Unsymmetrical decomposition

Another complication is that for many initiators, decomposition leads to two radicals of different structures and reactivities [1,2], The difference in reactivities between the radicals produced in an unsymmetrical decomposition is addressed in the following equations ... 

This means that in the case of unsymmetrical decomposition of initiator into two radicals with differing reactivity, the expression for the overall rate of initiator is actually a composite of two different reactions (Eq. 1.12). However, for the sake of simplicity, the two different initiation rate coefficients will be combined into an average rate constant to give the overall rate of initiation, Rr. 

Of these diacyl peroxides the ones that generate the most stable radicals (R ) are the most unstable diacyl peroxides. Most other diacyl peroxides decompose by competing free-radical and polar decomposition, ie, carboxy iaversion (188). Carboxy iaversion occurs to a much greater extent with certain diacyl peroxides having unsymmetrical diacyl peroxide stmctures (52,187,188,199) ... 

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

Both symmetrical and unsymmetrical azo compounds can be made, so that a single radical or two different ones may be generated. The energy for the decomposition can be either thermal or photochemical. In the thermal decomposition, it has been established that the temperature at which decomposition occurs depends on the nature of the substituent groups. Azomethane does not decompose to methyl radicals and nitrogen until temperatures above 400°C are reached. Azo compounds that generate relatively stable radicals decompose at much lower temperatures. Azo compounds derived from allyl groups decompose somewhat above 100°C for example ... 

Arylcopper intermediates can be generated from organolithium compounds, as in the preparation of cuprates.95 These compounds react with a second aryl halide to provide unsymmetrical biaryls in a reaction that is essentially a variant of the cuprate alkylation process discussed on p. 680. An alternative procedure involves generation of a mixed diarylcyanocuprate by sequential addition of two different aryllithium reagents to CuCN, which then undergo decomposition to biaryls on exposure to oxygen.96 The second addition must be carried out at very low temperature to prevent equilibration with the symmetrical diarylcyanocuprates. 

Similar results were obtained when o-phenylenediamine was reacted with mb. A range of low boiling compounds were separated from the reaction mixture after 10-12 hr heating at 220°C. These have been identified as diazadimethylsilaindane Vb (m.p. 76-78°C lit. (321 m.p. 75-83°C) and bis(phenylamino)dimethylsilane VIb (m.p. 46-48°C lit. (331 m.p, 45 2°C). They are probably formed by thermal decomposition of the intermediate unsymmetrical disilazane IVb". At 190-225°C/0.3 torr, a crystalline fraction with... 

It is considered that the decomposition of symmetrical azo-compounds proceeds via the concerted mechanism, and some unsymmetrical azo-compounds are decomposed by the concerted and nonconcerted mechanisms simultaneously. Phenyl-substituted azo-compounds are decomposed by the nonconcerted mechanism [3]. 

This decomposition is somewhat analogous to the polar decomposition of suitably unsymmetrical diacyl peroxides. However, it has not been possible to force the complementary mode of decomposition of a hydroxamic acid in which the nitrogen moiety departs with the electrons and leaves the oxygen electron-deficient.816... 

Diacetoxyscirpenol (DAS) immunoassay, 14 144-147 Diacetyl, 23 483 Diacetyl control, 10 293 Diacetyl peroxide, 1 148 Diacetyl rest, in beer making, 3 584 Diacrylamide, 1 293 Diacylglycerols, 10 802 Diacyl hydrazines, 13 599 Diacyl peroxides, 14 281 18 467 decomposition of, 18 473 as free-radical initiators, 14 282-284 hydrolysis and perhydrolysis of, 18 466, 473 preparation of, 18 476 properties of, 18 468-469t reaction with amines, 18 474 reduction of, 18 474 symmetrical or unsymmetrical, 18 477... 

Dialkyl (C12-C18) dimethylammonium chloride, 24 147 hair cleaner ingredient, 7 850t Dialkyldimethylammonium quaternary higher alcohols, 2 21 Dialkylgold(III) halides, 12 708 Dialkylhydrazines, unsymmetrical, 13 571 Dialkyl peroxides, 14 281 18 434, 436-448 a-oxygen-substituted, 18 448-460 chemical properties of, 18 439-444 decompositions of, 18 441 physical properties of, 18 436-439 primary and secondary, 18 439,... 

Decomposition of this equilibrium mixture with catalytic amounts of CuOTf affords a mixture of all three possible biaryls. The formation of the unsymmetrical biaryl 2-Me2NCH2C6H4C6H4Me-4 can only be explained by the occurrence of aggregated copper species in which both the C6H4CH2NMe2-2 and the C( H4Me-4 groups are bound to the same copper core [77]. It was furthermore observed that the ratio of the formed biatyls is not statistical, which points to significant differences in the thermodynamic stabilities of the various mixed aggregates present in solution. 

The thermal decomposition of azoalkanes bearing geminal a-cyano and a-trimethylsiloxy groups has been the subject of a report. The symmetrical compound (107) decomposes near room temperature to afford entirely C—C dimers, whereas the unsymmetrical azoalkane (108) requires heating to 75 °C. A NMR product study of photolysed (107) in the presence of TEMPO showed that the fate of caged t-butyl-l-trimethylsiloxy-l-cyanoethyl radical pairs is disproportionation (17%), cage recombination (20%), and cage escape (63%). 

Zeolites have been employed in the preferential synthesis of optically active sites and in determining the particular products formed from certain reactions. In looking at the products formed from the decomposition and reformations involving an unsymmetrical ketone, the major products are a combination of products listed below ... 

However, subsequent studies provided unequivocal evidence for consecutive three-centre rearrangements, the first step of which is a normal HERON reaction. Independent studies of decomposition of unsymmetrical hydrazines (227a and 227b) produced esters... 

McEwan and co-worker" examined the acid-catalyzed decomposition of unsymmetrical benzhydryl azides 18 and some related species (Fig. 13.11). The aryl migration step showed very little discrimination between aryl rings with electron-donating and those with electron-withdrawing substituents. This low selectivity was judged to be more consistent with formation of a discrete nitrenium ion intermediate (19). These workers reasoned that a concerted migration would exhibit higher selectivity toward donor-substituted arenes, because in that mechanism the electrons from the arene would participate in the reaction. 

Thermal decomposition of oxetanes proceeds at elevated temperatures, usually in the range of 300-450 °C, to give an alkene and a carbonyl compound in practically quantitative yield. Gas phase thermolyses of several alkyl-substituted oxetanes have been studied in detail, because such compounds occur as intermediates in the oxidation of alkanes (Section 5.13.3.3). When the oxetane is unsymmetrically substituted, as in the case of 2,4-dimethyl-oxetane (20), two modes of cleavage, to give two different sets of products, are possible and are generally observed. In fact, when only hydrogen and alkyl substituents are present,... 

No cross ozonide was formed from unsymmetrical alkenes. The authors theorized628 that the carbonyl oxide zwitterionic species formed on wet silica gel immediately adds water followed by rapid decomposition of the intermediate hydroxyalkyl hydroperoxide to carboxylic acid and water. It means that water on silica gel acts as participating solvent. In the absence of adsorbed water, rapid recombination of the adsorbed aldehyde and carbonyl oxide due to a favorable proximity effect gives normal ozonide. The low mobility of adsorbed species on the silica surface accounts for the absence of cross ozonides. 

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