Bond dissociation enthalpies substitution

In conclusion, therefore, a judicious use of CV methodology may lead to absolute thermodynamic data that are accurate to ca. 15 kJ mol-1. Relative values (i.e., differences between bond dissociation enthalpies in similar compounds) can be more reliable, but the approximations described suggest that some caution be exercised when using the results to draw conclusions that rely on small differences between bond dissociation enthalpies. This is the case, for example, for ring substituent effects on the O-H bond dissociation enthalpies in substituted phenols [346,349],... 

F. G. Bordwell, X.-M. Zhang. Acidities and Homolytic Bond Dissociation Enthalpies of 4-Substituted-2,6-di-tert-Butylphenols. J. Phys. Org. Chem. 1995, 8, 529-535. 

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.

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). 

Recently, Ribeiro da Silva et al. have described another QSAR study. The authors studied the substitution influence on the N - O bond dissociation enthalpies of a series of QDO and its relationship to antitumor activity [163]. 

Enthalpy required to form a free radical. Bond-dissociation enthalpies AH° = 381 kJ (91 kcal) show that more highly substituted... 

Both radicals and carbocations are electron deficient because they lack an octet around the carbon atom. Like carbocations, radicals are stabilized by the electron-donating effect of alkyl groups, making more highly substituted radicals more stable. This effect is confirmed by the bond-dissociation enthalpies shown in Figure 4-7 Less energy is required to break a C—H bond to form a more highly substituted radical. 

Triarylstannyl radicals with bulky ortfto-substituted phenyl groups are in thermal equilibrium wilh the corresponding hexaaryldistannanes, and (he lowest temperatures at which the radical can be detected by ESR spectroscopy in solutions of the distannanes, and the bond dissociation enthalpies, are shown in Table 18-2 the -values of these radicals lie between 2.0057 and 2.0079.17... 

Lithoxoidou, A.T., and Bakalbassis, E.G., PCM study of the solvent and substituent effects on the conformers, intramolecular hydrogen bonds and bond dissociation enthalpies of 2-substituted phenols, J. Phys. Chem. A, 109, 366-377 (2005). 

Reactions. - The equilibrium acidities (pKha) of six p-substituted benzyltriphenylphosphonium salts, and also those of related allylphosphonium salts, have been determined, together with the homolytic bond dissociation enthalpies of the acidic C-H bonds. A study of the data available in the Cambridge Structural Database reveals that tetraphenylphosphonium cations in crystals associate through phenyl-phenyl non-bonded interactions which are attractive, concerted, and widespread in nature. An attractive force of 60-85 kJmoP has been calculated. ... 

Table 3. Relatives O-H bond dissociation enthalpies (kcal/mol) for para-substituted...

In principle, the pK of the toluene radical cation can be estimated from the one-electron reduction potential of the radical cation and the C-H bond dissociation enthalpy for toluene (368 kj mor )[62] using equation (5). The resulting pR is ca -10, i.e., considerably more acidic than the phenol radical cation. Nicholas and Arnold have estimated the pfC of the toluene radical cation to between -9 and -13 in acetonitrile which is weU in line with the estimate given here. [32] Since the C-H bond dissociation enthalpies of substituted toluenes seem to be almost invariant with substituent,[63-66] the substituent effect on the pK of... 

The constancy of silicon-hydrogen bond dissociation enthalpies in methyl substituted silanes is supported by a thermochemical analysis which does not rely on the enthalpies of formation of the corresponding silyl radicals. 

A.K. Chandra, P.-C. Nam, and M.T. Nguyen, The S-H bond dissociation enthalpies and acidities of para and meta substituted thiophenols A quantum chemical study, J. Phys. Chem. A 107 (2003), pp. 9182-9188. 

P) substitution reaction (q) activation energy (r) bond-dissociation enthalpy... 

Younker JM, Beste A, Buchanan III AC. Computational study of bond dissociation enthalpies for substituted 3-0-4 lignin model compounds. ChemPhysChem. 2011 12 3556-3565. 

Bond dissociation energies (BDEs) provide a measure of both the reactivity of a compound (with respect to homolytic bond rupture) and the stability of the corresponding radical. There have been many theoretical investigations of BDEs for a wide variety of species [36], In particular, the C-H BDE for a substituted methane is given by the enthalpy change for the reaction ... 

Alternatively, the BDE values may be reported relative to the C-H bond dissociation energy in methane (3) as the reference. This is quantitatively described in Equation 5.3 as a formal hydrogen transfer process between methane (3) and a substituted carbon-centered radical 2. The reaction enthalpy for this process is often interpreted as the stabilizing influence of substituents Rj, R2, and R3 on the radical center and thus referred to as the radical stabihzation energy (RSE). When defined as in Equation 5.3, positive values imply a stabilizing influence of the substituents on the radical center. The RSE energies are connected to the BDE values in Equations 5.land 5.2 as described in Equation 5.4. 

The RSE is calculated here as the difference between the homolytic C-C bond dissociation energy in ethane (5) and a symmetric hydrocarbon 6 resulting from dimerization of the substituted radical 2. By definition the C-C bonds cleaved in this process are unpolarized and, baring some strongly repulsive steric effects in symmetric dimer 6, the complications in the interpretation of substituent effects are thus avoided. Since two substituted radicals are formed in the process, the reaction enthalpy for the process shown in Equation 5.5 contains the substituent effect on radical stability twice. The actual RSE value is therefore only half of the reaction enthalpy for reaction 5.5 as expressed in Equation 5.6. 

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