Bond dissociation enthalpies heterolytic

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 ...

Derived bond dissociation enthalpies for metallocene ions are given in Table I. These values assume that aS is negligible and, for the heterolytic D(M-Cp) values, that aH for Cp /" = -40 5 kcal mol ( 7). Corrections for junction potentials in electrochemical data were not applied. 

The bond-dissociation enthalpy (BDE, also called bond-dissociation energy) is the amount of enthalpy required to break a particular bond homolytically, that is, in such a way that each bonded atom retains one of the bond s two electrons. In contrast, when a bond is broken heterolytically, one of the atoms retains both electrons. 

When a bond is broken or formed, the energy required is known as the bond dissociation enthalpy (D° or, more commonly, H° for a bond broken or formed in a reaction). It is also called bond dissociation energy for convenience, the values listed will be used for both heterolytic and homolytic bond cleavages. 

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

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. 

The standard enthalpy change for the reverse of equation 1 is the heterolytic bond dissociation energy for BH (equal by convention to the proton affinity of B). The homolytic bond dissociation energy for BH", i.e. the standard enthalpy change for reaction 5, has been obtained [17,18] for some bases from the proton affinity and the... 

The standard Gibbs free enthalpy change, A G", for the heterolytic dissociation of a P-H bond acoordlng to... 

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