Radical-sensitized decomposition

It should also be mentioned that Allen and Pitts (1) studied the CH3-radical-sensitized decomposition of trans-crotonaldehvde to show that the 2-butene formation observed in the earlier photolysis studies is due to methyl-radical displacement of the formyl group. 

Both CII2O and CH3CHO undergo very fast radical-sensitized decompositions such as... 

The minor products undoubtedly indicate the occurrence of some free radical sensitized decomposition. 

There is no evidence in any of the gas phase systems for initial multiple bond rupture (i.e., fragmentation reactions). Because of the low reaction temperatures, the alkoxy radical intermediates of the bond fission reactions (or radicals resulting from alkoxy radicals) are mainly involved in radical-radical termination processes ( 0) rather than participating in hydrogen abstraction from the parent peroxide E oi 6-8). Thus it has been commonly believed that the peroxide decompositions were classic examples of free radical non-chain processes. Identification of the rate coefficients and the overall decomposition Arrhenius parameters with the initial peroxide bond fission kinetics were therefore made. However, recent studies indicate that free radical sensitized decompositions of some peroxides do occur, and that the low Arrhenius parameters obtained in many of the early studies (rates measured by simple manometric techniques) were undoubtedly a result of competitive chain processes. The possible importance of free radical reactions in peroxide decompositions is illustrated below with specific regard to the dimethyl peroxide decomposition. 

A complete quantitative mechanism cannot be constructed with confidence on the basis of the products since not all of them were determined. A variety of secondary processes is likely. Hydrogen abstraction is expected to involve mainly the N-bound H atom. The activation energy for abstraction by methyl is given as 4.8 kcal/mol (277). Reaction 85 has been found (277) to yield ethylene and nitrogen in the radical-sensitized decomposition of ethyleneimine. Reaction 70 (see above) could explain the production of hydrocyanic acid. 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

Alkyl peroxyesters are much less sensitive to radical-induced decompositions than diacyl peroxides. Induced decomposition is only significant in peroxyesters containingnonhindered a-hydrogens or a, P-unsaturation (213,242). 

While di-i-butyl (34) and dicumyl hyponitrites (35) have proved convenient sources of Tbutoxy and cumyloxy radicals respectively in the laboratory,71 72 115"117 the utilization of hyponitrites as initiators of polymerization has been limited by difficulties in synthesis and commercial availability. Dialkyl hyponitrites (16) show only weak absorption at A>290 ntn and their photochemistry is largely a neglected area. The triplet sensitized decomposition of these materials has been investigated by Mendenhall et a .11 s... 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

Clearly, this discovery has an important bearing on the kinetics of the decompositions of chlorine oxides and of the halogen-sensitized decompositions of oxides, e.g. 03 and N20. Johnston et alf21 have recently shown that an additional, pressure-dependent reaction occurs between two CIO radicals, viz... 

It has been shown that the benzophenone sensitized decomposition of benzoyl peroxide is due in part to formation of the benzophenone ketyl radical, which induces decomposition.98,99 Hydrocarbon sensitized peroxide decomposition is discussed in Section IV.A.4. The formation of benzonitrile from the benzophenone sensitized irradiation of benzalazine, which was originally attributed to hydrogen abstraction by benzophenone,100 actually results from a photooxidation.101... 

The discovery that azo compounds undergo singlet sensitized decomposition is particularly relevant to the problem of spin correlation effects in free radical reactions. Any radical pair precursor that gives a difference in products depending upon whether it is produced as a singlet or triplet excited state is said to show a spin correlation effect. 

Whatever the exact nature of these sensitized decompositions, they can be useful in generation of radicals by cleavage of certain peroxides, such as 4-r-butylcyclohexanepercarboxylate esters, which rearrange by nonradical paths during thermal decomposition.489 They can also be a nuisance, as evidenced by the quenching of triplet triphenylene observed in plastics when peroxides were used to polymerize the samples.470... 

There has been some speculation as to whether the mercury-sensitized decomposition of cyclopropane involves energy transfer or radical formation in the primary process.508... 

Heicklen195 examined the room-temperature, mercury-sensitized decomposition of C2F4 and found that the presence of a few torr of NO does not affect the c-C3F6 yield. It can be concluded that the singlet CF2 radicals produced do not react readily with NO. 

Further evidence for this step was provided by Knight and Gunning.259 In their study of the Hg-sensitized decomposition of C2H5OH in the presence of NO, they found that no CH3CHO was formed, yet 0(C2HBONO) was only 0.25. To explain the deficiency of products from C2H50 radicals, they suggested reaction (24). 

An entirely different consequence of electron (or hole) transfer to addends is provided by semiconductor-sensitized decomposition of electron acceptors A-X (or electron donors, i.e., hole acceptors, D-Y) [17]. If organic molecules whose redox states (anion radical or cation radical) are unstable with respect to scission into a free radical and an ion [Eqs. (5,6)] are adsorbed onto a wide band gap semiconductor such as Ti02, etc., then back electron (hole) transfer can be inhibited, if the scission process is rapid. 

Another synthetic approach based on pyridium salt photochemistry involves the use of alkoxy radicals which are formed in both direct and sensitized decomposition of pyridinium ions in free radical polymerization [78]. Obviously, polytetrahydrofuran (PTHF), terminated by JV-alkoxy pyridinium ions, can act as macrophotoinitiator for the polymerization of monomers such as methyl methacrylate (MMA) that readily polymerize by a free-radical mechanism. PTHF macrophotoinitiators were prepared by termination of living polymerization of THF by the corresponding IV-oxides, The well-defined macrophotoinitiators with exact functionalities, confirmed by H-NMR, UV-visible and g.p.e. analysis, were obtained. Upon irradiation of macroinitiators at suitable wavelengths, polymeric alkoxy radicals are produced. The overall process is shown for the pyridinium macrophotoinitiator in the following Scheme 21. 

Chemical Sensitization. Equally valuable for the demonstration of the existence of free radicals and for their study are methods of chemical sensitization. Free radicals may be demonstrated to exist in a reaction by their ability to produce a sensitized decomposition of a material normally inert at the temperature employed. Thus, it has been demonstrated that, whereas acetaldehyde does not decompose at an appreciable rate at 300 0, a fast decomposition can be induced at that temperature by adding azomethane (CH3)2N2 in small amounts. The role of the azomethane is to produce methyl radicals which can then start a chain decomposition. Oxygen is similarly a chemical sensitizer for the decomposition of many hydrocarbons and aldehydes. 

The extreme sensitivity of CH3CHO to chain decomposition makes it very susceptible to free radical sensitization. Thus OH3 radicals from the pyrolysis of azomethane can induce the chain decomposition in CIIsCHO at 300°C, the chain length being as great as 500. The photolysis of azomethane at room temperature can also sensitize the decomposition. Letort showed that CH3 radicals from dtBP will decompose as many as 50 molecules of CH3CHO per molecule of dtBP at 160 0. 

Fig. 6-6. The reaction scheme of the singlet-sensitized decomposition of dibenzolyperoxide (DBP) in toluene. In RPb, the g-value of the benzoyloxy radical (2.0123) is much larger than that of the phenyl radical (2.0024). (Reproduced from Ref. [34] by permission from The Chinese Chemical Society)...

Biacetyl has often been used for sensitizing the pyrolysis of organic compounds" . Radicals, resulting from the thermal decomposition of biacetyl, initiate the chain decomposition of such substances. However, NO causes no inhibition of notable significance in such systems . Nevertheless, this cannot be considered as an evidence against the occurrence of chains, since the sensitized decompositions are definitely inhibited by, for instance, propene . 

Considerable amounts of hydrogen and ketene were observed by Lossing among the products of the mercury-sensitized photolysis at 55 °C. He concluded that these were formed as the result of the mercury-sensitized decomposition of the acetyl radicals rather than in primary processes analogous to IV and V. 

Static system. Rates determined in the presence of nitric oxide, propene and toluene. Uninhibited rates were 10 % faster. Origin of the free radical sensitization was attributed to the formaldehyde product decomposition. 

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