Bond dissociation enthalpies table

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

A great number of studies related to thermochemical properties of QDO and PDO derivatives have been recently described by Ribeiro da Silva et al. [98-103]. These studies, which have involved experimental and theoretical determinations, have reported standard molar enthalpies of formation in the gaseous state, enthalpies of combustion of the crystalline solids, enthalpies of sublimation, and molar (N - O) bond dissociation enthalpies. Table 5 shows the most relevant determined parameters. These researchers have employed, with excellent results, calculations based in density functional theory in order to estimate gas-phase enthalpies of formation and first and second N - O dissociation enthalpies [103]. 

By studying the heats of reaction for many different reactions, chemists have developed reliable tables of bond-dissociation enthalpies. Table 4-2 gives the bond-dissociation enthalpies for the homolysis of bonds in a variety of molecules. 

Use bond-dissociation enthalpies (Table 4-2, p. 143) to calculate values of AH° for the following reactions. 

We can understand why bromination is more selective than chlorination by using bond dissociation enthalpies (Table 4.3) to calculate the energy changes for the propagation step in which each halogen atom abstracts a hydrogen from ethane. 

TABLE 2. Bond dissociation enthalpies in organogermanium compounds (kJmol-1)... 

The only experimental values of bond dissociation enthalpies that have been reported in the literature are given in McMillen and Golden s review32d D(Me3Pb—Me) = 238 17 kJmol-1 and D(Et3Pb—Et) = 230 17 kJmol-1. These values and the data in Table 6 imply Ai °(PbMe3, g) = 227 18 kJmol-1 and AF °(PbEt3, g) = 221 18 kJmol-1. 

Table 6. Mean bond dissociation enthalpy D (M-L) kJ mol 1 for homoleptic metal compounds ML ...

Table 9. Valence m.o. electron configuration Ofg e"g e%g, enthalpy of sublimation, AHSU, and standard enthalpy of formation, (AHf, g), of metallocenes. Mean bond dissociation enthalpy, D (M-Cp) (Refs.68 69 ) and (Metal-cyclopentadienyl ring) bond length, r(M-Cp). (Ref.72 ).

Note that the values in table 5.2 are not absolute solution phase bond dissociation enthalpies. Because the purpose of the exercise is to probe the substituent... 

Table 5.2 ArO-H bond dissociation enthalpies ofpora-monosubstituted phenols in benzene (I = 298.15 K) relative to DH° n(PhO-H) [73], Data in kJ mol-1.

Though some more traditional thermodynamicists will be dismayed by the concept of solution phase bond dissociation enthalpy, the fact is that the database involving these quantities is growing fast. When used judiciously, they may provide important chemical insights—as is indeed the case for the stability of the O-H bond in phenolic compounds. Although solution phase bond dissociation enthalpies are not true bond dissociation enthalpies, because they include some contribution from intermolecular forces, a series of solution values like those in table 5.2 may be (and often is) taken as a good approximation of the trend in the gas-phase. 

Table I includes the relative bond dissociation enthalpies obtained for some group 14 hydrides by photoacoustic calorimetry,7 10 The data demonstrate that, for the trialkyl-substituted series, the bond strengths decrease by 6.5 and 16.5 kcal/mol on going from silane to germane and to stannane, respectively. The silicon-hydrogen bonds can be dramatically weakened by successive substitution of the Me3Si group at the Si-H functionality. A substantial decrease in the bond strength is also observed by replacing alkyl with methylthio groups.

Table 2.2. Homolytic bond dissociation enthalpies, AHohom. and heats of formation, AHf(x.), of some selected bonds and radicals [kJ mol ] ...

Table 2.3. Heterolytic bond dissociation enthalpies, AHnhet, molecules and ions [kJ mol" ] and heats of formation of some ...

The relative bond enthalpies from the photoacoustic calorimetry studies can be placed on an absolute scale by assuming that the value for D//(Et3Si—H) is similar to D/f(Me3Si—H). In Table 2.2 we have converted the D/frei values to absolute T>H values (third column). On the basis of thermodynamic data, an approximate value of D//(Me3SiSiMc2—H) = 378 kJ/mol can be calculated that it is identical to that in Table 2.2 [1]. A recent advancement of photoacoustic calorimetry provides the solvent correction factor for a particular solvent and allows the revision of bond dissociation enthalpies and conversion to an absolute scale, by taking into consideration reaction volume effects and heat of solvation [8]. In the last colunm of Table 2.2 these values are reported and it is gratifying to see the similarities of the two sets of data. 

Table 2.2 Relative and absolute bond dissociation enthalpies (kJ/mol) ...

In Table 2.4, we have collected background information for discussion in the following chapters. Recommended C—H bond dissociation enthalpies of selected organic compounds are reported in the first two columns, followed by a variety of heteroatom-hydrogen bond strengths including N—H, O—H, S—H, Ge—H, and Sn—H bonds. 

Table 2.5 Electron affinities of silyl radicals together with gas-phase acidities and bond dissociation enthalpies of silanes"...

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