reactions are commenced by the addition of compounds that promote the formation of radicals. These compounds are called initiators, and often contain a weak bond that is easily broken, e.g. peroxides. Similarly, there are some species that scavenge free radicals and these are called inhibitors. Such species include molecular oxygen, nitric acid and benzoquinone. The addition of either initiators or inhibitors will greatly affect the rate of the radical reaction. 

The first method is more perspective, as stabilizers may inhibit destruction reaction may directly influence the mechanism of destruction with the purpose of decreasing undesirable products yield and so on. It is supposed that stabilizers may act by means of 1) blocking of active centres (weak bonds) 2) filtration of ultra-violet radiation 3) breaking of peroxides 4) interaction with free radicals 5) suppression of excited states. 

A covalent bond is generally cleaved to its radical fragments at temperatures higher than 800 °C. Covalent bonds that can be cleaved at <150°C are limited to weak bonds whose dissociation energies are under 30-40 kcal/mol [1]. Azo compounds, peroxides, nitrite esters, ester of A-hydroxy-2-thiopyridone etc. fit into this group... 

Organic compounds can generate the initiators of free radical sequences through the primary photochemical processes homolytic dissociation into radicals, hydrogen-atom abstraction, photoionization, and electron transfer reactions. The homolytic dissociation reactions are limited to compounds containing relatively weak bonds (<98 kcal), such as sulfides, peroxides, and some halides and ethers. Representatives of all of these classes of compounds are certainly present in seawater, but the limited information on the qualitative and quantitative aspects of their occurrence does not allow for an estimate of their importance in the promotion of free radical reactions. The same is true for electron transfer reactions, which may be an important photochemical process for organic transition metal complexes. 

Radicals can be generated by homolysis of weak a-bonds. Homolysis is effected by photochemical, thermal or redox (electron transfer) methods. A common method to initiate a radical reaction is to warm a peroxide such as benzoyl peroxide or azobi-sisobutyronitrile (AIBN) 1 (4.2). The radical -C(CN)Me2 generated from AIBN is rather umeactive, but is capable of abstracting a hydrogen atom from weakly bonded molecules such as tributyltin hydride (4.3). The resulting tributyltin radical reacts readily with alkyl halides, selenides and other substrates to form a carbon-centred radical. 

The general kinetics for this mechanism (Eastmond, 1976b) involve the usual three primary steps of any chain reaction, i.e., initiation, propagation, and termination, as shown below. Initiation generally occurs by the formation of free radicals through the homolytic dissociation of weak bonds (e.g., in peroxides or azo compounds) or by irradiation. Termination reactions for vinyl polymers can occur either by combination (coupling), by disproportionation, or by a combination of both reactions (discussed next). 

Radicals may be generated by thermal or photochemical processes that accomplish homolytic dissociation of a two-electron bond. Organic peroxides (equation 5.10) and azo compounds (equation 5.11) have weak bonds that undergo dissociation to radicals relatively easily. Chemical or electrochemical oxidation or reduction of stable molecules can produce radicals as well. One approach for generating ethyl radicals is to add triethylborane to a reaction mixture from which not all the oxygen has been removed. ° Radicals may also be produced by photoinduced electron transfer (Chapter 12). Single electron transfer processes initially generate radical... 

The azo compound and peroxides contain weak valence bonds in their structures. Heating causes weak bonds in these compounds to cleave and to dissociate into free radicals as follows ... 

The molecule types listed (alkane, ether, alcohol, etc.) are introduced in Chapter 4 and Chapter 5, Section 5.7.). Note that the shortest bonds in Table 3.1 that involve carbon are the C-C and C-H bonds. When an atom other than carbon or hydrogen is attached to carbon, the bond is generally longer and often weaker (this will be explained in Section 3.7). Both the OH and NH bonds are rather short, relative to bonds of various atoms to carbon. The 0-0 bond (a peroxide) is not particularly long, but it is a very weak bond. 

From the material cited above, the negative significance of the fact that antioxidants not only terminate chains, but also take part in the initiation of oxidation is clear. Moreover, an important role is played by the formation of hydroperoxides in the reaction of peroxide radicals RO2 with antioxidants, the molecules of which contain weakly bonded hydrogen. Such antioxidants include widely used derivatives of phenols and aromatic amines. 

A variety of compounds have been created by chemists to initiate radical reactions. These initiators can be used not only in addition reactions, but also radical substitutions and polymerizations. The common design principle is the incorporation of a weak bond that undergoes homolysis thermally or with light. Organic peroxides and azo compounds are common examples that can decompose by both means (Eqs. 10.41-10.43). The decomposition of benzoyl peroxide (a common light-activated acne medicine Eq. 10.42) leads to phenyl radical and carbon dioxide. 

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