Benzoyl radicals, decomposition

Initiators are introduced into the reactant, as a rule, in very small amounts. The initiator produces free radicals, most of which react with the reactant or solvent or recombine with other free radicals. Radicals formed from the initiator or reactant react with the initiator very negligibly. However, systems (initiator reactant) are known where free radicals induce the chain decomposition of initiators [4,13-15]. Nozaki and Bartlett [16,17] were the first to provide evidence for the induced decomposition of benzoyl peroxide in different solvents. They found that the empirical rate constant of benzoyl peroxide decomposition increases with an increase in the peroxide concentration in a solution. The dependence of the rate of peroxide decomposition on its concentration was found to be... 

These substitution reactions were discovered by Nozaki and Bartlett [57,58] in their study of benzoyl peroxide decomposition in different solvents. When benzoyl peroxide is decomposed, the formed benzoyloxyl radical attacks the solvent (RH), and the formed alkyl radical (R ) induces the chain decomposition of the peroxide (see Chapter 3). 

Bartlett and Nozaki [65] discovered the chain reaction of benzoyl peroxide decomposition in the presence of alcohols where the chain propagating step is the reaction of the ketyl radical with peroxide. 

The unimolecular decompositions are nevertheless not without their complications. Consider, for example, the unimolecular decomposition of benzoyl peroxide in benzene or carbon tetrachloride, where good first-order kinetics indicate that the contribution of induced decomposition is small.58 Initial homolysis of the O—O bond leads to two benzoyl radicals (Equation 9.21), which can fragment according to Equation 9.22 to yield phenyl radicals and carbon... 

Smith and Hinshelwood - interpreted the maximally inhibited reaction as the intramolecular decomposition of the aldehyde. As already mentioned, such a conclusion must be treated with reserve. The participation of free radicals has been confirmed experimentally by Ingold and Lossing , who identified the phenyl and benzoyl radicals by mass-spectrometry. 

Inii/ioHon-. Any material which decomposes spontaneously or under external stimidus into free radicals may be used as an initiator for polymerization at the double bond A variety of peroxides satisfy this condition, such as, for instance, benzoyl peroxide and tertiary butylhydroper-oxide in systems where the initiator is dissolved in the monomer itself or in a monomer solution also hydrogen peroxide and potassium persulfate in emulsion polymerizations where the initiator is dissolved in an aqueous medium. Benzoyl peroxide decomposition occurs by Unimolecular reao tion ... 

In contrast, it is well known that styrene is a useful radical polymerizable monomer, and diacyl peroxide such as benzoyl peroxide and alkanoyl peroxide are widely used as conventional radical initiators. Since the interaction between theLUMO of diacyl peroxide (e.g., butyryl peroxide) and the HOMO of styrene become extremely weak compared to that of perfluorobutyryl peroxide (see Figure 4.3), the interaction of the SOMO of acyloxy radicals, which produce in the radical decomposition of diacyl peroxides, should interact with the HOMO of styrene to afford the radical polymerization of styrene. In contrast, styrene reacted with fluoroalkanoyl peroxides to afford not the radical polymerizable products but 1 1 adducts [PhCH(OCORp)CH2Rp] via... 

In a broad sense radical-induced decomposition may be considered as the reaction of any substrate promoted by a free radical, but the more common usage is to restrict consideration to those cases in which the molecule undergoing reaction is a free radical initiator (1, ). Thus radical-induced decomposition may be de-fTned as reaction of a free radical with the primary source of the radical. This phenomenon is particularly noticeable when a free radical initiator does not react with first-order kinetics, but is consumed by the radicals it generates in chain processes. This case was originally elucidated for the kinetics of benzoyl peroxide decomposition in solvents such as ethyl ether ( - ), and was found to occur with radical displacement (the SH2 reaction) (S) by solvent derived radicals on the peroxidic oxygen of benzoyl peroxide as shown in Fig. 1. 

Free-radical carboxymethylation of several aromatic compounds has been reported, " the -CHaCOOH radical being produced by the thermal decomposition of benzoyl peroxide in acetic acid. More recently the carboxymethylation of dibenzofuran brought about by the thermal decomposition of chloroacetylpolyglycolic acid (41) has... 

Diacyl peroxides undergo thermal and photochemical decomposition to give radical intermediates (for a recent review, see Hiatt, 1971). Mechanistically the reactions are well understood as a result of the many investigations of products and kinetics of thermal decomposition (reviewed by DeTar, 1967 Cubbon, 1970). Not surprisingly, therefore, one of the earliest reports of CIDNP concerned the thermal decomposition of benzoyl peroxide (Bargon et al., 1967 Bargon and Fischer, 1967) and peroxide decompositions have been used more widely than any other class of reaction in testing theories of the phenomenon. 

Since the benzene emission in the thermal decomposition of benzoyl peroxide results from radical transfer by the phenyl component of a benzoyloxy-phenyl radical pair, phenyl benzoate produced by radical combination within the same pair should appear in absorption. A weak transient absorption has been tentatively ascribed to the ester (Lehnig and Fischer, 1970) but the complexity of the spectrum and short relaxation time (Fischer, personal communication) makes unambiguous assignment difficult. Using 4-chlorobenzoyl peroxide in hexachloro-acetone as solvent, however, the simpler spectrum of 4-chlorophenyl 4-chlorobenzoate is clearly seen as enhanced absorption, together with... 

In the decomposition of benzoyl peroxide, the fate of benzoyloxy radicals escaping from polarizing primary pairs remains something of a mystery. Benzoic acid is formed but shows no polarization in and C-spectra, and the carboxylic acid produced in other peroxide decompositions behaves similarly (Kaptein, 1971b Kaptein et al., 1972). Some light is shed on the problem by studies of the thermal decomposition of 4-chlorobenzoyl peroxide in hexachloroacetone containing iodine as... 

Few CIDNP studies on free radical reactions with olefins and related unsaturated molecules have been reported, and relatively little chemically useful information seems to have been derived, despite the potential relevance in polymerizing systems. Thus CIDNP has been reported in the decomposition of benzoyl peroxide in the presence of styrene and... 

The last-mentioned compound has been chosen in preference to benzoyl peroxide in many of the more recent kinetic investigations on account primarily of the freedom of its decomposition from side reactions such as radical-induced decomposition (see p. 113). It may also serve as a photoinitiator under the influence of near ultraviolet radiation, which... 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

The accepted kinetic scheme for free radical polymerization reactions (equations 1-M1) has been used as basis for the development of the mathematical equations for the estimation of both, the efficiencies and the rate constants. Induced decomposition reactions (equations 3 and 10) have been Included to generalize the model for initiators such as Benzoyl Peroxide for... 

The very intensive chain decomposition of benzoyl peroxide was found in alcoholic solutions [16,18,19]. This is the result of the very high reductive activity of ketyl radicals formed from alcohol. They cause the chain decomposition of peroxide by the following mechanism ... 

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