Homolytic decomposition

Any one of these expressions gives the rate of initiation Rj for the particular catalytic system employed. We shall focus attention on the homolytic decomposition of a single initiator as the mode of initiation throughout most of this chapter, since this reaction typifies the most widely used free-radical initiators. Appropriate expressions for initiation which follows Eq. (6.6) are readily derived. 

Other Hydroperoxides. Several hydrotrioxides including alkyl hydrotrioxides, R—OOOH, have been reported (63,64). There is strong spectroscopic evidence that a-cumyl hydrotrioxide [82951-48-2] is produced in the low temperature ozonization of cumene. Homolytic decomposition of a-cumyl hydrotrioxide in cumene/acetone-hindered phenol resulted in cumyl alcohol as the only organic product (65). Based on the... 

Propagation. Propagation reactions (eqs. 5 and 6) can be repeated many times before termination by conversion of an alkyl or peroxy radical to a nonradical species (7). Homolytic decomposition of hydroperoxides produced by propagation reactions increases the rate of initiation by the production of radicals. 

Autoca.ta.Iysis. The oxidation rate at the start of aging is usually low and increases with time. Radicals, produced by the homolytic decomposition of hydroperoxides and peroxides (eqs. 2—4) accumulated during the propagation and termination steps, initiate new oxidative chain reactions, thereby increasing the oxidation rate. 

Thermally induced homolytic decomposition of peroxides and hydroperoxides to free radicals (eqs. 2—4) increases the rate of oxidation. Decomposition to nonradical species removes hydroperoxides as potential sources of oxidation initiators. Most peroxide decomposers are derived from divalent sulfur and trivalent phosphoms. 

Obviously, if the step in which T reacts is rate-controlling, then the overall reaction rate depends in some way upon the concentration and identity of T. This dependence may take the form of a direct proportionality to [T], but more complex forms have been given in the preceding paragraphs. If the reaction of an intermediate with a trap is faster than the step generating the intermediate, on the other hand, then the rate may not depend on either the concentration or the identity of the trap. If, for example, an organometal (RM) undergoes a slow, homolytic decomposition,... 

Many reactions that take place in a solvent do not occur in the gas phase. This statement is generally true for ionic reactions. Homolytic decompositions of neutral molecules can sometimes be observed in both. One reaction that can be studied in the gas phase and in a variety of solvents is the decomposition of N2O5,... 

A further limitation with regard to the suitability of peroxides concerns the efficiency of cross-linking. Higher efficiencies are observed for those peroxides that form one of the following radicals during homolytic decomposition [48] (Figure 14.12). 

Complementary to the work with aqueous acidic media is the study of the homolytic decompositions of Co(III) carboxylates in carboxylic acid media by Lande and Kochi . For example, Co(III) is reduced in pivalic acid media with first-order kinetics with E = 30.6 kcal.mole , AS = 8 eu and k ko = 1.28+0.10 (69 °C). The main oxidation products were found to be isobutylene and tert-butyl pivalate, which suggests that (CH3)3C- is an intermediate. Oxidative decarboxylation is the probable course in the analogous oxidations of n-butyric and isobutyric acids, in view of the production of propane and CO2 under normal... 

The bonds cleaved in the rate determining step of homolytic decomposition of 11-19 are indicated in the formula. 

Only a limited number of peroxides are suitable for crosslinking purposes. The most suitable are those which form radicals through homolytic decomposition. These radicals are so aggressive that they can split hydrogen atoms from the polymer chain, producing stable peroxide decomposition products and polymer radicals. These polymer radicals can then be recombined, producing stable C-C crosslink bonds. 

This mechanism of homolytic decomposition of formed tetroxide explains all known facts about secondary peroxyl radicals disproportionation ... 

Due to the unimolecular and bimolecular homolytic decomposition of hydroperoxide, such as... 

The homolytic decomposition of diacyl peroxides proceeds via splitting of the weakest O—O bond. The acyloxy radicals formed are very unstable and a cascade of cage reactions follows this decomposition [4,42-46] ... 

ET Denisov. Mechanisms of Homolytic Decomposition of Molecules in the Liquid Phase, Itogi Nauki i Tekhniki, Kinetika i Kataliz, vol 9. Moscow VINITI, 1981, pp 1-158 [in Russian]. 

The rate constants of homolytic decomposition of the two hydroperoxides in butylacetamide media are given in Table 9.8. 

Along with homolytic decomposition, hydroperoxides are decomposed in an acetamide solution by the chain mechanism under the action of formed free radicals [33,35]. 

Another probable reaction of homolytic decomposition of ester hydroperoxide is the intramolecular interaction of the hydroperoxide group with the carbonyl group of ester with the formation of labile hydroxyperoxide succeeded the splitting of the weak O—O bond (see decomposition of hydroperoxides in oxidized ketones in Chapter 8). 

The homolytic decomposition of hydroperoxides was proved to be catalyzed by Bronsted as well as Lewis acids (for example, BF3, A1C13, SbCls) [230]. Experimental data on acid catalysis of the homolytic decomposition of hydroperoxides are collected in Table 10.9. 

Acid Catalysis of the Homolytic Decomposition of Hydroperoxides (Experimental Data)... 

In addition to homolytic decomposition, hydroperoxyl groups of polymer are decomposed in reactions with alkyl radicals (see Chapter 4). Such induced decomposition of POOH groups... 

The formation of the H0S 02 radical was demonstrated by EPR spectroscopy [51]. The homolytic decomposition of hydroperoxide catalyzed by S02 underlies an oscillating pattern of the M-hcxylbenzcne oxidation inhibited by thiophene or BaS04 [48]. Radicals can also be... 

Reactivity toward nucleophiles and comparison with other electrophilic centers 152 Paths for nucleophilic substitution of sulfonyl derivatives 156 Direct substitution at sulfonyl sulfur stereochemistry 157 Direct substitution at sulfonyl sulfur stepwise or concerted 158 The elimination-addition path for substitution of alkanesulfonyl derivatives 166 Homolytic decomposition of a-disulfones 172 10 Concluding remarks 173 Acknowledgement 174 References 174... 

Second, partial decomposition of an aryl thiolsulfinate specifically labeled (35S) at the sulfinyl sulfur indicates (a) some incorporation of label in ArSSAr, which would not be expected if (75) were the only path for formation of disulfide (b) a significantly unequal distribution of label beween the two sulfurs of ArS02SAr [in its simple form (76) predicts both sulfurs should be equally labeled] (c) some incorporation of label into the sulfenyl sulfur of the recovered unreacted thiolsulfinate. The specific reactions responsible for these variations of the 35S-distribution from the pattern predicted by (74)-(76) cannot be pinpointed with certainty, although Koch et al. (1970) considered that a homolytic decomposition (80) of sulfenyl sulfinate [20], competitive with its... 

The generation of the benzoyloxyl radical relies on the thermal or photoinitiated decomposition [reaction (49)] of dibenzoyl peroxide (DBPO). An early study (Janzen et al., 1972) showed that the kinetics of the thermal reaction between DBPO and PBN in benzene to give PhCOO-PBN" could be followed by monitoring [PhCOO-PBN ] from 38°C and upwards. The reaction was first order in [DBPO] and zero order in [PBN], and the rate constants evaluated for the homolysis of the 0—0 bond in DBPO (k = 3.7 x 10-8 s-1 at 38°C) agreed well with those of other studies at higher temperatures. Thus in benzene the homolytic decomposition mechanism of DBPO seems to prevail. 

This acid undergoes homolytic decomposition to OH and N02 radical species ... 

---