vollhardt3_c3 - Reactions of Alkanes Bond-Dissociation...

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93 Reactions of Alkanes 3 Bond-Dissociation Energies, Radical Halogenation, and Relative Reactivity C ombustion of alkanes releases most of the energy that powers modern industri- alized society. How does this process occur? Is it related to the chemistry that converts alkanes into other organic molecules? We will find that both transformations rely on the same essential step: the breaking of a bond, or bond dissociation. Many liquid and solid alkanes are obtained cheaply from petroleum (Section 3-3). Thus, nature has given us large quantities of hydrocarbons that can be used as “chem- ical feedstocks,’’ or starting materials, for the synthesis of other organic molecules. Alkanes are also produced naturally by the slow decomposition of animal and veg- etable matter in the presence of water but in the absence of oxygen, a process last- ing millions of years. The smaller alkanes—methane, ethane, propane, and butane— are present in natural gas, methane being by far its major component. In the United States, natural gas is a major source of energy, with annual production in the hun- dreds of millions of tons. As stated in Chapter 2, alkanes are organic chemicals that lack functional groups. Chapter 3 begins by explaining how bond dissociation in alkanes can be made to oc- cur. Next, we learn how to introduce functional groups, to turn alkanes into com- pounds useful for synthesis, a process called functionalization. An important func- tionalization reaction of alkanes is halogenation, in which a hydrogen atom is replaced by a halogen. For each of these processes, we shall use a description of its mechanism to explain the conditions under which each takes place. You will see that these mechanistic concepts also explain the effects of halogen-containing chemicals on the stratospheric ozone layer. Finally, a discussion of alkane combustion leads to a description of the methods used to establish the heat contents and relative stabili- ties of molecules. Reactive odd-electron species such as halogen atoms and hydroxy radicals (shown here) are involved in the oxidative degradation of organic materials and contribute to weathering.
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94 Chapter 3 REACTIONS OF ALKANES 3-1 Strength of Alkane Bonds: Radicals Chapter 1 explained how bonds are formed and that energy is released on bond for- mation. For example, bringing two hydrogen atoms into bonding distance produces 104 kcal mol 2 1 of heat (refer to Figures 1-1 and 1-12). H ? 1 H ? 7777n H O H D H ° 52 104 kcal mol 2 1 Consequently, breaking such a bond requires heat, in fact the same amount of heat that was released when the bond was made. This energy is called bond-dissociation energy, DH °, or bond strength. H O H 7777n H ? 1 H ? D H ° 5 DH ° 5 104 kcal mol 2 1 Radicals are formed by homolytic cleavage In our example, the bond breaks in such a way that the two bonding electrons divide equally between the two participating atoms or fragments. This process is called ho- molytic cleavage or bond homolysis.
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This note was uploaded on 04/27/2008 for the course CHEM 266 taught by Professor Forsey during the One '08 term at University of Wollongong, Australia.

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vollhardt3_c3 - Reactions of Alkanes Bond-Dissociation...

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