Heterolytic bond-dissociation

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

Photolysis, 350 nm, CH3CN, H2O. The by-product from the reaction is 2-phenyl-benzo[ ]furan. Cleavage with TBAF and PhCH2SH has been demonstrated (70-94% yield). The related 3,5-dimethoxybenzoin analogue is cleaved with a rate constant of >10 sec Photolytic cleavage occurs by heterolytic bond dissociation. ... 

Reactions between cations and anions in the gas phase generally proceed with no activation energy. The simplest example is heterolytic bond dissociation. 

Heterolytic Bond Dissociation. A process in which a bond is broken and a cation and anion result. 

Heterolytic bond dissociation (heterolysis) electronically unsymmetrical bond... 

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 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 acidity of a substrate AH in the gas phase is measured using the standard enthalpy change A//°cid for the heterolytic bond dissociation ... 

X. This quantity is defined in eq 5 and is essentially the heterolytic bond dissociation enthalpy of the X-CH bond. 

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

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. 

This chapter assesses the performance of quantum chemical models with regard to the calculation of reaction energies. Several different reaction classes are considered homolytic and heterolytic bond dissociation reactions, hydrogenation reactions, isomerization reactions and a variety of isodesmic reactions. The chapter concludes with a discussion of reaction energies in solution. 

Less disruptive are reactions in which the total number of electron pairs is maintained, but a chemical bond is converted to a non-bonded lone pair or vice versa. Heterolytic bond dissociation reactions provide an example. 

Perhaps the two most important heterolytic bond dissociation reactions are those used to define "absolute" acidity and basicity. 

Both reactions involve dissociation of a polar covalent bond to hydrogen and both lead to a "free" proton. While absolute acidities and basicities are rarely if ever (directly) measured experimentally, they provide a good opportunity to assess the performance of different models with regard to the energetics of heterolytic bond dissociation. 

Another important type of isodesmic reaction compares acid (or base) strength to that of a closely-related standard compound, for example, the basicity of trimethylamine relative to that of ammonia as a standard. This differs fundamentally from absolute acid (or base) strength comparisons, which are heterolytic bond dissociations and which significantly alter overall bonding. ... 

Heterolytic Bond Dissociation. A process in which a bond is broken and a cation and anion result. The number of electron pairs is conserved, but a non-bonding electron pair has been substituted for a bond. 

Two classes of systems illustrate cases for which heterolytic bond dissociation lies lower than the homolytic products. The first involves transition metal dimer cations, M2+. Especially for metals to the right side of the periodic table, such cations can be considered to have ground-state electron configurations with o2dndn+1 character, where the d electrons are not heavily involved in the bonding and the o bond is formed primarily from the metal atom s orbitals. If the o bond is homolytically broken, one forms X + Y = M (s1dn+1)... 

A number of reactivity studies have been performed on 6 and 8 and indicate a strongly polar (if not ionic) Mn—E bond Mn "—E,+ (E = In, Tl). Thus heterolytic bond dissociation occurs in polar ligating solvents such as MeCN or DMF, and halogens, hydrogen halides, and alkyl halides readily add across the metal-metal bond in a manner consistent with the polarity described above (13,13a,18). In the thallium example, however, the reactions are generally more complicated and result in T1(I) salts [e.g., Eq. (3)], and metal exchange reactions are also more facile, e.g., the synthesis of 6 from 8 and indium metal. In general, therefore, the chemistry of 6 and 8 is consistent with predominantly ionic behavior. 

A tetracobalt anionic complex, viz. [In Co(CO)4 4] (27) (37,37a), has been briefly described together with the thallium analogue (28) (37a), both formed by addition of [Co(CO)4] to either 25 or 26. No structural details have been reported although the indium and thallium centers are presumably tetrahe-drally coordinated by the four cobalt atoms. Mention is also made (37a) of the facile heterolytic bond dissociation (In—Co or Tl—Co) observed in polar solvents. Little has been reported about the reactivity of these complexes, although a discussion on the use of 25 as a catalyst in the dimerization of norbornadiene has appeared (58). 

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. 

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