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44 Chapter 3 REACTIONS OF ALKANES alkyl groups effectively take some of the electron deficiency onto themselves, spreading it out, or "delocaliz- ing" it. Delocalization of an electron deficiency or an electron excess over more than just the atom on which it is nominally located is often an energetically favorable, stabilizing process. It effectively allows the "prob- lem" to be diluted over a larger area, rather than being the concentrated burden of a single atom. The ability of alkyl groups to stabilize electron-deficient centers like radicals is often taken to imply that they are better donors of electrons than hydrogen atoms. Alkyl groups are therefore referred to as electron donating. 3-3. Conversion of Petroleum: Pyrolysis The practical, "real-world" aspects of bond cleavage and radical formation are explored. Pyrolysis is a process that often gives mixtures of many products, and methods have been developed (mostly within the petroleum industry) to control this reaction somewhat. We will frequently explore the issue of reaction control: the mod- ification of conditions under which a chemical transformation is carried out in order to give a desired mole- cule as a major or exclusive product. 3-4. Chlorination of Methane: The Radical Chain Mechanism In this section the reaction of methane with chlorine molecules is discussed. The process 3 CH₄ + Cl₂ HCI + is important because it converts a nonfunctionalized molecule (an alkane) into a molecule containing a func- tional group (a haloalkane). Once the functional group is present, many more kinds of chemical reactions be- come possible. This section also presents the mechanism of this reaction in full detail. Pay close attention not only to the steps of the reaction (initiation, propagation, and termination) but also to the finer details relating Eₐ, and transition state structure for each step. Although some of the terminology introduced here is ap- propriate only for radical mechanisms and not for the majority of reactions to come later, the type of information that the mechanism contains is critical to an understanding of how and why organic reactions occur. Take some time in this section to study each reaction step. What are its energetic circumstances, un- der what conditions does it occur, what role does it play in the overall process? Try to establish a feeling for the species involved as "stable" or "unstable," "reactive" or "unreactive," relatively speaking. Reaction mech- anisms are intended to allow one to make sense out of organic chemistry. Give this one the time to do that for you. Be sure that you understand the procedure for calculating the value for a chemical reaction from the DH° values of the bonds taking part in the transformation. The general formula is = (bonds broken) (bonds formed) Energy input Energy output To illustrate with a reaction different from those in the text, let us calculate for the process C₂H₆ + H₂ 2 Using the data from Tables 3-1 and 3-2 in the text. DH° = 90 104 105 kcal mol⁻¹ reaction = [90 + 104] [2(105)] = 16 kcal mol⁻¹ Note 2 methane bonds Comment: This is a "hydrocracking" process that, although exothermic, requires very high temperatures to occur (cleavage of bond is necessary).