1559T_ch02_18-3710/22/051:11Page23 EQA Keys to the Chapler·23 used for convenience or by force of habit.A number of compounds whose systematic names are very com the uninitiated.Several vprovides perspectiven en g L of th n the HO- CH2OH Q H H OH 个OH 0 0 CH-OH end of this ould sit do with the handbook of rule vith the cyclohexane (and even this is only partially complete.lacking certain indicators that dist nguish it from othe is a lo the name used by genera which isnone other thar Fear not,oddsare you will never,ever,have to give IUPAC name toa molecule like this.Inever did.at least until I had to write this study guide 2-6.Physical Properties Every time we encounter ordinary conditions (e.g..diethylamine,colorless liquid,smells like something died.or,2-hydroperoxy-2-iso propoxypropane,colorless crystalline soli blows up like n A-bomb if you look at it )The pur gases or liquids.with rather light odors.or white.waxy solids (candle wax is mainly alkanes) ore specinc discuss on wIl nd phy or ccules is presented.Alkanes,lacking charged atoms or highly polarized bonds,do are attracte clectrons are always moving.Even though the average location of the electron pair is exactly half- icular moment in time te slects my c AA:AsAA一 Average situation During these moments.the bond is polarized.Because this polarization is not permanent.the partial charges e,thus other end and attracted toward its end.The positions and movements of all the electrons are said to be"correlated
Keys to the Chapter • 23 The notes above refer to the systematic nomenclature method as it is currently used. Please note, however, that there are many nonsystematic names in common use that are holdovers from the olden days and are still used for convenience or by force of habit. A number of compounds whose systematic names are very complicated have been given names that are well understood by people in the business but may seem random to the uninitiated. Several of these are mentioned in the text. One more example provides perspective in this area. Illustrated below is a compound that we eat every day. By the end of this course, you could sit down with the handbook of IUPAC rules and come up with the name 1-[3,4-dihydroxy-2,5-bis(hydroxymethyl)oxacyclopent-2-oxy]-3,4,5-trihydroxy-6-(hydroxymethyl)oxacyclohexane (and even this is only partially complete, lacking certain indicators that distinguish it from other known isomers!). Fortunately, the name used by general consent for this molecule, which is none other than ordinary table sugar, is a lot shorter: sucrose. See, even chemists use common sense sometimes. Fear not, odds are you will never, ever, have to give an IUPAC name to a molecule like this. I never did, at least until I had to write this study guide. 2-6. Physical Properties Every time we encounter a new class of compounds, we will briefly discuss common “physical properties” of members of that compound class. These will include general comments on the nature of the compound under ordinary conditions (e.g., diethylamine, colorless liquid, smells like something died, or, 2-hydroperoxy-2-isopropoxypropane, colorless crystalline solid, blows up like an A-bomb if you look at it cross-eyed). The purpose of these comments is to give you a feeling for what these materials are really like (as well as alerting you to the fact that some organic molecules may not be your friends). For the record, alkanes are colorless gases or liquids, with rather light odors, or white, waxy solids (candle wax is mainly alkanes). More specific discussion will focus on relationships between molecular structure and physical properties for the class of compounds as a whole. In this chapter a brief summary of the kinds of forces that attract molecules to each other is presented. Alkanes, lacking charged atoms or highly polarized bonds, do not exhibit either ionic or dipolar forces. As nonpolar molecules, alkane molecules are attracted to each other by only the rather weak London forces. These can be understood fairly simply. In even a totally unpolarized bond, the electrons are always moving. Even though the average location of the electron pair is exactly half-way between the atoms, at any particular moment in time, the electrons may be closer to one atom or the other: During these moments, the bond is polarized. Because this polarization is not permanent, the partial charges associated with it are only transient, or fleeting in nature, thus the name fleeting dipoles. When two nonpolar molecules are close to each other and a bond in one of them exhibits a fleeting dipole, the electrons in a nearby bond of the other molecule will be pushed away from the fleeting dipole’s “” end and attracted toward its “” end. The positions and movements of all the electrons are said to be “correlated”: A: A A vs. vs. A : A :A “Fleeting dipole” “Fleeting dipole” Average situation 0 0 HO HO O O O OH OH OH H H H CH2OH CH2OH CH2OH 1559T_ch02_18-37 10/22/05 1:11 Page 23
1559Tch0218-3710/22/051:11Page24 EQA 24.chapter 2 STRUCTURE AND REACTIVITY Molecule 1: A:A Repel↑ Molecule2: A:A 人Ea电p道 ecules nonpola ims out that the od always favor the presence of some fleeting dipoles in a mol kanes can display.This subject will be taken up in the next chapter. 2-7and2-8. Conformations atoms together.The bonds are therefore somewhat flexible and are subject to some degree of bending or stretch ing.So,even in esimplest molecules like 10 degree of nt with on"view of these conformations X.Y=substituents 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: Each CH3-H eclipsing is 0.3 kcal mo worse than an H-H eclipsing (relative to corresponding 3.Each CHyCH eclipsing is 20 kcal molworse than an H-H eclipsing. 4.Each CHa-CHa gauche is 0.9 kcal mol worse than CH-CH3 anti. With these individual estimates,the graph of energy vs.rotational angle can be readily sketched for simple alkanes.Note:These "energy"values are actually enthalpies (heat content.or AH values)
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 molecules—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 molecule, 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 conformations 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 “endon” view of these conformations: 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 mol1 higher in energy (less stable) than staggered for ethane. 2. Each CH3–H eclipsing is 0.3 kcal mol1 worse than an H–H eclipsing (relative to corresponding changes in staggered conformation energies). 3. Each CH3–CH3 eclipsing is 2.0 kcal mol1 worse than an H–H eclipsing. 4. Each CH3–CH3 gauche is 0.9 kcal mol1 worse than CH3–CH3 anti. With these individual estimates, the graph of energy vs. rotational angle can be readily sketched for simple alkanes. Note: These “energy” values are actually enthalpies (heat content, or H° values). A: A A : A A :A Original fleeting dipole New fleeting dipole, “induced” by the original one Result Repel Attract Molecule 1: Molecule 2: Attract Attract A: A Electrons will move 24 • Chapter 2 STRUCTURE AND REACTIVITY 1559T_ch02_18-37 10/22/05 1:11 Page 24