Heterolytic bond enthalpies

Table 2.4 Selected hemolytic and heterolytic bond enthalpies (kJ mo -1)3-7 ...

Figure 4. Average heterolytic bond enthalpies for M(acac)3 and M(acac)3 complexes. Dashed lines connect those complexes with no net ligand field stabilization (dashed line on anion data drawn arbitrarily for purposes of illustration). Values for M(acac)3 taken from Ref. 27.

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 ...

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. 

Some homolytic (between the same element) and heterolytic (between different elements) bond enthalpies are summarized in Table 2.4. 

Again assuming that aG is a good estimate for aH, average heterolytic bond dismption enthalpies, -(acac)"), can be derived for... 

A covalently bonded complex M-X can be envisioned to undergo M-X bond cleavage by the three modes depicted in Scheme 1. Here, Equation (4) represents the homolytic bond cleavage that defines the BDE, normally defined in enthalpy terms - as will be the case in all tabulated values here. Equations (5) and (6) in Scheme 1 are the two corresponding heterolytic bond cleavage processes. The processes are interrelated by the electron affinities (EAs) and ionization potentials (IPs) of the radicals M and X in the gas phase. In solution, EA and IP are replaced by the corresponding electrode potentials (multiplied by the Faraday constant). 

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 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). 

M. Xian, X.-Q. Zhu, J. Lu, Z. Wen, J.-P Cheng. The First O-NO Bond Energy Scale in Solution Heterolytic and Homolytic Cleavage Enthalpies ofO-Nitrosyl Carboxylate Compounds. Org. Lett. 2000, 2, 265-268. 

Because most heteroatom-carbon single bonds are less stable than carbon-carbon bonds, traceless linkers can be synthesized on the basis of nearly all heteroatoms. The enthalpies of C-X bonds are, however, only relevant for homolytic bond scission. Many linkers are cleaved heterolytically, and kinetic stability toward hetero-lytic bond cleavage is decisive in these linkers. 

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