Unimolecular peroxide decomposition

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

The thermal decompositions described above are unimolecular reactions that should exhibit first-order kinetics. Under many conditions, peroxides decompose at rates faster than expected for unimolecular thermal decomposition and with more complicated kinetics. This behavior is known as induced decomposition and occurs when part of the peroxide decomposition is the result of bimolecular reactions with radicals present in solution, as illustrated below specifically for diethyl peroxide. 

Ideally all reactions should result from unimolecular homolysis of the relatively weak 0-0 bond. However, unimolecular rearrangement and various forms of induced and non-radical decomposition complicate the kinetics of radical generation and reduce the initiator efficiency.46 Peroxide decomposition induced by radicals and redox chemistry is covered in Sections 3.3.2.1.4 and 3.3.2.1.5 respectively. 

Correlated or geminate radical pairs are produced in unimolecular decomposition processes (e.g. peroxide decomposition) or bimolecular reactions of reactive precursors (e.g., carbene abstraction reactions). Radical pairs formed by the random encounter of freely diffusing radicals are referred to as uncorrelated or encounter (P) pairs. Once formed, the radical pairs can either collapse, to give combination or disproportionation products, or diffuse apart into free radicals (doublet states). The free radicals escaping may then either form new radical pairs with other radicals or react with some diamagnetic scavenger... 

On the basis of mechanistic studies, mainly on these isolable cychc four-membered peroxides (1 and 2), two main efficient chemiexcitation mechanisms can be defined in organic peroxide decomposition (i) the unimolecular decomposition or rearrangement of high-energy compounds leading to the formation of excited-state products, exemplified here in the case of the thermal decomposition of 1,2-dioxetane (equation i)". 5,i9. 

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) . 

There is some difficulty with the energetics of unimolecular hydroperoxide decomposition. The endothermicity for the reaction ROOH RO + OH is of the order of 50 kcal., whereas the observed activation energy is as low as 30 kcal. The question is, therefore, bound to arise To what extent is decomposition trace metal--catalyzed It can be demonstrated that ferrous phthalocyanine, even at concentrations below lO M, is a most powerful activator of hydroperoxides—e.g., in the oxidation of quercetin, rhamnetin, or 8-carotene. The action of ferrous phthalocyanine is in principle similar to that of ferrous ion with hydrogen peroxide, already discussed. It may be described as reduction activation. 

Important evidence in favor of reaction (iv) as the key reaction in the catalytic peroxide decomposition is the fact that the rate of formation of the primary hydrogen peroxide complex is high enough to account for the entire catalytic activity, provided a mechanism for its rapid decomposition is available. The bimolecular formation velocity constant has a value of about 3 X 107 M.-1 see.-1 as compared with the value of the overall bimolecular constant for the catalytic reaction of 3 — 3.5 X 107 M.-1 sec.-1. Any simple mechanism with consecutive reactions including the unimolecular decomposition must be excluded, for the decomposition rate of the primary complex is extremely low, the first order constant being about 0.02 sec.-1 (Chance, 69). 

Inii/ioHon-. Any material which decomposes spontaneously or under external stimidus into free radicals may be used as an initiator for polymerization at the double bond A variety of peroxides satisfy this condition, such as, for instance, benzoyl peroxide and tertiary butylhydroper-oxide in systems where the initiator is dissolved in the monomer itself or in a monomer solution also hydrogen peroxide and potassium persulfate in emulsion polymerizations where the initiator is dissolved in an aqueous medium. Benzoyl peroxide decomposition occurs by Unimolecular reao tion ... 

CL measurements on peroxide decay in polyethylene [80M1] or polypropylene [91B1, 97B1] have related this process to the proportionality between hydroperoxide concentration and CL emission intensity. The first order analysis of CL emission (Fig. 69a), the decay rate of hydroperoxides (Fig. 69b) and the rate constants of peroxide decomposition (Fig. 69c) emphasize the unimolecular decomposition of isolated hydroperoxides. 

The thermal decompositions described above are unimolecular reactions that should exhibit first-order kinetics. Under many conditions, peroxides decompose at rates faster than expected for unimolecular thermal decomposition, and with more... 

The amount of induced decomposition that occurs depends on the concentration and reactivity of the radical intermediates and the susceptibility of the substrate to radical attack. The radical X- may be formed from the peroxide, but it can also be derived from subsequent reactions with the solvent. For this reason, both the structure of the peroxide and the nature of the reaction medium are important in determining the extent of induced decomposition, relative to unimolecular homolysis. 

The thermal decomposition of diacyl peroxides has been the most frequently employed process for the generation of alkyl radicals. The rate and products of the unimolecular decomposition of acetyl peroxide have been the subject of many studies. Acetyl peroxide decomposes at a convenient rate at 70-80°C both in the solution and in the gas... 

The kinetic form of the decomposition in various solvents indicates competing unimolecular homolysis of the peroxide link (a) and radical induced decomposition (b). Other diacyl peroxides behave similarly, except that, in the case of acetyl peroxide, induced dceomposition is much less important. More highly branched aliphatic or a-phenyl-substituted diacyl peroxides decompose more readily, partly because induced decomposition is more important again and partly because of the occurrence of decomposition involving cleavage of more than one bond (for a mechanistic discussion of these cases, see Walling et al., 1970). 

The inhibited unimolecular decomposition of symmetrically di-substituted benzoyl peroxides into radicals also obeys the Hammett rho-sigma relationship. Unfortunately, no extensive activation parameter data are available. The effect of the substituent changes on the rates at the single temperature has been explained in terms of dipole-dipole repulsion in the peroxide.122... 

Fig. 2. Rate coefficients for the low-pressure region of the unimolecular decomposition of hydrogen peroxide. A, Ref. 10 , ref. 15 O, refs. 16,17 (total density 10"2 mole.I-1) , refs. 16, 17 (total density 10 1 mole.l 1).

The catalytic decomposition of hydrogen peroxide at the surface of platinum foil investigated by Bredig and Tetelow (Zeit. Elehtro-chem. XII. 581, 1906) was found to obey a unimolecular law and the seat of the reaction was shown to be the thin saturated layer at the surface of the metal. 

The unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones (a-peroxylac-tones) is the simplest and most exhaustively studied example of a thermal reaction that leads to the formation, in this case in a single elementary step, of the electronically excited state of one of the product molecules. The mechanism of this transformation was studied intensively in the 1970s and early 1980s and several hundreds of 1,2-dioxetane derivatives and some 1,2-dioxetanones were synthesized and their activation parameters and CL quantum yields determined. Thermal decomposition of these cyclic peroxides leads mainly to the formation of triplet-excited carbonyl products in up to 30% yields. However, formation of singlet excited products occurs in significantly lower yields (below... 

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