Forças de London e Conformações

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Keys to the Chapter 27 Original δ⁺ δ⁺ fleeting Molecule 1: A: A A: A dipole Result Repel Attract Attract Attract Molecule 2: A A A :A δ⁺ New fleeting dipole, Electrons "induced" by will move the original one The result will be a new dipole in the second molecule's bond, "induced" by the original fleeting dipole in the first molecule. As the diagram shows, the polarizations that result lead to an attractive force between the mol- ecules-the so-called London forces. Even though the dipoles involved have only transient existence and all the bonds are nonpolar, it turns out that the odds always favor the presence of some fleeting dipoles in a mol- ecule, and the net result is this weak, but real, London attraction. Because of the weakness of this attraction, alkanes exhibit relatively low melting points and boiling points relative to those of more polar or charged molecules. The nonpolar nature of alkanes results in other physical consequences, such as rather limited ability to serve as solvents for polar compounds (remember "like dissolves like" from freshman chemistry?). Lack of polarized bonds also very much limits the chemistry that alkanes can display. This subject will be taken up in the next chapter. 2-7 and 2-8. Conformations Although we generally draw pictures of molecules in a single geometrical representation, the fact is that no molecule has a single rigid geometry. The electrons in bonds can be viewed as an elastic glue holding the atoms together. The bonds are therefore somewhat flexible and are subject to some degree of bending or stretching. So, even in the simplest molecules like H₂, the atoms are capable of some degree of movement with respect to one another. In more complicated molecules, additional forms of internal motion become possible. The con- formations of ethane and larger alkanes are a result of rotation about carbon-carbon single bonds, a relatively easy process. This section describes the energetics associated with this rotation and the names associated with the various shapes of the molecules as this rotation occurs. Newman projections provide an "end-on" view of these conformations: X Y Y = substituents Y Eclipsed Staggered Gauche Anti At this point you should take a look at a set of molecular models so that you can become familiar with these conformations in three dimensions. Conformational energetics can be summarized for alkanes as follows: 1. Eclipsed is 2.9 kcal mol⁻¹ higher in energy (less stable) than staggered for ethane. 2. Each CH₃-H eclipsing is 0.3 kcal mol⁻¹ worse than an H-H eclipsing (relative to corresponding changes in staggered conformation energies). 3. Each CH₃-CH₃ eclipsing is 2.0 kcal mol⁻¹ worse than an H-H eclipsing. 4. Each CH₃-CH₃ gauche is 0.9 kcal mol⁻¹ worse than CH₃-CH₃ anti. With these individual estimates, the graph of energy rotational angle can be readily sketched for simple alkanes. Note: These "energy" values are actually enthalpies (heat content. or values).