Dissociation energies, heterolytic

Table 1.4. Heterolytic Bond Dissociation Energies for Some C-H and C-Cl Bonds S...

Heats of reaction and bond dissociation energies allow the estimation of the feasibility of homolytic processes, as these are largely — but not solely — governed by thermochemical effects. The quantitative treatment of heterolytic processes, however, presents a far more difficult problem. Basic electrostatic considerations indicate that the dissociation of a covalent bond into positive and negative ions is inherently a highly endothermic process. It will be facilitated by any mechanism that allows dissipation or stabilization of the incipient charges. Chemists have come to differentiate these... 

Bromonium ion stabilities in the gas phase have also been measured in ion cyclotron resonance experiments by Beauchamp s group (Staley et al, 1977). The heterolytic bond dissociation energies shown in Table 3 are taken as a... 

With respect to the parent ethylenebromonium ion and expressed as heterolytic bond dissociation energies D (R +—Br"). Data from Staley et al. ( 1977). Calculations (Galland et al., 1990) indicate that this ion is not bridged but open. 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

The next question which presents itself is whether we can explain why in some systems solvent co-catalysis occurs, whereas in others, apparently similar, it does not. Let it be said first that in fact there is very little experimental evidence on this point. From the thermochemical point of view one can say that alkyl halide co-catalysis is the more probable, the lower the heterolytic bond dissociation energy of the alkyl halide, the more stable the cation derived from the monomer, and the smaller the anion derived from the metal halide. It must, however, be remembered that the non-occurrence of alkyl halide co-catalysis may be due to a kinetic prohibition, i.e., an excessively high activation energy for a reaction which is thermodynamically possible. 

Table 1 Some heterolytic bond dissociation energies (all figures in kcal/mole) ...

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

Michael additions to benzotriazole-stabilized carbanions have been reviewed. review of the structural dependence of heterolytic bond dissociation energy of carbon-carbon a-bonds in hydrocarbons has summarized the synthesis and behaviour of molecules in which highly stable cationic and anionic hydrocarbon moieties have apparently been combined. 

The second type of system in which heterolytic cleavage is favored arises with a metal-ligand complex having an atomic metal ion (with a s°dI1+1 configuration) and a two electron donor, L 2. A prototype is (Ag C6H6)+ which was observed to photodissociate to form X + Y = Ag(2S, s M10) + C6H6+(2B j) rather than the lower energy (heterolytically cleaved) dissociation limit Y + X2 =... 

Fio. 1. Schematic potential energy diagrams for the homolytic (I) and heterolytic (II) splitting of hydrogen by Ag+. All processes and energy terms are in solution. D = dissociation energy I = ionization potential. [Webster, A. H., and Halpern, J., J. Phys. Chem. 61, 1239 (1967).]... 

The most widely studied reference acid is the proton. Proton affinity, PA(B), is defined for a base B as the heterolytic bond dissociation energy for removing a proton from the conjugated acid BH+ (equation 20). The homolytic bond dissociation energy D(B+—H) defined by equation 21 is related to PA(B) and the adiabatic ionization potentials IP(H) and IP(B) (equation 22) are derived from the thermochemical cycle shown in Scheme 6. 

Gas-phase methods also constitute a source of important information on basic physical properties of silylenium ions. In particular, the thermochemical behavior is well characterized (30,33,34,47,61). Thermochemical data are applied for the evaluation of relative thermodynamic reactivities of silylenium ions in some systems. For example, affinities of R3Si+ and R3C+ toward various bases may be compared as the heterolytic dissociation energies of corresponding bonds [Eq. (12)] (47,61). It was shown that... 

Bond dissociation energies, described in Chapter 5, measure the energy required to homolytically break a bond. They are not the same as dissociation enthalpies, which measure the ability of a compound to dissociate heterolytically. Bond dissociation energies can be used to calculate dissociation enthalpies in the gas phase if other quantities are also known. 

An alternative view of the interaction of an alkali metal cation with a fluoride-containing anion is one of Lewis acid/base competition. The reactions discussed in the preceding section involved the reaction of an alkali fluoride salt with a Lewis acid with subsequent fluoride ion transfer to the Lewis acid. However, the alkali metal cation is a Lewis acid as well, and the degree of perturbation of the anion by the cation may be dependent on the differences in fluoride ion affinity of the Lewis acid and the alkali metal cation. The fluoride ion affinities for a variety of Lewis acids are well known from ICR (53,54,64) studies, while the fluoride ion affinities for alkali metal cations are the heterolytic bond dissociation energies of the gas phase alkali fluoride molecules... 

In view of the coordination pattern in protein-bound methylcobalamin (see below), the thermodynamic studies on the effect of the coordination of the dimethylbenzimidazole base to the a-side of the cobalt center on the homolytic and heterolytic (Co/3-C)-bond dissociation energy in (4) (thermodynamic effect of the trans hgand or trans influence ) were extended to corresponding investigations with CojS-methyl-imidazolylcobamides, such as (9), where imidazole replaces the dimethylbenzimidazole these studies showed this change of the nature of the axial base to have httle effect on the two relevant bond-dissociation energies of the corresponding methylcobamide. ... 

SO far as ionization reactions are concerned, bond strength should be characterized by heterolytic dissociation energies (X)jiet) rather than by Dhom 7). Heterolytic dissociation refers to the gas phase reaction (30)... 

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