Bond dissociation energy order

What are the reasons for the observed reactivity order of alkane hydrogens toward radical chlorination A look at the bond dissociation energies given previously in Table 5.3 on page 156 hints at the answer. The data in Table 5.3 indicate that a tertiary C—H bond (390 kj/mol 93 kcal/mol) is weaker than a secondary C-H bond (401 kj/mol 96 kcal/mol), which is in turn weaker than a primary C H bond (420 kj/mol 100 kcal/mol). Since less energy is needed to break a tertiary C-H bond than to break a primary or secondary C-H bond, the resultant tertiary radical is more stable than a primary or secondary radical. 

Bond dissociation energies qualitatively predict the order of reactivity of X-H bonds shown in Figure 1.7 (for examples see Table 1.1). However, as will become apparent, a variety of factors may perturb this order. 

The effect of TOS on the product distribution during the pyrolysis of R22 over CU-AIF3 catalyst is shown in Fig. 3. The amoimt of halogen ion trapped in NaOH solution was determined by IC. The concentration of Cl formed during the pyrolysis of R22 was higher than the concentration of F at all TOS. This result is a consequence of the facile cleavage of the C-Cl bond in comparison to the C-F bond. Bond dissociation energy for the C-element of R22 is followed by the order C-C1[Pg.235]

Electrical discharges through samples of helium gas generate He cations, some of which bond with He atoms to form Hc2 cations. These fall apart as soon as they capture electrons, but they last long enough to be studied spectroscopically. The bond dissociation energy is 250 kJ/mol, approximately 60% as strong as the bond in the H2 molecule, whose bond order is 1. 

The phenanthrene system appears to be no more easily cleaved than the naphthalene system however, ethyl anthracene is clearly destabilized significantly more than the other compounds in the table. The large decrease in bond-dissociation energy for the anthracene system is reflected in the increase by three to four orders of magnitude in the rate of scission at conversion temperatures, as shown in the table. 

The electrochemical behaviour of the compounds containing bonds between silicon and other group-14-metals is also interesting. Mochida et al. reported the electrochemical oxidation potentials of group-14-dimetals [66], As shown in Table 8, there is a good correlation between the oxidation potentials and the ionization potentials which decrease in the order Si-Si > Si-Ge > Ge-Ge > Si-Sn > Ge-Sn > Sn-Sn in accord with the metal-metal ionic bond dissociation energy. 

The thermal decomposition is first-order and the rate coefficient is given by k = 1.7 x 1014 exp(—51,000/i T) sec-1. The mean metal-carbon bond dissociation energy in this alkyl is 58.0 kcal.mole-1. In view of the normal frequency factor, it might seem reasonable to relate the observed activation energy to... 

The apparent first-order rate coefficient is 1.5x 1010 exp(—28,200/RT ) sec-1. This expression has undoubtedly been obtained for a pressure-dependent region. If, as an extreme case, it is assumed that the unimolecular process occurred in the second-order region and if approximately one half of the classical degree of vibrational freedom are active, an upper limit of kx — 1.5 x 1015 exp(—46,000/Rr) sec-1 is obtained. The mean Pb-CH3 bond dissociation energy in tetramethyl lead19,142 is 37.6 kcal.mole-1. Dx should therefore be about 40 kcal.mole-1. 

RSE values can also be calculated from experimentally measured X - H bond dissociation energies or heats of formation (where available). In order to be directly comparable to the RSE values calculated at the ROMP2 or G3(MP2)-RAD level described above, this requires thermochemical data for the species in Eqs. 1-4 at 0 K. One straightforward approach is the back correction of experimentally measured heats of formation at 298.15 K to 0 K values using thermochemical corrections calculated using the rigid ro-tor/harmonic oscillator model in combination with scaled DFT or UMP2 frequencies [19,23]. 

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