Dissociation energy Lewis structure

Calculate CH bond dissociation energies in propene and in toluene, leading to allyl and benzyl radicals, respectively. (The energy of hydrogen atom is given at right.) Is bond dissociation easier or more difficult in these systems relative to bond dissociation in 3-ethylpentane (methyl CH) Examine spin density surfaces for allyl and benzyl radicals. Draw Lewis structures that account for the electron distribution in each radical. Does spin delocalization appear to stabilize radicals in the same way charge delocalization stabilizes ions ... 

As is the case for the examples considered in the previous sections, and below, the VB formulation to be preferred for dissociation will depend on the relative energies of the Kekule and Dewar-type Lewis structures. 

From Table 2.4, the average bond dissociation energy of a N=0 bond is 630 kJ mof, which is right in line with the Lewis structure of NO. The bond dissociation energy of each NO bond in NO2 is 469 k J mof, which is about half-way between a N-0 double and an N-0 single bond, suggesting that the resonance picture of NO2 is a reasonable one. 

An alternative Lewis structure, C 0 , yields formal charges equal to zero for the two atoms and suggest that the CO bond should be of the same length and strength as in CO2. The bond dissociation energy of CO is in fact 1077 kJ mol" 34% larger than the MBE of 805 kJ moF in CO2. The bond distance is 113 pm as compared to 116 pm in CO2. The electric dipole moment is very small (tXei = 0.11 Debye). 

The large difference between the dissociation energy of CO and the mean bond energy in CO2 is by itself sufficient to justify a description of the CO bond as triple in accordance with the Lewis structure C 0 . 

The structures shown above the dotted line illustrate the implications of these criteria and suggest more stable structures. In addition although the stmcture shown for N2O2 is consistent with the Lewis formalism, the weak N-N single bond means that the structure is only observed at low temperatures. The dissociation energy of the NO dimer is only 8.3 kJ moP and it represents an example of a molecule which is not adequately represented by Hartree-Fock molecular orbital calculations [76]. The structures below the dotted line are disfavoured because of they are dipolar or have identical charges on adjacent atoms. 

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