Reaction acylperoxyl radicals with

Chain propagation in an oxidized aldehyde is limited by the reaction of the acylperoxyl radical with the aldehyde. The dissociation energy of the O—H bond of the formed peracid is sufficiently higher than that of the alkyl hydroperoxide. For example, in hydroperoxide PhMeCHOOH, Z)0 H = 365.5 kJ mol-1 and in benzoic peracid... 

The chain unit in the thermal and photochemical oxidation of aldehydes by molecular dioxygen consists of two consecutive reactions addition of dioxygen to the acyl radical and abstraction reaction of the acylperoxyl radical with aldehyde. Experiments confirmed that the primary product of the oxidation of aldehyde is the corresponding peroxyacid. Thus, in the oxidation of n-heptaldehyde [10,16,17], acetaldehyde [4,18], benzaldehyde [13,14,18], p-tolualdehyde [19], and other aldehydes, up to 90-95% of the corresponding peroxyacid were detected in the initial stages. In the oxidation of acetaldehyde in acetic acid [20], chain propagation includes not only the reactions of RC (0) with 02 and RC(0)00 with RC(0)H, but also the exchange of radicals with solvent molecules (R = CH3). 

There are two channels of the reaction of acylperoxyl radicals with olefins hydrogen atom abstraction and addition to the double bond with epoxide formation [5,35] ... 

Another factor that influences the reactivity of two polar reactants, acylperoxyl radical with aldehyde, is the polar interaction of carbonyl group with reaction center in the transition state. Aldehydes are polar compounds, their dipole moments are higher than 2.5 Debye (see Section 8.1.1). The dipole moment of the acylperoxyl radical is about 4 Debye (/jl = 3.87 Debye for PhC(0)00 according to the quantum-chemical calculation [54]). Due to this, one can expect a strong polar effect in the reaction of peroxyl radicals with aldehydes. The IPM helps to evaluate the increment Ain the activation energy Ee of the chosen reaction using experimental data [1], The results of Acalculation are presented in Table 8.10. 

The ho value also depends on the ionization potential of the double bond the higher the ionization potential, the lower It is most likely that in the transition state a considerable transfer of the electron density occurs from the double bond to the oxygen atom of the attacking peroxyl radical. For the same reason, acylperoxyl radicals with a higher electron affinity add to the double bound by 2— 3 orders of magnitude more rapidly that alkylperoxyl radicals do. The steric factor also affects addition the bulky substituent in the a-position to the double bond impedes the addition of RO 2. In the reactions of RO 2 with polyfimctional unsaturated esters, the effect of multidipole interaction is manifested polyesters (calculated per fragment) react more slowly than monoesters (see Chapter 6). 

El-Agamey, A. and McGarvey, D.J. 2003. Evidence for a lack of reactivity of carotenoid radicals towards oxygen A laser flash photolysis study of the reactions of carotenoids with acylperoxyl radicals in polar and non-polar solvents. J. Am. Chem. Soc. 125 3330-3340. 

Carbonyl group of the aldehyde decreases the BDE of the adjacent C—H bond. This is due to the stabilization of the formed acyl radical, resulting from the interaction of the formed free valence with Tr-electrons of the carbonyl group. For example, DC—H = 422kJmol 1 in ethane and D( n 373.8 kJ mol 1 in acetaldehyde. The values of Dc H in aldehydes of different structures are presented in Table 8.1. In addition, the values of the enthalpies of acylperoxyl radical reactions with aldehydes were calculated (D0 H= 387.1 kJ mol-1 in RC(0)00 H). 

The Values of the C—H Bond Dissociation Energies in Aldehydes DC—h and Enthalpies AH of the Reaction of Acylperoxyl Radical (RC(O)OO ) with Aldehydes [2]... 

The important role of reaction enthalpy in the free radical abstraction reactions is well known and was discussed in Chapters 6 and 7. The BDE of the O—H bonds of alkyl hydroperoxides depends slightly on the structure of the alkyl radical D0 H = 365.5 kJ mol 1 for all primary and secondary hydroperoxides and P0—h = 358.6 kJ mol 1 for tertiary hydroperoxides (see Chapter 2). Therefore, the enthalpy of the reaction RjOO + RjH depends on the BDE of the attacked C—H bond of the hydrocarbon. But a different situation is encountered during oxidation and co-oxidation of aldehydes. As proved earlier, the BDE of peracids formed from acylperoxyl radicals is much higher than the BDE of the O—H bond of alkyl hydroperoxides and depends on the structure of the acyl substituent. Therefore, the BDEs of both the attacked C—H and O—H of the formed peracid are important factors that influence the chain propagation reaction. This is demonstrated in Table 8.9 where the calculated values of the enthalpy of the reaction RjCV + RjH for different RjHs including aldehydes and different peroxyl radicals are presented. One can see that the value A//( R02 + RH) is much lower in the reactions of the same compound with acylperoxyl radicals. 

The cross-coupling with acylperoxyl radicals was shown to lead to high-valent metal species and reactive organic intermediates (144). The Craq002+/CMe3C(0)00 reaction appears to be the sole example of such chemistry reported so far. Extension to other metals and types of radicals is essential before one can even begin to understand whether such reactions take place in catalytic oxidation systems and/or in aerobic organisms, and whether or how to exploit or suppress them. 

The acyltrialkylstannanes are readily hydrolysed to the parent aldehydes, and are oxidised in the air to give the corresponding carboxylates, R3SnOCOR 63 in a radical chain reaction which presumably is similar to that of the autoxidation of an aldehyde, and involves as a key step the Sh2 reaction of an acylperoxyl radical at tin rather that at hydrogen (equation 6-26). The trialkylstannyl peroxyester which is formed then reacts with the parent stannyl ketone to give the trialkyltin carboxylate. 

A procedure widely applied for effecting O-atom insertions (olefin epoxidation, hydrocarbon hydroxylation or ketonization) with cobalt(II) catalyts is the addition of sacrificial 2-methylpropanal. It is converted via H-atom abstraction to an acyl radical, which upon reaction with O2 produces an acylperoxyl radical. H-atom abstraction by the latter leads to a hydroperoxide, which is capable of effecting O-atom insertions via free-radical chain reactions. The sacrificial aldehyde is lost via oxidation to an acid or an ester. 

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