有机化学结构与功能5th答案sg_ch02

1、18 2 Structure and Reactivity: Acids and Bases, Polar and Nonpolar Molecules Organic chemistry is largely a study of chemical reactions involving organic molecules. The textbook chapter therefore begins with a review of the principles of kinetics and thermodynamics, which apply to all reactions. The

2、n we proceed to discuss acids and bases, which serves two purposes: It provides us with good examples of thermodynamics as applied to reactions, and it illustrates a process that is actually closely related to most of the reactions of polar organic molecules. After an introduction to functional grou

3、ps and the classes of or- ganic compounds, the chapter turns to a consideration of the simplest of those classes, the nonpolar alkanes. These sections cover (1) how to name organic molecules (nomenclature), (2) the relation of the physical prop- erties of molecules to their molecular structure, (3)

4、flexibility and shape of molecules (conformation), and (4) applications of kinetics and thermodynamics to changes in molecular shape. In Chapter 3 we cover chemi- cal reactions of alkanes. Outline of the Chapter 2-1 Kinetics and Thermodynamics The energetic factors that govern the transformations of

5、 molecules. 2-2 Acids and Bases; Electrophiles and Nucleophiles Reviewing chemistry that youve seen before, with an eye to chemistry you will see a lot of in the chapters to come. Also, a first look at the notation used to describe how organic reactions take place. 2-3 Functional Groups The “busines

6、s ends” of molecules: where reactions are likely to occur. 2-4 Straight-Chain and Branched Alkanes Alkanes of various structures. Isomers. 2-5 Nomenclature The first of a group of rules used to unambiguously name any organic compound. 2-6 Physical Properties A topic that is usually not emphasized ve

7、ry much but that does reveal several useful, generalizable points about molecules. 2-7 and 2-8 Conformations A discussion of the spatial arrangements that are possible for atoms in alkanes and the energy changes associated with their interconversions. 1559T_ch02_18-37 10/22/05 1:11 Page 18 Keys to t

8、he Chapter 19 Keys to the Chapter 2-1.Kinetics and Thermodynamics This section introduces ideas associated with energy changes in organic chemistry. Even though some of the terminology may be somewhat familiar to you from freshman chemistry, a few comments may be useful for orientation purposes. In

9、this course you are going to encounter a lot of discussion concerning the energy con- tent of molecules or other species. This term will refer in general to what is called potential energy in physics: energy that is stored in some way and can potentially be released in some process later on. Discuss

10、ions involving energy will often refer to the stability or instability of various substances or systems. Energy and stability are related in the following way: A species with high-energy content will tend to want to get rid of some of its energy somehow. So, relatively speaking, high-energy species

11、are generally unstable. Heat and energy are also related, so high-energy species will have a tendency to undergo processes that give off lots of heat. However, that a substance is capable of doing such a thing doesnt necessarily mean that it will do it quickly. The point here is that the rate of a p

12、rocess is the subject of kinetics, whereas the energetic favorability is one of thermodynamics, and the two are very different. Energetically favorable processes can take place at fast rates, slow rates, or in some cases, hardly seem to take place at all. A wooden match in the presence of air is a g

13、ood chemical example of the latter. The reactions with oxygen of the compounds in the wood as well as on the head of the match are all extremely energetically favorable (thermodynamics), but noth- ing perceptible happens at room temperature. Why not? The rate of the reaction is too low: The number o

14、f molecules actually reacting with the oxygen at room temperature is so small that nothing seems to be hap- pening at all (kinetics). However, when we strike the matchheat the match head with frictionit starts to burn and continues until the whole thing has burned up. The reaction of most organic mo

15、lecules with oxygen requires energy input to get started even though it ultimately results in net energy output after the reaction has finished. The reason is as follows: In most reactions, old bonds are broken and new ones are formed, but not exactly simultaneously. Some partial breaking of old bon

16、ds has to take place before anything else, and that re- quires an input of energy. Once this process has started, it can lead to the formation of new bonds, and the re- lease of energyenough to make more old bonds break plus extra in the form of the flame and heat of burn- ing. This initial energy i

17、nput is the activation energy of the reaction, and it is a key factor governing kinetics: rates of reactions. This section provides a brief mathematical description of each of the main concepts involved in thermody- namics and kinetics as applied to organic chemistry. The equations are generally fai

18、rly straightforward in their application. The problems will give you several chances to use them. 2-2.Acids and Bases; Electrophiles and Nucleophiles The beginning of this text section covers the material you are most likely to have encountered in freshman chemistry: the mechanics of acid-base chemi

19、stry. The guiding principle is that such processes are reversible and are governed by thermodynamics. Notions of strong and weak acids and bases are based upon the posi- tions of equilibria of the general sort stronger acid ? stronger base weaker acid ? weaker base?G ? 0, where the thermodynamic dri

20、ving force favors conversion of the stronger acid and stronger base into the weaker ones. While this concept may be familiar to you, you may not be quite as used to the relative nature of the terms strong and weak. In other words, a compound that acts as the strong acid in one such equation may be t

21、he weak partner in another, or may even play the role of a base. After all, the range of known acid strengths covers sixty orders of magnitude, and in organic chemistry we will encounter examples from every part of the scale. Water is the most familiar of substances to show such varied behavior. Wat

22、er acts as a weak acid (the way we normally think of it): 1559T_ch02_18-37 10/22/05 1:11 Page 19 HCl ? NaOH NaCl? H2O strongerstrongerweakerweaker acidbasebaseacid But water may act as the strong acid (and conversely, hydroxide as the weak base!): H2O? NaNH2NaOH ? NH3 strongerstrongerweakerweaker ac

23、idbasebaseacid And finally, water acts as a base when it encounters a strong enough acid: HCl? H2OCl? H3O? strongerstrongerweakerweaker acidbasebaseacid In describing acid-base reactions, we define a very simple relationship: the one between an acid and its con- jugate base (or, conversely, a base a

24、nd its conjugate acid). Through this relationship it is possible to estimate the strength of acids and bases that weve never seen before by making structural comparisons with species with which we are more familiar. We use the notion that, relatively speaking, strong acids have weak conju- gate base

25、s, and vice versa. Through this relationship, we may use an analysis of either component of a con- jugate acid-base pair to find the strengths of both, relative to other acids and bases. The most common appli- cation is to determine the strength of an acid by evaluating effects that stabilize (and m

26、ake weaker) its conjugate base: increased size and increased electronegativity of the negatively charged atom, and any effects that dis- perse negative charge away from the negatively charged atom, such as resonance.* By comparing the degree to which these properties are present in each of a pair of

27、 conjugate bases, you can usually tell which of the corresponding conjugate acids is stronger or weaker. This text section also reviews the definitions of Lewis acids and bases and compares them with their analogs in organic chemistry: electrophiles and nucleophiles. The latter are the two terms tha

28、t we use to describe electron-poor and electron-rich atoms in molecules, respectively. Such atoms possess partial or full electrical charges and, as a result, are places where chemical reactivity is usually high. Many of the functional groups are characterized by the presence of electrophilic or nuc

29、leophilic carbon atoms, for example. The analogy be- tween a simple inorganic acid-base reaction and an organic nucleophilic substitution is illustrative of these principles. It also utilizes the “curved arrow” notation that we first presented in Chapter 1 when we discussed the shifting of electron

30、pairs to interconvert resonance forms. Here, however, we use the arrows to show the electron movement that takes place when bonds break or form in the course of a chemical reaction. As your first example of a polar organic reaction, pay close attention to the details here: We will be returning to th

31、ese principles repeatedly. The curved arrow convention is an especially powerful tool to help you understand how and why chemical reactions of organic compounds take place. The more you understand, the less you will have to memorize. 2-3.Functional Groups One look at the 16 classes of organic compou

32、nds in Table 2-3 (and these are only some of the most common ones!) will immediately tell you how complicated organic chemistry can become. At the same time, however, closer inspection reveals features of these categories that can greatly simplify learning in this course. Each 20 Chapter 2 STRUCTURE

33、 AND REACTIVITY * You may have learned in freshman chemistry that the bond strength between H and A in HA relates inversely to its acid strength. This correlation is not as general as you may have been led to believe: It holds only when the acids being compared are all from the same column of the pe

34、riodic table, such as the hydrogen halides. It fails, for instance, in the series CH4, NH3, H2O, HF, where the acid strength increases as the bond strength goes up! The reason? Acidity relates to heterolytic bond cleavage to give ions, whereas bond strength relates to homolytic bond cleavage to give

35、 uncharged species. The two processes are very different. Differences in atomic electronegativity affect heterolytic bond cleavage (and therefore acidity) much more. 1559T_ch02_18-37 10/22/05 1:11 Page 20 Keys to the Chapter 21 compound class is characterized by a specific atomic grouping called a f

36、unctional group. Notice that only nine different elements are represented: C, H, S, N, O, and the four halogens. In fact, 11 of these classes contain only carbon, hydrogen, and oxygen. Knowledge of the characteristics of these atoms and the bonds between them, as we will see, will tell us the proper

37、ties of the functional groups in which they appear. The functional groups will, in turn, provide the key to understanding the chemistry of all the members of the category. Thus, all members of the “alcohol” class of compounds, for instance, have certain common physical and chemical properties, resul

38、ting from the presence of the OH group in all of them. This kind of generalizable, qualitative similarity among compounds in any given class allows organic chemistry to be learned in a structured, orga- nized, and, above all, logical way. Functional groups consist either of polarized bonds, whose at

39、oms can attract other polarized or charged species, thereby leading to reactions, or of multiple (double or triple) bonds that also show reactivity for rea- sons well explore later. Functional groups are the parts of molecules that most often take part in chemical re- actions of those molecules. The

40、y are the “centers of reactivity” of moleculeswhere the action is. The most fundamental feature of alkanes relates to this concept of functional groups: Alkanes dont have any. Well see the consequences of this in the next chapter. 2-4 and 2-5.Structures and Names for Alkanes There are a lot of organ

41、ic compounds. Table 2-4 lists the numbers of isomers of just alkanes, and only goes up to 20 carbons, and already over half a million structures are possible! Imagine how many more structures can be manufactured when functional groups are present, or when the molecules get larger. Obviously not all

42、these possible structures exist in nature or have been prepared in laboratories. Nonetheless, over 80 million different compounds are known at present, and nomenclature is the language that allows anyone interested in any of these materials to communicate about them in a clear and sensible way. The

43、text presents brief descriptions of the problems associated with naming compounds before the systematic procedures of the IUPAC were developed. It then goes on to introduce just the rules necessary for naming sim- ple alkanes: molecules containing only carbon and hydrogen atoms and having only singl

44、e bonds holding the atoms together. Only four rules are needed at this stage: 1. Identify the longest carbon chain (the parent chain) in the molecule and name it. 2. Name all groups attached to this chain as substituents. 3. Number the carbon atoms of the parent chain from the end that gives the one

45、 containing the first substituents the lowest possible number. 4. Assemble the name, using the proper format. Although examples are given in the text and there are lots of problems for you to practice on, here are four additional worked-out examples to further clarify some fine points of the procedu

46、re. Example 1. Analysis: The longest chain contains seven carbons, so this is a heptane. However, there are two ways to iden- tify a seven-carbon chain (see numbering). Which one is the parent? The rules specify that, in case of a tie for longest chain, the one with the most substituents is chosen a

47、s the parent. The seven-carbon chain labeled “a” CH3CH2CH2CHCH2CH2CH3 CHCH2CH3 CH3 Name: 7654321 321 Chain “b” (Proper parent stem) Improper numbering Chain “a” Proper numbering 1559T_ch02_18-37 10/22/05 1:11 Page 21 has one substituent (a sec-butyl group on carbon 4). The seven-carbon chain labeled

48、 “b” has two substituents (a methyl on carbon 3 and a propyl on carbon 4), so it wins. The molecule is called 3-methyl-4-propylheptane. Example 2. Analysis: The main chain here is unambiguous and 14 carbons longthe parent is tetradecane. Which is the correct numbering direction, however? Most of the

49、 groups are close to the right-hand end and will have low numbers if we number right-to-left. But that is not the criterion for determining which way to number the chain. The rule says to number in the direction that gives the carbon containing the first substituent the low- est possible number. If

50、we number from right-to-left, the first substituted carbon is C3; if left-to-right, it is C2. So, left-to-right is correct, and the molecules name is 6,10-diethyl-2,8,8,10,11,12-hexamethyltetradecane. Even though the name that comes from numbering the other way has mostly low numbers, (5,9-diethyl-3

51、,4,5,7,7,13- hexamethyltetradecane), it is wrongits lowest number is a “3,” and the correct names lowest number is a “2.” Example 3. Analysis: A hexane. Numbering left-to-right gives 2,3,4,4,5-pentamethylhexane; right-to-left gives 2,3,3,4,5- pentamethylhexane. The choice is made by comparing substi

52、tuent numbers from lowest to highest. The name with the lower number at the first point of difference is the winner. So 2,3,3,4,5 is preferred over 2,3,4,4,5. Example 4. Analysis: A nonane with a complicated substituent on carbon 5. Rule 3 illustrates what to do. Number the sub- stituent carbons fro

53、m the point of attachment to the main chain, outward along the substituents longest chain. The substituent has three carbons, so it has a name based on propyl. Then add appropriate numbers and names for groups attached to the substituent chain. So, 1,1,2-trimethylpropyl is the complete name of the s

54、ubstituent. Now, attach the substituents name to the name of the main chain to get the name of the whole molecule: 5-(1,1,2-trimethylpropyl)nonane. Note punctuation. Its not hard, but it does take some careful analysis. CH3CH CH3 CH3 CH3CH3CH3CH3 CHCHCName: CH3CHCH2CH2CH2CHCH2CCH2C CH3CH3CH3CH3 CH3

55、CH3 CH2CH3CH3CH2 CHCHCH2CH3Name: 231 12 22 Chapter 2 STRUCTURE AND REACTIVITY 1559T_ch02_18-37 10/22/05 1:11 Page 22 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

56、 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 com- plicated have been given names that are well understood by people in the business but may seem random to the uninitiated. Several of these are m

57、entioned 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-tri

58、hydroxy-6-(hydroxymethyl)oxa- cyclohexane (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. S

59、ee, 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-iso-