Decomposition, heterolytic

The homolysis of tertiary hypochlorites for the production of oxy radicals is well known." The ease with which secondary hypohalites decompose to ketones has hampered the application of hypohalites for transannular reactions. However the tendency for the base-catalyzed heterolytic decomposition decreases as one passes from hypochlorites to hypobromites tohypoidites. Therefore the suitability of hypohalites for functionalization at the angular positions in steroids should increase in the same order. Since hypoidites (or iodine) do not react readily with ketones or carbon-carbon double bonds under neutral conditions hypoiodite reactions are more generally applicable than hypochlorite or hypobromite decompositions. 

Diacyl peroxides may also undergo non-radical decomposition via the carboxy inversion process to form an acylcarbonate (Scheme 3.27).46 The reaction is of greatest importance for diaroyl peroxides with electron withdrawing substituents and for aliphatic diacyl peroxides (36) where R is secondary, tertiary or ben/,yl.157 The reaction is thought to involve ionic intermediates and is favored in polar solvents 57 and by Lewis acids.158 Other heterolytic pathways for peroxide decomposition have been described.150... 

A number of mechanisms for thermal decomposition of persulfate in neutral aqueous solution have been proposed.232 They include unimolccular decomposition (Scheme 3.40) and various bimolecular pathways for the disappearance of persulfate involving a water molecule and concomitant formation of hydroxy radicals (Scheme 3.41). The formation of polymers with negligible hydroxy end groups is evidence that the unimolecular process dominates in neutral solution. Heterolytic pathways for persulfate decomposition can he important in acidic media. 

In ATRP, the initiator (RX) determines the number of growing chains. Ideally, the degree of polymerization is given by eq. 7 and the molecular weight by cq. 8. Note the appearance of the initiator efficiency (/ ) in the numerator of these expressions. In practice, the molecular weight is ofien higher than anticipated because the initiator efficiency is decreased by side reactions. In some cases, these take the form of heterolytic decomposition or elimination reactions. Further redox chemistry of the initially formed radicals is also known. The initiator efficiencies are dependent on the particular catalyst employed. 

Competition between Homolytic and Heterolytic Catalytic Decompositions of Hydroperoxides... 

In the oxidized hydrocarbon, hydroperoxides break down via three routes. First, they undergo homolytic reactions with the hydrocarbon and the products of its oxidation to form free radicals. When the oxidation of RH is chain-like, these reactions do not decrease [ROOH]. Second, the hydroperoxides interact with the radicals R , RO , and R02. In this case, ROOH is consumed by a chain mechanism. Third, hydroperoxides can heterolytically react with the products of hydrocarbon oxidation. Let us consider two of the most typical kinetic schemes of the hydroperoxide behavior in the oxidized hydrocarbon. The description of 17 different schemes of chain oxidation with different mechanisms of chain termination and intermediate product decomposition can be found in a monograph by Emanuel et al. [3]. 

The rate constant is an effective characteristic of the heterolytic decomposition of hydroperoxide. Its value can depend on the concentration of acid, for example, and increase during oxidation. 

Acids are the final products of all hydrocarbon oxidations. They catalyze the heterolytic decomposition of hydroperoxides. The sharp decrease in the hydroperoxide concentration in oxidizing the hydrocarbon is observed as soon as acids are formed in the oxidized hydrocarbon. Consequently, the rate of initiation and the rate of... 

So, these reactions cannot lead to effective chain termination in oxidized alcohol. The decomposition of tetroxides depends on pH and apparently proceeds homolytically as well as heterolytically in an aqueous solution. The values of the rate constants (s 1) of tetroxide decomposition at room temperature in water at different pH values are given below [38,39],... 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

Acids are well known as efficient catalysts of various heterolytic reactions (hydrolysis, esterification, enolyzation, etc. [225,226]). They catalyze the heterolytic decay of hydroperoxides formed during oxidation. For example, they catalyze the decomposition of cumyl hydroperoxide into phenol and acetone (important technological reaction) [5]. 

All schemes presented are similar and conventional to a great extent. It is characteristic that the epoxidation catalysis also results in the heterolytic decomposition of hydroperoxides (see Section 10.1.4) during which heterolysis of the O—O bond also occurs. Thus, there are no serious doubts that it occurs in the internal coordination sphere of the metal catalyst. However, its specific mechanism and the structure of the unstable catalyst complexes that formed are unclear. The activation energy of epoxidation is lower than that of the catalytic decomposition of hydroperoxides therefore, the yield of oxide per consumed hydroperoxide decreases with the increase in temperature. 

The reverse emulsion stabilized by sodium dodecylsulfate (SDS, R0S03 Na+) retards the autoxidation of dodecane [24] and ethylbenzene [21,26,27]. The basis for this influence lies in the catalytic decomposition of hydroperoxides via the heterolytic mechanism. The decay of hydroperoxides under the action of SDS reverse micelles produces olefins with a yield of 24% (T=413 K, 0.02mol L 1 SDS, dodecane, [ROOH]0 = 0.08 mol L 1) [27], The thermal decay gives olefins in negligible amounts. The decay of hydroperoxides apparently occurs in the ionic layer of a micelle. Probably, it proceeds via the reaction of nucleophilic substitution in the polar layer of a micelle. 

Subsequent studies suggested [138,139] that the decomposition of ONOOCOO may proceed by both homolytic and heterolytic ways ... 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

Polymer anion radicals are usually less reactive than the cation radicals, and are often stable at 77K, but they are usually unstable at room temperature. Excited state species can undergo decomposition by a variety of routes including (i) homolytic cleavage to form two neutral radicals, (ii) heterolytic cleavage to form an anion and a cation, or (iii) bond rupture with the formation of two neutral molecules. 

Attempts at 4-exo nucleophilic cyclization failed, presumably because of a heterolytic fragmentation of the intermediate radical cation (Scheme 32) [139], not unlike that proposed (Scheme 15) for the decomposition of a mannose-derived alkene radical cation. 

This view has been challenged with more recent evidence indicating that AT-[(acyloxy)methyl] derivatives of both primary and secondary amides (8.170, Fig. 8.21) undergo decomposition by the same mechanisms, namely a) an acid-catalyzed process involving protonation followed by formation of an /V-acyliminium species (Fig. 8.21, Reaction a) b) a pH-independent heterolytic cleavage forming the same /V-acyliminium species (Fig. 8.21, Reaction b) and c) a base-catalyzed pathway, which for /V-[(acyloxy)methyl] derivatives of AT-methylamides is the normal mechanism (Fig. 8.21, Reaction c), but for AT-[(acyloxy)methyl] derivatives of primary amides involves substrate deprotonation followed by /V-acy limine formation (Fig. 8.21, Reaction d) [218],... 

Heterolytic decomposition of the acid is also possible Pryor and Squadrito (1995) connect this direction with the generation of a high-energy intermediate [ONOOH] N02 + OH —> [ONOOH] NO + OOH. Interactions between such generated species eventually lead to the formation of NO and NO3 ions ... 

Similar results were obtained for tert-butyl hydroperoxide and perchloric acid in 2-propanol. Thus, it is evident from the decomposition of hydrogen peroxide into free radicals that both heterolytic and homolytic reactions may be catalyzed by hydrogen ions. Further research is needed to investigate proton catalysis in certain homolytic reactions. 

The different oxidative pathways are possible because tbe 0-0 bond of peroxynitrite can react as if it was either cleaved homolytically into HO + N02 or heterolytically into HO and N02. The third possibility of separating peroxynitrite into HO+ and NO2 is too energetically costly to be considered, requiring 105 20 kcal/mol. Koppenol et al. (1992) have calculated the energetic cost of separating peroxynitrite at pH 7.0 homolytically to be 21 3 kcal/mol, which compares favorably with the experimental activation energy for spontaneous decomposition of 18 2 kcal/mol. The complete separation of peroxy-... 

Studies of the heterolysis reaction for a series of Cr a-hydroxyalkyl complexes established that the presence or absence of a cis-aqua ligand has a negligible effect on the specific rate of the heterolytic decomposition (44,107). Thus it was suggested that the electrophilic attack of a solvent water molecule at the carbon center of the chromium carbon -bond is the rate determining step in the heterolysis process (44,107). 

Measurements of the volumes of activation (108) and solvent H/D isotope effect (50) of some of the heterolytic decomposition reactions suggest that they proceed via a transition state of the type (108) ... 

Furthermore, the heterolysis reaction is catalyzed by the addition of anions (organic and inorganic oxy anions, e.g., acetate) (44,107, 110,111). Comparing the acetate effect in the presence of different chelating ligands ([15]aneN4 and nta) to those of the aquated system led to the conclusion that the oxy anions have to occupy the trans position to the R group in order to labilize the M-R bond (trans labilization effect) and thus catalyze the heterolytic decomposition (44,107,110,111). 

In the presence of efficient scavengers for M"Lm and/or R the mechanism of decomposition of complexes with metal-carbon -bonds, which decompose heterolytically under routine conditions, can be shifted towards the homolytic decomposition (49,50,124). 

Kinetic Evidence The homolytic decomposition follows a rate law, which is second order in the LlMre + 1-CR1R2R3 intermediate and shows an inverse dependence on the MreL m concentration, whereas a heterolytic decomposition mechanism obeys a first order dependence on the LlMn+1-CR1R2R3 compound and is independent of the MKL m concentration. 

Product Analysis The products of a homolytic decomposition are expected to contain dimers of the organic radical or their disproportionation products, whereas in the heterolytic decomposition process the only expected organic products in aqueous solutions are RH (reduced organic radical) or ROH (oxidized organic radical). 

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