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Keys to the Chapter 43 3-7 Selectivity with Other Halogens Energetic comparisons of the reactions involving and Br₂. 3-8 Synthetic Aspects More practical considerations. 3-9 Synthetic Chlorine Compounds and the Stratospheric Ozone Layer Halogens in the "real world." 3-10 Combustion and the Relative Stability of Alkanes Included is a detailed introduction to the evaluation of the energetics associated with a chemical reaction. Keys to the Chapter 3-1. Strength of Alkane Bonds: Radicals A minor but annoying point of confusion is often encountered when one discusses bond strengths. A bond's strength, or more properly, bond-dissociation energy is defined as the energy released when a bond forms or, equivalently, the energy input required to break a bond: A-B = DH° Energy is released. A-B = DH° Energy is put in. Inspection of these two equations shows that the bonded molecule A-B is more stable-lower in energy content-than the separated atoms A and B by an amount equal to DH°. When the linkages in a molecule are strong (high DH°), the molecule is usually relatively low in energy content (e.g., stable). As long as you remember that DH° is the energy that has to be put in to break a bond, you won't fall into the common trap of associating large DH° values with high-energy species. Large DH° values imply low energy, strongly bonded, stable species. The tables and figures in this section should further help you develop a comfortable under- standing of the meaning of DH° values, in preparation for their use later on. 3-2. Alkyl Radicals and Hyperconjugation Homolytic cleavage of any bond in an alkane generates radicals: species with a single unpaired electron where an attached group used to be. The section illustrates four such examples: methyl, ethyl, CH₂CH₃; isopropyl, CH(CH₃)₂; and tert-butyl, C(CH₃)₃. Several points are made in the section. First, radical carbons are sp² hybridized (planar), not sp³ hybridized (tetrahedral, as in alkanes). Why should this be? A partial reason goes back to basic electrostatics. The shape of a species will be that which minimizes repulsion between electrons around a central atom (remember va- lence shell electron pair repulsion, or VSEPR?). In ammonia, the four electron pairs around N are best accommodated by a pyramidal shape based on sp³ hybridization: Repulsion between the lone pair and the elec- trons in the N-H bonds is important in causing this geometry to be preferred. Reducing the number of non- bonded electrons from two to one as in methyl radical, CH₃, changes the situation. Now, electron repulsion between the pairs in the C-H bonds dominates, a situation leading to sp² hybridization and trigonal planar geometry, which allows the C-H bonding electrons to spread out far away from one another. The second main point in the chapter is the stabilization of a radical center by the presence of alkyl groups attached to the radical carbon. So, tert-butyl radical is more stable than isopropyl, which is better than ethyl; methyl radical is the least stable. Hyperconjugation is one concept often used to explain this stabilization. Phys- ically, and electrostatically, the radical carbon can be viewed as somewhat electron deficient (7 valence elec- trons instead of an octet). Hyperconjugation provides a means for bonds in neighboring alkyl groups to "lend" a little electron density to the radical center, thereby making it feel a little less electron-poor. In doing so, the