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O.1Organic Chemistry 1. The Concept of Valence Atoms combine in various ways to form an enormous variety of molecules. Chemists have discovered simple rules governing these combinations of atoms – rules that bring order to what would otherwise be an impossible task of rote memorization and library searching. As we will see, rules of atomic combination, combined with understanding of molecular interactions, enable us to make sense of the material universe in which we find ourselves. That is what Chemistry 110H is about. In this supplement to Chemistry 110H, we will focus on organic chemistry, a topic with which many of you will not be familiar. We do this because the subject provides excellent examples with which we can illustrate many of the concepts that come later in the course. Think of this topic as providing a toolkit that we will draw upon throughout the next two semesters. For those of you planning to take organic chemistry next year, this approach will help prepare you for that experience. Organic chemistry is also relevant to biology and biologically-related courses, and is important in understanding polymers and new materials. For those of you not going on in chemistry, this section will provide you with some background in one of chemistry’s most important branches. It is a topic that will continually come up throughout your life, no matter what your profession. We start by reviewing the elementary rules of covalent atomic combination which we assume you learned in an earlier course. These are the rules that refer to the number of covalent bonds an atom of each element forms as it combines with another atom of the same element or of a different element. You will most likely remember these covalent bonds as “sticks” connected to each atom in a diagram showing how the atoms are connected in a molecule. For example, the structural formula for a molecule of water is: H−O−H structural formula for a water molecule Note that the oxygen atom is connected to two sticks, or covalent bonds, and each hydrogen is connected to one bond. This is the normal number of covalent bonds for each of these atoms – two for oxygen and one for hydrogen. These numbers are sometimes called the valences of the atoms. The normal valences for some other atoms of interest to us in this chapter are: one for fluorine (F); three for nitrogen (N), and four for carbon (C). Generally, we expect elements in the same family of the periodic table (i.e., in the same vertical column) to share the same valence. Thus, the valence for chlorine (Cl) is one because chlorine is in the same family as fluorine, that for sulfur (S) is two because it is in the same family as oxygen, etc. (You may have learned in an earlier course that the most common valence number associated with each family in the periodic table reads as follows as we go from left to right across the table: 1, 2, 3, 4, 3, 2, 1, 0.)O.2SAMPLE EXERICE Draw structural formulas for the simplest molecule you can construct from (a) oxygen and chlorine, (b) sulfur and hydrogen, (c) carbon and fluorine. Cl−O−Cl H−S−H CFFFF chlorine with oxygen sulfur with hydrogen carbon with fluorine The solutions to the Sample Exercise show each atom connected to each of its bonded neighbors by a single bond. You probably recall from earlier courses that this is not the only possibility. Some molecules involve multiple bonds, that is, cases where neighbors are connected by two or even three bonds. Here are two examples, resulting when a pair of oxygen atoms combine to form an oxygen molecule or when two nitrogen atoms combine to from a nitrogen molecule. O=O N≡N oxygen molecule (O2) nitrogen molecule (N2) Observe that the rules of valence are still satisfied. Each oxygen has satisfied its valence of two and each nitrogen has satisfied its valence of three. The oxygen molecule has a double bond and the nitrogen molecule has a triple bond. The existence of this option accounts for some of the variety in molecular compounds. Oxygen, for example, can form a double bond with itself, as shown above, but can also form a single bond with itself if it can find some way to bond to another atom to satisfy its valence of two. Hydrogen peroxide is an example. H−O−O−H hydrogen peroxide (H2O2) The valence requirements guides us to what is possible and exclude what is not possible. Thus, we expect NH3 to be a stable molecule, but not NH2. More accurately, we are not surprised that NH3 (ammonia) is stable. But if we were to discover that NH2 is a stable molecule, we would be surprised and investigate to discover where our simple scheme is incorrect. All sciences proceed in this manner: we develop a scheme, or set of rules, or theory, and then make corrections or refinements or extensions on the basis of the situations where it does not work. The rules of valence, clearly, go a long way toward making sense of the choices that atoms make when combining to form molecules. They also serve as a source of useful clues about molecular structure and bond energy. For instance, the distance between oxygen atoms is shorter in the doubly-bonded O2 than in the singly bonded H2O, and more energy is needed to break the double bond in O2 than the O-O single bond in H2O2.O.3There is one more combining rule that we need before we venture into organic chemistry. It has to do with the ability of an atom to combine with others of its own kind. One can imagine bonding together oxygen atoms to form long chains, as illustrated in this structure. H−O−O−O−O−O−O− . . . –O−O−O−O−O−O−H A proposed long chain of OnH2 This structure satisfies the rules of valence, but the molecule shown does not exist. Chain formation between atoms of the same type does not occur to significant extents for most elements to produce stable molecules. A few elements, such as sulfur, are moderately successful at forming chains (the stable form of sulfur is a ring of eight atoms), but there is one element that is supremely successful at forming chains by bonding to itself, and that is carbon. Carbon forms bonds with itself that are very stable and that go on and on over chain lengths of tens of thousands of atoms. This is a key factor in the ability of compounds of carbon to form a limitless number of compounds. It is also the reason that carbon compounds are the natural realm for life processes, for only here is the


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PSU CHEM 110 - Organic Chemistry

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