Peroxides concerted decomposition

The rates of thermal decomposition of diacyl peroxides (36) are dependent on the substituents R. The rates of decomposition increase in the series where R is aryl-primary alkyKsecondary alkyKtertiary alkyl. This order has been variously proposed to reflect the stability of the radical (R ) formed on (i-scission of the acyloxy radical, the nucleophilicity of R, or the steric bulk of R. For peroxides with non-concerted decomposition mechanisms, it seems unlikely that the stability of R should by itself be an important factor. 

Goldstein and co-workers have pointed out that this oxygen exchange can take place at least partly through nonradical rearrangements 9.25 or 9.26,82 and isotope effects in acetyl peroxide decomposition are consistent with significant concerted decomposition.63... 

In the Criegee proposals the initial product is stipulated to have a 1,2,3-trioxolane structure (I) which is also known as a molozonide structure. This undergoes a concerted decomposition to give a zwit-terion (II) and a carbonyl compound (III). The decomposition products (II) and (III) in most cases recombine to give the normal ozonide (IV). Other possible reactions include dimerization of the zwitterion to yield a diperoxide (V) or a higher peroxide (VI) whilst in the presence of methanol as a reactive solvent a methoxy-hydroperoxide (VII) may be produced. 

Many organic molecules are stable in the liquid state and decompose to radicals only in the gas phase. Their decomposition occurs often as chain reactions (see Chapter 10). A restricted scope of compounds decompose with a noticeable rate in the liquid phase. They are compounds with rather weak 0—0, C— N, and S—S bonds. The decomposition of such compounds as peroxides and azo compounds, which are widely used in technology and research practice as initiators of chain liquid-phase reactions, was studied in detail. Using these compounds, the following three mechanisms of molecule decomposition were found and studied decomposition with the cleavage of one bond, concerted decomposition with the cleavage of several bonds, and decomposition with chimerical interaction. 

If this peroxide decomposes according to the mechanism of concerted decomposition, we would deal with the following mechanism of the formation of alcohol and acetone ... 

The cleavage of one bond in the molecule is always accompanied by an increase in its sizes. Therefore, always higher than zero, and an increase in pressure retards decomposition. For the decomposition of peroxide compounds with one bond cleavage A = 10 3 cm /mol. Concerted decomposition occurs with the formation of a more compact transition state. Therefore, its AP is lower than that for the decomposition of one bond, in some cases AP < 0. The pressure accelerates or weakly affects concerted decomposition. 

The transition state at the concerted decom sition of peresters and diacyl peroxides has the polar structure of the R. ..C02...0R type. Therefore, the concerted decomposition of peroxide compounds depends on the solvent polarity the higher the polarity, the faster the decomposition. 

Generally speaking, the substance can decompose in parallel via two and more routes through different transition states. For example, benzoyl peroxide decomposes mainly with the cleavage of one O—O bond. However, as shown by the method of chemical nuclei polarization, its concerted decomposition also occurs in part... 

The solvent has a strong eifect on concerted decomposition, which was proved for the decomposition of isobutyiyl peroxide, whose decomposition rate constant varies from 310 in isooctane to 5810 s in nitrobenzene (313 K). A linear dependence between logk and ( -l)/(2e+l) is fulfilled for polar solvents, and nonpolar solvents are characterized by a linear dependence of logk and solvent polarizability. The solvent effect on the decomposition rate is related to the polar structure of the transition state... 

X = Cl) was based independently on the dioxetanone (61) and concerted peroxide decomposition (6,8,62) theories. Possible examples of dioxetanones in bioluminescence are discussed later. 

Special review articles published since 1968 on these topics are one by E. H. White and D. F. Roswell 2> on hydrazide chemiluminescence M. M. Rauhut 3) on the chemiluminescence of concerted peroxide-decomposition reactions and D. M. Hercules 4 5> on chemiluminescence from electron-transfer reactions. The rapid development in these special fields justifies a further attempt to depict the current status. Results of bioluminescence research will not be included in this article except for a few special cases, e.g. enzyme-catalyzed chemiluminescence of luminol, and firefly bioluminescence 6>. 

Hydrogen peroxide was identified as the product of secondary peroxyl radical disproportionation [187-192], It cannot be explained by the concerted mechanism of tetroxide decomposition. 

Such decay is known as concerted fragmentation. Peroxides have the weak O—O bond and usually decompose with dissociation of this bond. The rate constants of such decomposition of ROOR into RO radicals demonstrate a low sensitivity of the BDE of the O—O bond to the structure of the R fragment [4], Bartlett and Hiat [8] studied the decay of many peresters and found that the rate constants of their decomposition covered a range over 105 s-1. The following mechanism of decomposition was proposed in parallel with a simple dissociation of one O—O bond [3,4] ... 

Two extreme mechanisms have been proposed for the unimolecular dioxetane decomposition the concerted mechanism , whereby cleavage of the peroxide and the ring C—C bond occurs simultaneously, and the biradical mechanism whereby the initial cleavage of the 0—0 bond leads to the formation of a 1,4-dioxy biradical whose subsequent C—C bond cleavage leads to the formation of the two carbonyl fragments (Scheme 8). Although the biradical mechanism adequately explains the activation parameters obtained for most of the dioxetanes smdied, it appears not to be the appropriate mechanistic model for the rationalization of singlet and triplet quanmm yields. Therefore, an intermediate mechanism has been proposed, whereby the C—C and 0—0 bonds cleave in a concerted, but not simultaneous, manner (Scheme 8) . 

The experimentally observed substituent effect on the triplet and singlet quantum yields in the complete series of methyl-substituted dioxetanes, as well as the predicted C—C and 0—0 bond strength for the four-membered peroxidic rings , have led to the hypothesis that a more concerted, almost synchronized, decomposition mechanism should lead to high excitation quantum yields (as in the case of tetramethyl-l,2-dioxetane), whereas the biradical pathway presumably leads to low quantum yields (as in the case of the unsubstituted 1,2-dioxetane)" . However, it appears that this criterion of concertedness is difficult to apply generally to structurally dissimilar dioxetane derivatives. 

Unimolecular peroxide decomposition chemiluminescence, 1227-31 asynchronous concerted mechanism, 1230 biradical mechanism, 1181-2, 1227-31 concerted mechanism, 1227, 1228-9, 1230... 

CIEEL mechanism was originally proposed by Schuster for intense luminescence (or bioluminescence) in the decomposition of endoperoxides and dioxetanes. The relevance and importance of ET in the latter has recently been discussed. While the mechanism of reduction of peroxides in these early studies was described essentially as a concerted dissociative process, little insight into the fine detail of the mechanism could be provided. 

Decomposition of oxetanes is still another chemiluminescent reaction. On the basis of Woodward-Hoffman rule of conservation of orbital symmetry, the concerted bond cleavage of dioxetane, a 4-membered ring peroxide, should yield one carboxyl moiety in the excited state... 

The oxidation of cyclohexene using hydrogen peroxide was chosen as a test reaction for the catalytic evaluation of the titanium modified hexagonal NaY sanq)les. Scheme I illustrates some of the typical products of cyclohexene oxidation. The epoxide and the diol which is a hydrolysis product of the epoxide, generally reflect a concerted process. In contrast the allylic alcohol and ketone are often ascribed to an autoxidation or radical process. We anticipated that some homolytic decomposition of the peroxide may be observed with these acidic zeolites. In fact, there was -74% conversion of H2O2 over calcined hexagonal NaY after heating at 55 C for 24 hours. This resulted in only a 1% conversion of... 

Rauhut, M. M., Chemiluminescence from Concerted Peroxide Decomposition Reactions, Acc. Chem. Res. 1969, 2, 80 87. 

The polar nature of the catalytic decomposition of cumene hydroperoxide with added terf-butyl terf-butanethiolsulfinate has been clearly established (I). The thiolsulfinate is converted into an active peroxide decomposer capable of destroying many moles of hydroperoxide per mole of sulfur compound. The acidic character of the active species was demonstrated by its effective neutralization with the added base calcium carbonate. Formation of the active peroxide decomposer may be envisaged as involving one or more of the following three reaction types concerted process, ionic processes, and free-radical processes. 

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