Organic 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 1 SAMPLE EXERICE Draw structural formulas for the simplest molecule you can construct from a oxygen and chlorine b sulfur and hydrogen c carbon and fluorine F Cl O Cl H S H F C F F 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 2 There 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 variety of compounds available for the range of functions that nature requires and modifies in life forms It was the recognition of the central role of carboncontaining compounds that led early chemists to call these compounds organic and to propose that such compounds had a
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