Homolytic Bond Dissociation Energies DH

This behavior is characteristic of radical reactions. Consider another example, one that shows another way in which radicals can react They can combine with a compound containing a multiple bond to produce a new radical, which goes on to react further. (We shall study reactions of this type in Section 10.10.) 

When atoms combine to form molecules, energy is released as covalent bonds form. The molecules of the products have lower enthalpy than the separate atoms. When hydrogen atoms combine to form hydrogen molecules, for example, the reaction is exothermic it evolves 436 kJ of heat for every mole of hydrogen that is produced. Similarly, when chlorine atoms combine to form chlorine molecules, the reaction evolves 243 kJ moF of chlorine produced  

Reactions in which only bond breaking occurs are always endothermic. The energy required to break the covalent bonds of hydrogen or chlorine homolytically is exactly equal 

The homolytic bond dissociation energies of hydrogen and chlorine, for example, can be written in the following way  

The homolytic bond dissociation energies of a variety of covalent bonds have been determined experimentally or calculated from related data. Some of these DH° values are listed in Table 10.1. 

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The relative reactivity of a FR can be estimated from its homolytic bond dissociation energy (DH) if it is tended to hydrogen [79]. Examples are shown in Table 1. 

Homolytic bond dissociation energy, DH° (Section 10.2) The enthalpy change that accompanies the homolytic cleavage of a covalent bond. 

Bond dissociation energy (DH°) The energy rec tiired to break a chemical bond through homolytic cleavage, or the energy released in forming that bond. 

The energies required to break covalent bonds homolytically are called homolytic bond dissociation energies, and they are usually abbreviated by the symbol DH°. 

Homolytic bond dissociation energies also provide us with a convenient way to estimate the relative stabilities of radicals. If we examine the data given in Table 10.1, we find the following values of DH° for the primary and secondary C—H bonds of propane ... 

TABLE 10.1 SINGLE-BOND HOMOLYTIC DISSOCIATION ENERGIES (DH°) AT 25 °C ... 

Table 10.1 Single-bond homolytic dissociation energies DH° at 25°C...

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