Radical benzoyloxy

4 and Table 3.8). They also display a marked propensity for aromatic substitution. On the other hand, compared with alkoxy radicals, they show little tendency to abstract hydrogen.  

The rate constant for P-scission is dependent on ring substituents. Rate constants for radicals X-Ct,H4C02 are reported to increase in the series where X is p-F -CH20 /7-CH5 /5-Cl H m-Cl. There is qualitative evidence that the relative rates for p-scission and addition are insensitive to solvent changes. For benzoyloxy radicals, similar relative reactivities are obtained from direct competition experiments as from studies on individual monomers when p-scission is used as a clock reaction.  

The rate constants for benzoyloxy and phenyl radicals adding to monomer arc high ( 10 M s for S at 60 C - Table 3.7), In these circumstances primary radical termination should have little importance under normal polymerization conditions. Some kinetic studies indicating substantial primary radical termination during S polymerization may need to be re-evaluated in this light. Secondary benzoate end groups in PS with BPO initiator may arise by head addition or transfer to initiator (Section 8.2.1). 

The chemistry of atkoxycarbonyloxy radicals in many ways parallels that of the aroyloxy radicals (e.g. benzoyloxy, see 3.4.2.2.1). Products attributable to the reactions of alkoxy radicals generally arc not observed. This indicates that the rate of p-scission is slow relative to the rate of addition to monomers or other 

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These results show that in the phenylation of thiazole with benzoyl peroxide two secondary reactions enter in competition the attack of thiazole by benzoyloxy radicals, leading to a mixture of thiazolyl benzoates, and the formation of dithiazolyle through attack of thiazole by the thiazolyl radicals resulting from hydrogen abstraction on the substrate and from the dimerization of these radicals. This last reaction is less important than in the case of thiophene but more important than in the case of pyridine (398). 

Benzoyl peroxide has been the most common source of phenyl radicals. But in reaction with thiazoles the benzoyloxy radical abstracts a hydrogen atom from the thiazole nucleus or from a methyl group in the case of methylthiazoles, giving by-products such as dithiazolyls or 2.2 -dithiazolylethane (183). The results obtained with benzoyl peroxide are summarized in Tables III-23, III-24. and III-25. 

Aromatic diacyl peroxides such as dibenzoyl peroxide (BPO) [94-36-0] may be used with promoters to lower the usehil decomposition temperatures of the peroxides, although usually with some sacrifice to radical generation efficiency. The most widely used promoter is dimethylaniline (DMA). The BPO—DMA combination is used for hardening (curing) of unsaturated polyester resin compositions, eg, body putty in auto repair kits. Here, the aromatic amine promoter attacks the BPO to initially form W-benzoyloxydimethylanilinium benzoate (ion pair) which subsequentiy decomposes at room temperature to form a benzoate ion, a dimethylaniline radical cation, and a benzoyloxy radical that, in turn, initiates the curing reaction (33) ... 

This last result bears also on the mode of conversion of the adduct to the final substitution product. As written in Eq. (10), a hydrogen atom is eliminated from the adduct, but it is more likely that it is abstracted from the adduct by a second radical. In dilute solutions of the radical-producing species, this second radical may be the adduct itself, as in Eq. (12) but when more concentrated solutions of dibenzoyl peroxide are employed, the hydrogen atom is removed by a benzoyloxy radical, for in the arylation of deuterated aromatic compounds the deuterium lost from the aromatic nucleus appears as deuterated benzoic acid, Eq. (13).The over-all reaction for the phenylation of benzene by dibenzoyl peroxide may therefore be written as in Eq, (14). 

I Initiation The polymerization reaction is initiated when a few radicals are generated on heating a small amount of benzoyl peroxide catalyst to break the weak 0-0 bond. A benzoyloxy radical then adds to the C=C bond of ethylene to generate a carbon radical. One electron from the C=C bond pairs up with the odd electron on the benzoyloxy radical to form a C-O bond, and the other election remains on carbon. 

For reactions with S, specificity is found to decrease in the series cyanoisopropyl mcthyl Fbutoxy>phcnyl>bcnzoyloxy. Cyanoisopropyl (Scheme 3.3),7 f-bntoxy and methyl radicals give exclusively tail addition. Phenyl radicals afford tail addition and ca l% aromatic substitution. Benzoyloxy radicals give tail addition, head addition, and aromatic substitution (Scheme 3.4). ... 

Benzoyloxy radicals decompose to phenyl radicals and carbon dioxide (Scheme 3.7). 

Only a few diacvl peroxides see widespread use as initiators of polymerization. The reactions of the diaroyl peroxides (36, R=aryl) will be discussed in terms of the chemistry of BPO (Scheme 3.25). The rate of p-scission of thermally generated benzoyloxy radicals is slow relative to cage escape, consequently, both benzoyloxy and phenyl radicals are important as initiating species. In solution, the only significant cage process is reformation of BPO (ca 4% at 80 °C in isooctane) II"l only minute amounts of phenyl benzoate or biphenyl are formed within the cage. Therefore, in the presence of a reactive substrate (e.g. monomer), tire production of radicals can be almost quantitative (see 3.3.2.1.3). 

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

It has been suggested that the amine radical cation (46) is not directly involved in initiating chains and that most polymerization is initiated by benzoyloxy radicals.179 However, Sato et a ." employed spin trapping (3.5.2.1) to demonstrate that the anilinomethyl radical (45) was formed from the radical cation (46) by loss of a proton and proposed that the radical 45 also initiates polymerization. Overall efficiencies for initiation by amine-peroxide redox... 

The rate of fl-scission of benzoyloxy radicals is such that in most polymerizations initiated by these radicals both phenyl and benzoyloxy end groups will be formed (Scheme 3.4). A reliable value for the rate constant for p-xcission would enable the absolute rates of initiation by benzoyloxy radical to be estimated. Various values for the rale constant for p-scission have appeared. Many of the early estimates are low. The activation parameters (in CCI4 solvent) determined by Chateauneuf et a(.m are log]0 A = 12.6 and Ea = -35.97 kJ mol 1 which corresponds to a rate constant of 9xl06 s 1 at 60 °C. 

Grant et a/.397 examined the reactions of hydroxy radicals with a range of vinyl and a-methylvinyl monomers in organic media. Hydroxy radicals on reaction with AMS give significant yields of products from head addition, abstraction and aromatic substitution (Table 3.8) even though resonance and steric factors combine to favor "normal tail addition. However, it is notable that the extents of abstraction (with AMS and MMA) arc less than obtained with t-butoxy radicals and the amounts of head addition (with MMA and S) are no greater than those seen with benzoyloxy radicals under similar conditions. It is clear that there is no direct correlation between reaclion rale and low specificity. 

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... 

The initiator-derived radical products generate a-tocopheroxyl radicals (2) from a-tocopherol (1). The radicals 2 are further oxidized to ort/io-quinone methide 3 in a formal H-atom abstraction, thereby converting benzoyloxy radicals to benzoic acid and phenyl radicals to benzene. The generated o-QM 3 adds benzoic acid in a [ 1,4] -addition process, whereas it cannot add benzene in such a fashion. This pathway accounts for the observed occurrence of benzoate 11 and simultaneous absence of a 5 a-phenyl derivative and readily explains the observed products without having to involve the hypothetical C-centered radical 10. 

Diacyl peroxides exhibit behavior somewhat like that of the dialkyl peroxides. Benzoyl, peroxide decomposes into radicals partly by a unimolecular and partly by a chain mechanism. In at least the case of benzoyl peroxide itself, if not for similar compounds, the first step is the production of benzoyloxy radicals rather than phenyl radicals and... 

For example, when benzoyl peroxide is allowed to decompose in the presence of an olefin and iodine a high yield of the olefin dibenzoate is formed And very little carbon dioxide. Since Hammond has shown that the rate of the decomposition is independent of the iodine concentration, the iodine must not participate in the rate-determining initial step. It probably reacts with the benzoyloxy radicals to form benzoyl hypoiodite.U8>11 ... 

Benzoylacetanilide couplers, 79 253 Benzoyl chloride, 3 595, 634 end use of chlorine, 6 134t physical constants of ring-chlorinated derivatives, 6 333t Benzoylcyclohexane-diones, 73 294 Benzoyloxy radicals, 74 281 Benzoyl peroxide (BPO), 3 634 20 105 hazards associated with, 78 491 uses for, 78 496... 

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