Benzoyl radical fragmentation

Further evidence for the formation of alkene radical cations derives from the work of Giese, Rist, and coworkers who observed a chemically induced dynamic nuclear polarization (CIDNP) effect on the dihydrofuran 6 arising from fragmentation of radical 5 and electron transfer from the benzoyl radical within the solvent cage (Scheme 6) [67]. 

The chromophore in this type of photoinitiator is frequently an aromatic carbonyl. The benzoyl radical is the major initiating species, while the other fragment may also contribute to the initiation, in some cases. The most efficient type I initiators are benzoin ether derivatives, benzil ketals, hydroxyl-alkylphenones, a-aminoketones, and acylphosphine oxides. Substituents on the aromatic carbonyl influence the absorption. 

Among the most widely used photofragmenting initiators are alkoxyaceto-phenones and hydroxy-alkylacetophenones (Figure 1). The primary reaction of these initiators is a Norrish Type I cleavage leading to the formation of a benzoyl radical and a fragment radial moiety both of which may initiate polymerization. Sander and Osborn (15) have shown that 2,2-dimethyoxy-2-... 

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... 

Cotter and Knight have reported the ionic and thermal fragmentation of 2,5-diphenyl-l,3,4-oxadiazole 24. The major fragmentation pathways for the molecule ion 8) and neutral 27) are shown in Eq. (13) and (14) respectively. The benzoyl cation mje 105) is certain to be relatively more stable than its neutral analog the reverse is true of the benzonitrile molecule. It seems likely that products which result from loss of H2, including benzoyl radicals will be obtained from 24 on photolysis. 

Benzil has frequently been used as a means of generating free radicals in polymerization systems subjected to ultra-violet irradiation 11, 16, 56—58). In studies of the benzil-photoinitiated polymerizations of methyl methacrylate, and vinyl acetate, Melville (16) assumed that initiation was brought about by fragmentation of photoexcited benzil into two benzoyl radicals. However a survey of the photochemistry of benzil 34) indicates that such a cleavage does not in fact take place in solution studies of the products formed on irradiation of benzil in cyclohexane (59), cumene and isopropanol (60) can be rationalised on the basis of initial hydrogen abstraction from solvent by photoexcit i benzil, e.g. 

Acyl radicals can fragment by loss of carbon monoxide. Decarbonylation is slower than decarboxylation, but the rate also depends on the stability of the radical that is formed. For example, rates for decarbonylations giving tertiary benzylic radicals are on the order of 10 s whereas the benzoyl radical decarbonylates to phenyl radical with a rate on the order of 1 s (see also Table 11.3, Entries 45 to 48). When reaction of isobutyraldehyde with carbon tetrachloride is initiated by f-butyl peroxide, both isopropyl chloride and isobutyroyl chloride are formed, indicating that decarbonylation is competitive with the chlorine atom transfer. 

Fragmentation processes of photoinitiators form a number of volatile products, which may contribute to indoor air pollution. Benzaldehyde and alkyl-substituted benzalde-hydes are usual components, because Norrish-I is the most important reaction for cleavage. Awell known example is l-phenyl-2-hydroxy-2-methyl-propane-l-one (PHMP). a-Cleavage generates two radicals in the first step. The benzoyl radical may recombine to form benzil, reduction of PHMP leads to l-phenyl-2-methy 1-1,2-propane and acetone, and recombination of the 2-hydroxypropyl radical gives 2,3-dimethyl-2,3-butanediol... 

In this case, the benzoyl radical and a methyl radical produced through the fragmentation of a,a-dimethoxy benzyl radical both initiate pol5unerization. Other families include benzoin ethers, acetophenone derivatives, amino ketones, and phosphine oxide derivatives (1 ) (3,11). 

The monomers used in chain polymerisations are unsaturated, sometimes referred to as vinyl monomers. In order to carry out such polymerisations a small trace of an initiator material is required. These substances readily fragment into free radicals either when heated or when irradiated with electromagnetic radiation from around or just beyond the blue end of the spectrum. The two most commonly used free radical initiators for these reactions are benzoyl peroxide and azobisisobutyronitrile (usually abbreviated to AIBN). They react as indicated in Reactions 2.1 and 2.2. 

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. 

The oxygen transfer to the -acetylenic carbon results in the very intense benzoyl cation, whereas the transfer to the a-carbon, via a series of fragments corresponding to the loss of OH , CO and CO2, respectively, leads to annelated heterocycles such as the radical cation of carbazole, as a result of elimination of CO81. The generation of the benzoyl cation was rationalized as shown in Scheme 1281. 

The thermolysis of benzoyl peroxide in thiophene gives a complex mixture, with the phenylation products accounting for only 3%. Considerable amounts of 2,2 -bithienyl and products containing the benzoyloxy group are formed. The suggested mechanism is shown in Scheme 59, according to which the benzoyloxy radicals are efficiently trapped before fragmentation to C02 and phenyl radicals. 

Mass spectra of hydroxy- and alkoxy-coumarins have been very intensively studied. The decomposition sequence of 3-hydroxycoumarin is initiated by carbon monoxide loss from the molecular ion giving a 2-hydroxybenzofuran ion. Subsequent fragmentation occurs by two major pathways, involving a further loss of CO and expulsion of a formyl radical. The former leads to the base peak, and thence by another loss of CO to give the abundant benzene radical cation at m/e 78. The other main pathway gives a benzoyl cation which leads to the phenonium ion at m/e 77 (77IJC(B)816). 

The reaction is called an addition reaction because two monomers are added to each other with the elimination of a double bond. This is also called a chain growth polymerization reaction. However, the reaction as such does not go without the help of an unstable molecule, called an initiator, that starts the reaction. Benzoyl peroxide or i-butyl benzoyl peroxide are such initiators. Benzoyl peroxide splits into two halves under the influence of heat or ultraviolet light and thus produces two free radicals. A free radical is a molecular fragment that has one unpaired electron. Thus, when the central bond was broken in the benzoyl peroxide, each of the shared pair of electrons went with one half of the molecule, each containing an unpaired electron. 

---