Sigma bond dissociation energy

Many polymerizations are carried out at temperatures between 0 and 100°C. Initiation at the required rates under these conditions is confined to compounds with activation energies for thermal homolysis in the range 1(X)-165 kJ/mol. If the decomposition process is endothermic, the activation energy can be considered to be approximately equal to the dissociation energy of the bond which is being split. It can be expected, then, that useful initiators will contain a relatively weak bond. (The normal C—C sigma bond dissociation energy is of the order of 350 kJ/mol, and alkanes must be heated to 3(X)-500°C to yield radicals at the rates required in free-radical polymerizations.)... 

I2 has a low bond dissociation energy (151 kJ/mol) and forms 21- on heating. I- adds to the C=C to form a carbon radical which rotates about its sigma bond and assumes a different conformation. However, the C—I bond is also weak (235 kJ/mol) and the radical loses I- under these conditions. The double bond is reformed and the two conformations produce a mixture of cis and trans isomers. 

The calculated bond dissociation energy for N2 of 10.28 eV is 5% greater than the experimentally determined value of 9.759 eV. This is the same error found in the analysis of the H2 molecule. The sigma contribution to the overall bond strength is 4.20 0.05 eV and is greater than the single 7t contribution of 3.04 0.03 eV, as theory would predict. 

Over the years we came up with methods to calculate bond dissociation energies, charge distribution in sigma-bonded systems, charge distribution in pi-bonded systems, the inductive effect, the resonance effect, the hyperconjugation effect and the polarisability effect. ... 

In Figure 11 various single bonds of acetaldol are shown in a reactivity space spanned by the polarity in the sigma electron distribution, the bond dissociation energy, BDE, and the resonance effect, R. 

For each bond of aldol, calculations were made by the methods developed in our group for the bond dissociation energy, the sigma charge distribution and the amount of resonance stabilisation of the charges generated by polar breaking of that bond. The two ways of heterolysis of a bond are shown with the points 4 and 7. Point 4 corresponds to the dissociation of OH , whereas point 7 represents the loss of OH. ... 

The quintessential example of sigma-pi rehybridization due to structural modification is the structural consequences of replacing H by F in ethylene (C2H ). As the sigma bonds become stronger, the C-C pi bond becomes weaker as the following pi Bond Dissociation Energies (BDE s) indicate ... 

Recall that the energy required to break the sigma bond homolytically is called the bond dissociation energy (BDE, p. 337). Note that the bond breaks to give two neutral species, two radicals, and not to give two polar species, a cation and an anion (Fig. 11.3). 

What is the difference between a sigma bond and a pi bond How many of each type of bond are found in a triple bond What is bond dissociation energy ... 

Figure 1.4 shows sp hybridization in ammonia. In this case one of the sp orbitals has a pair of electrons, so only three sigma bonds can be formed. The structure affects the sigma bond length (0.96 A in ammonia, compared to 1.01 A in methane) and the bond angle (107° vs 109°). This in turn affects the energy of dissociation of the sigma bond 103... 

---