Radical abstraction reactions

The parameter AH for a free-radical abstraction reaction can be regarded simply as the difference in D values for the bond being broken and the one formed. 

Geometry of the Transition State of Radical Abstraction Reaction... 

The IPM as a semiempirical model of an elementary bimolecular reaction appeared to be very useful and efficient in the analysis and calculation of the activation energies for a wide variety of radical abstraction and addition reactions [108-113]. As a result, it became possible to classify diverse radical abstraction reactions and to differentiate in each class the groups of isotypical reactions. Later this conception was applied to the calculations of activation energies and rate constants of bimolecular reactions of chain generation [114]. In the IPM, the radical abstraction reaction, for example,... 

A clear-cut dependence of the activation energy on the heat (enthalpy) of the reaction, which is equal, in turn, to the difference between the dissociation energies of the ruptured (Z> ) and the formed (D j bonds, was established for a great variety of radical abstraction reactions [1,2,16]. In parabolic model, the values of Dei and Def, incorporating the zero-point energy of the bond vibrations, are examined. The enthalpy of reaction AHe, therefore, also includes the difference between these energies (see Equation [6.7]). 

As noted above, all radical abstraction reactions can be divided into groups and the activation energy Ee0 for a thermally neutral reaction can be calculated for each group (see Equation [6.11]). This opens up the possibility of calculating of the enthalpy contribution (A h) to the activation energy for the given (z th) reaction and a thermally neutral reaction characterized by the quantity fse0 [4,11] ... 

Another important characteristic of radical abstraction reactions is the force constants of the ruptured and the generated bonds. The dependence of the activation energy for the reactions of the type R + R X > RX + R1, where X = H, Cl, Br, or I, on the coefficients Ai and Af was demonstrated experimentally [17]. It was found that parameter re = const in these reactions, while the square root of the activation energy for a thermally neutral reaction is directly proportional to the force constant of the ruptured bond. The smaller the force constant of the C—X bond, the lower the Ee0, and the relationship Feo12 to A(1 I a) 1 is linear (see Figure 6.4). The same result was also obtained for the reactions of hydrogen atoms with RC1, RBr, and RI [17]. 

The role of triplet repulsion in radical abstraction can clearly be traced on comparing reactions in which the energies of the X—Y bonds differ significantly. The values of Ee0 and re for a series of radical abstraction reactions found by the parabolic method as well as the energies De of the X—Y bond are presented below. 

This formula makes possible the estimation of the contribution of each factor, i.e., triplet repulsion AET, electronegativity, A1sEa, and repulsion of the electron shells of the X and Y atoms A1sr in the TS, to the activation barrier Ec() for each class of radical abstraction reactions. Since Ee01,2 = bre (1 la) 1, it follows that, by employing the corresponding increments from Equation (6.31), it is possible to calculate the contribution of a particular factor. Table 6.8 presents the results of such calculation for 17 classes of radical abstraction reactions. 

AEba = —45 kJ mol 1 for the HO + SiH4 reaction and AEba = —43 kJ mol-1 in the reaction of hydrogen atom with water. The repulsion of the electron orbitals of the atoms forming the reaction center AER plays an important role in all the radical abstraction reactions. In the interaction of radicals with molecules the contribution of this repulsion ranges from 25 to 46 kJ mol-1. In reactions of molecules with hydrogen atoms the contribution is naturally smaller, varying from 8 to 16kJ mol-1. 

An aromatic ring and a double or triple bond in the a-position relative to the C—H bond weaken this bond by virtue of the delocalization of the unpaired electron in its interaction with the iT-bond. The weakening of the C—H bond is very considerable for example, D(C—H) is 422 kJ mol-1 in ethane [27], 368 kJ mol-1 in the methyl group of propene [27] (AD = 54 kJ mol-1), and 375 kJ mol-1 in the methyl group of toluene [27] (AD = 47 kJ mol-1). Such decrease in the strength of the C—H bond diminishes the enthalpy of the radical abstraction reaction and, hence, its activation energy. This effect is illustrated below for the reactions of the ethylperoxyl radical with hydrocarbons ... 

Contributions of the Solvation Effect AEso to the Activation Energies for Radical Abstraction Reactions [30]... 

GEOMETRY OF THE TRANSITION STATE OF RADICAL ABSTRACTION REACTION... 

Geometric Parameters of Transition State Calculated for Radical Abstraction Reactions sec-R02 + RH by Equations (6.11), (6.35)-(6.37) [35]... 

Radical abstraction reactions that involve molecules with u-bonds in the vicinity of the reaction center are characterized by higher Ee0 values than the corresponding reactions... 

As described earlier, the reaction enthalpy is a very important factor that influences the reactivity of alcohol in free radical abstraction reactions. The IPM model helps to estimate the increment of activation energy AEn which characterizes the influence of enthalpy on the activation energy (see Equation [6.20] in Chapter 6). The parameters bre and values ACH for reactions of different peroxyl radicals with alcohols are given in Table 7.8. The mean value... 

The reaction enthalpy is known as a very important factor that determines the reactivity of reactants in free radical abstraction reactions [71]. The IPM method helps to calculate the increment of AEfi that enthalpy determines in the activation energy of the individual reaction. This increment can be estimated within the scope of IPM through the comparison of activation energy Ee of the chosen reaction and activation energy of the thermoneutral reaction Ee0 (see Equation [6.18] in Chapter 6). This increment was calculated for several reactions of different peroxyl radicals with ethers (Table 7.19). 

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 comparison of ArO reactions with RH and ROOH illustrates a great role of the triplet repulsion in free radical abstraction reactions. The IPM method helps to clarify this important factor (see Ref. [33] and Chapter 6). The parameters of reactions of AriO and sterically hindered phenoxyls Ar20 with hydrocarbons (R1 , R2H, and R3H) and hydroperoxides ROOH are collected in Table 15.13. 

Kinetic Parameters of Radical Abstraction Reactions by Phenoxyl Radicals in IPM [4]... 

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