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Keys to the Chapter 45 Note on energetics: The overall enthalpy of a radical chain reaction is the sum of the values for only the propagation steps. If we "sum up" the species in these steps, we see that the free atoms and radicals "cancel out," leaving only the molecular species of the overall reaction: Propagation step 1 CH₄ + HCI + CH₃ = + 2 kcal Propagation step 2 CH₃ + Cl₂ CH₃Cl + = 27 kcal Sum: CH₄ + + + Cl₂ HCI + + + = 25 kcal Removing the and CH₃ that appear on both sides of the equation leaves just the molecules of the overall process. What about the initiation and termination steps and their They are separate. Their values are not a part of the enthalpy change as it is defined for the stoichiometric reaction. When we measure the heat of a radical reaction experimentally, the value we obtain will not be precisely equal to for the propagation steps alone; initiation and termination steps are occurring, too, and their will introduce an error. This deviation will usually be small, however, because initiation and termination steps occur only infre- quently relative to the propagations, and because the for the endothermic initiation are for the most part canceled out by those of the exothermic termination processes. 3-5. Other Radical Halogenations of Methane One of the best features of organic chemistry is the fact that one mechanism can hold for many individual re- actions. Thus, the same types of steps that occur in the chlorination of methane are followed in its reactions with the other halogens. The similarities are qualitative, however. Differences in energetics are significant, and as a result the diagrams differ in appearance in a way that will become important as we continue through the chapter. 3-6. Chlorination of Higher Alkanes The same mechanism also applies qualitatively to chlorination of other alkanes. The only difference is in the nature of the C-H bonds available in the alkane to be broken. They are generally less strong than those in methane, following a DH° order of CH₄ > 1° > 2° > 3°. (Note: 1° = primary, 2° = secondary, and 3° = ter- tiary. These are commonly used symbols.) The weakest (3°) are the most readily broken; thus, alkanes with different types of C-H bonds display a built-in selectivity of 3° > 2° > 1° in their reactions with chlorine. This section describes this selectivity quantitatively, illustrating how both reactivity differences and statistical factors combine to produce the observed ratios of products in several representative systems. 3-7. Selectivity with Other Halogens An extension of the previous sections. The most significant point is that reactivity and selectivity in radical halogenations are inversely related. Simply put, the more reactive halogens are less picky and show less pref- erence for 3° 2° vs. 1° C-H bonds relative to less reactive halogens. The reason lies in the different activation energies associated with the C-H bond-breaking step. The values for fluorine are all very small, and very similar to one another. Fluorine thus reacts very rapidly with any C-H bond in a molecule. The reactions for bromine have large activation energies, with significant differences associated with the different types of C-H bonds present. The result is that bromine is much, much slower than fluorine to react with any alkane, and bromine is much more discriminating (selective) in its reactions, too, greatly preferring 3° over 2° or 1° C-H bonds. The contrasts between reaction-coordinate diagrams for the two halogens provide a picto- rial representation of these differences. 3-8. Synthetic Aspects Synthesis is one primary function associated with organic chemistry. In synthesis, we strive to produce a de- sired product in good yield with high selectivity, to minimize the effort required to separate this material from side products. In particular, a reaction that gives rise to a hard-to-separate mixture of many components is syn- thetically useless. In this chapter we have seen a large number of possible permutations of a single reaction: alkane halogenation. Not all of the examples shown are equally useful synthetically. The best ones start with