22.3 1 FOCUS Objectives 22.3.1 Explain why structural isomers have different properties. 22.3.2 Describe the conditions under which geometric isomers are possible. 22.3.3 Identify optical isomers. Guide for Reading Build Vocabulary Word Parts Explain that the term isomer derives from Greek terms meaning same parts. Ask, What is another chemical term that begins with the prefix iso- and what does it mean? (Isotopes are atoms of the same elements with different masses.) Reading Strategy Identify Main Ideas ave students use headings and key concepts to identify the main ideas of this section. 2 INSTRUCT 22.3 Isomers Guide for Reading Key Concepts ow do the properties of structural isomers differ? What are the two types of stereoisomers? Vocabulary isomers structural isomers stereoisomers geometric isomers trans configuration cis configuration asymmetric carbon optical isomers Reading Strategy Building Vocabulary As you read, write definitions of the four types of isomers described. Be sure your definitions differentiate among the types. The retinal molecule in the rod and cone cells of your eye has a hydrocarbon skeleton. The first step in the process of vision occurs when light enters your eye and strikes a cell containing retinal. The light causes a change in the three-dimensional structure of the retinal molecule. The structures before and after the light strikes are examples of isomers. In this section, you will study different types of isomers. You may have noticed that the structures of some hydrocarbons differ only in the positions of substituent groups or of multiple bonds in their molecules. Look at the structural formulas for butane and 2-methylpropane shown below and at their ball-and-stick models in Figure 22.7. Even though both compounds have the formula C 4 10, their boiling points and other properties differ. Because their structures are different, they are different substances. Compounds that have the same molecular formula but different molecular structures are called isomers. C 2 C 2 C Butane (C 4 10 ) 2-methylpropane (C4 10 ) (bp 0.5 C) (bp 10.2 C) Butane and 2-methylpropane represent a category of isomers called structural isomers. Structural isomers are compounds that have the same molecular formula, but the atoms are joined together in a different order. Structural isomers differ in physical properties such as boiling point and melting point. They also have different chemical reactivities. In general, the more highly branched the hydrocarbon structure, the lower the boiling point of the isomer compared with less branched isomers. For example, 2-methylpropane has a lower boiling point than butane. ave students read the introduction. Ask, What do three-dimensional structures of molecules reveal? (how atoms are arranged in space) CLASS Activity Modeling Isomers Purpose Students use models to identify isomers Procedure ave students use model kits to construct isomers of pentane (C 5 12 ) and hexane (C 6 12 ). ave them draw the structural formulas for each isomer. Ask, ow does the number of pentane isomers compare with the number of hexane isomers? (There are more hexane isomers.) Figure 22.7 Both butane and 2-methylpropane have the molecular formula C 4 10. But the atoms in their molecules are arranged in a different order. So these compounds are structural isomers. Butane 2-methylpropane 704 Chapter 22 Section Resources Print Guided Reading and Study Workbook, Section 22.3 Core Teaching Resources, Section 22.3 Review Transparencies, T257 T258 Small-Scale Chemistry Laboratory Manual, Lab 37 Technology Interactive Textbook with ChemASAP, Simulation 28, Problem-Solving 22.18, Assessment 22.3 704 Chapter 22
Stereoisomers Remember that molecules are three-dimensional structures. So molecules with the same molecular formula and with atoms joined in exactly the same order may still be isomers. Stereoisomers are molecules in which the atoms are joined in the same order, but the positions of the atoms in space are different. Two types of stereoisomers are geometric isomers and optical isomers. Geometric Isomers The first category of stereoisomers is based on the presence of a double bond in a molecule. A double bond between two carbon atoms prevents them from rotating with respect to each other. Because of this lack of rotation, groups on either side of the double bond can have different orientations in space. Geometric isomers have atoms joined in the same order, but differ in the orientation of groups around a double bond. Look at the models of 2-butene in Figure 22.8. Two arrangements are possible for the methyl groups with respect to the rigid double bond. In the trans configuration, the methyl groups are on opposite sides of the double bond. In the cis configuration, the methyl groups are on the same side of the double bond. Trans-2-butene and cis-2-butene have different physical and chemical properties. For example, the density of cis-2-butene is 0.616 g/cm 3, and the density of trans-2-butene is 0.599 g/cm 3. The groups attached to the carbons of the double bond do not need to be the same. Geometric isomerism is possible whenever each carbon of the double bond has at least one substituent. C2 trans-2-pentene C2 C2 2-methyl-1-butene cis-2-pentene (no cis, trans isomers) Optical Isomers The second category of stereoisomers occurs whenever a carbon atom has four different atoms or groups attached. A carbon with four different atoms or groups attached is an asymmetric carbon. Look at the models in Figure 22.9. Because, F, Cl, and Br atoms are attached to a single carbon atom, the carbon is asymmetric. The relationship between the molecules is similar to the relationship between right and left hands. Think about an object placed in front of a mirror. If the object is symmetrical, like a ball, then its mirror image can be superimposed. That is, the appearance of the ball and its reflection are indistinguishable. By contrast, a pair of hands is distinguishable even though the hands consist of identical parts. The right hand reflects as a left hand and the left hand reflects as a right hand. The images cannot be superimposed. Many ordinary objects, such as ears, feet, shoes, and bird wings, are similarly related. Trans configuration Cis configuration Figure 22.8 There is a trans and a cis configuration of 2-butene because a methyl group is attached to each carbon of the double bond. Comparing and Contrasting ow are the trans and cis configurations different? Figure 22.9 In these models, four different atoms are attached to the same carbon atom, making the carbon atom an asymmetric carbon. Section 22.3 Isomers 705 Discuss Point out that the number of possible structural isomers for an alkane increases dramatically with increasing numbers of carbon atoms. With more carbon atoms there are more ways to arrange the atoms in space. Remind students that each isomer has a unique set of physical and chemical properties. Name the three isomers of pentane. ave students name the five isomers of hexane they constructed in the Modeling Isomers class activity. Point out that the existence of isomers is a major reason that so many different organic compounds exist. Stereoisomers Discuss Point out that the number of possible isomers for an alkene can be much greater than for an alkane. Draw the complete structural formulas of 1-butene, cis-2-butene, and trans-2- butene on the board to show the three possible isomers for butene (C 4 8 ). Introduce the term geometric isomers and explain how cis-2-butene and trans- 2-butene are different molecules due to the lack of free rotation about the carbon-carbon double bond. TEACER Demo Optical Isomers Purpose Students observe threedimensional models of optical isomers. Materials two ball-and-stick models of CFClBr Procedure Demonstrate how it is not possible to superimpose one model on the other. Answers to... Figure 22.8 The groups are on opposite sides of the double bond in the trans configuration; the groups are on the same side of the bond in the cis configuration. ydrocarbon Compounds 705
Section 22.3 (continued) CONCEPTUAL PROBLEM 22.3 Answers 18. a. * C CO CI b. no asymmetric carbon 19. a. C 2 * C Br F b. no asymmetric carbon of eptane Objective After completing this activity, students will be able to: build ball-and-stick models of heptane isomers and name them. Skills Focus Using models, observing, analyzing Prep Time 5 minutes Class Time 25 minutes Expected Outcome Students will model and name nine structural isomers of heptane. Analyze and Conclude 1. heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,2-dimethylpentane, 3,3-dimethylpentane, 3-ethylpentane, 2,2,3-trimethylbutane 2. 4 3. Each is a unique compound. For Enrichment L3 Students can model, draw, and name the 18 structural isomers of octane. Simulation 28 Play the isomer game of Pick the Pairs. CONCEPTUAL PROBLEM 22.4 Practice Problems Identifying Asymmetric Carbon Atoms Compounds with the following formulas are alcohols. The alcohol represented by the formula in a. is the alcohol in rubbing alcohol. Which compound has an asymmetric carbon? a. C b. O 706 Chapter 22 Facts and Figures CC 2 O Mirror Images Tell students that mirror images exist in nature. For example, 99% of the shells of some snails are right-handed spirals and 1% are left-handed spirals. The right- and lefthanded spirals are mirror images. Other species have shells that are normally left-handed. Pairs of molecules that differ only in the way that four different groups are arranged around a central carbon atom are called optical isomers. The term optical is used to describe this type of isomer because the term is related to the concept of objects and mirror images. The molecules shown in Figure 22.9 are optical isomers. The models cannot be superimposed because they are mirror images of each other. Try to visualize the models in three dimensions. Imagine rotating one model 180. At first, it might appear that the models are now identical. owever, the direction of each bond in space is not the same. For example, the carbon-bromine bond would be pointing away from you rather than toward you. No matter how you turn the models, you cannot make the models look exactly alike. Unless bonds are broken, these molecules cannot be made to look alike. Checkpoint Analyze Identify the relevant concepts. An asymmetric carbon atom has four different groups attached. Solve Apply concepts to this situation. Draw the structures in a more complete form to determine the presence of a carbon atom with four different groups attached. Practice Problems 18. Identify the asymmetric carbon, if any, in each of the following structures. a. CCO b. CO Cl 19. Identify the asymmetric carbon, if any, in each of the following structures. a. C 2 C Br b. C 2 Cl 2 F ow does the presence of a carbon-carbon double bond in a molecule result in geometric isomers? a. The central carbon has two groups attached. It is not asymmetric. b. The central carbon has one, one O, one, and one C 2 group attached. Because these four groups are different, the central carbon is asymmetric. In a structural formula, an asymmetric carbon is often marked with an asterisk. Problem-Solving 22.18 Solve Problem 18 with the help of an interactive guided tutorial. 706 Chapter 22
of eptane Purpose To build ball-and-stick models and name the nine structural isomers of heptane (C 7 16 ). Materials ball-and-stick molecular model kit ( Colors used to represent elements in the kit may not match colors used to represent elements in this book.) eptane 22.3 Section Assessment Procedure 1. Build a model for the straight-chain isomer of C 7 16. Draw the structural formula and write the IUPAC name for this isomer. 2. Move one carbon atom from the end of the chain and add a methyl substituent to the chain. Draw the structural formula and name this isomer. 3. Move the methyl group to a new position on the chain. Then draw and name this third isomer. Is there a third position that the methyl group can be moved to on the chain of six carbons to form yet another different isomer? 4. Make other structural isomers by shortening the longest straight chain and using the removed carbons as substituents. Draw the structural formulas and name each isomer. Analyze and Conclude 1. What are the names of the nine structural isomers of C 7 16? 2. What is the shortest possible straight carbon chain in the isomers? 3. Why does each structural isomer have its own unique name? 3 ASSESS Evaluate Understanding ave students explain what the term isomer means and then describe the differences among structural isomers, geometric isomers, and optical isomers. ave them draw examples of each type of isomer. Reteach L1 Use ball-and-stick models to review the requirements for classifying two molecules with the same molecular formula as structural, geometric, or optical isomers. Answers should include (1) same molecular formula, different structures; (2) atoms joined in a different order; (3) presence of a double bond; and (4) presence of an asymmetric carbon. 20. Key Concept Explain why you would expect two structural isomers to have different boiling points. 21. Key Concept Name the two types of stereoisomers. 22. What structure must be present in a molecule for geometric isomers to exist? 23. Key Concept ow can an asymmetric carbon be identified? 24. What is the relationship between two molecules that are optical isomers? 25. Draw structural formulas for the following alkenes. If a compound has geometric isomers, draw both the cis and trans forms. a. 1-pentene b. 2-hexene c. 2-methyl-2-hexene Writing Instructions Write instructions that could be used to decide whether two compounds are isomers and, if so, what type of isomers they might be. (1) ow can you tell if two compounds are isomers? (2) ow can you tell if two compounds are structural isomers? (3) ow can you tell if two compounds can be geometric isomers? (4) ow can you tell if two compounds can be optical isomers? Assessment 22.3 Test yourself on the concepts in Section 22.3. Section 22.3 Isomers 707 If your class subscribes to the Interactive Textbook, use it to review key concepts in Section 22.3. with ChemASAP 20. Structural isomers are different compounds with different physical properties. 21. geometric isomers and optical isomers 22. when each carbon in a double bond has at least one substituent 23. It has four different substituents. 24. The relationship is similiar to the relationship between right and left hands, which can t be superimposed. Section 22.3 Assessment 25. a. b. c. C 2 C 2 1-pentene C 2 C 2 trans-2-hexene C 2 C 2 2-methyl-2-hexene C 2 C 2 cis-2-hexene Answers to... Checkpoint The double bond prevents rotation around the carbon atoms, which allows for different spatial arrangements of substituents around the bond. ydrocarbon Compounds 707
Small-Scale LAB ydrocarbon Isomers Objective After completing this activity, students will be able to: write line-angle formulas for hydrocarbons. Prep Time 5 minutes Class Time 20 minutes Expected Outcome Students will learn to write line-angle formulas for hydrocarbons. Analyze 1. Students should draw formulas for pentane, 2-methylbutane, and 2,2-dimethylpropane. 2. Subtract from four the number of lines drawn to any point. 3. (C 2 ) 2 butane ( ) 2 C methylpropane For Enrichment L3 There are eight isomers of C 7 16. Draw their line-angle formulas and name them. 3-methylhexane 2,2-dimethylpentane 3,3-dimethylpentane heptane 2-methylhexane 2,3-dimethylpentane Small-Scale LAB ydrocarbon Isomers Purpose To draw line-angle formulas and name some of the isomers in gasoline. Materials toothpicks modeling clay Procedure Gasoline is a complex mixture of hydrocarbon molecules. Each molecule contains between five and ten carbon atoms. Many of the components of gasoline are isomers with the same molecular formula. These components include the isomers of pentane. Study the formulas and names of the isomers of C 5 12 in the chart below. Make a model of each isomer using toothpicks and modeling clay. Compare the models and make accurate drawings on your paper. Formulas Representing Isomers of C 5 12 Condensed Line-angle Space-filling C 2 C 2 C 2 pentane CC 2 2-methylbutane C 2, 2-dimethylpropane Analyze Using your experimental data, answer the following questions. 1. Complete structural formulas include all the atoms and all the chemical bonds in a molecule. Draw the complete structural formula for each isomer of C 5 12 in the chart. 2. In a line-angle formula, each line represents a carboncarbon bond. Each end of a line, as well as the intersection of lines, represents a carbon atom. All hydrogen atoms are understood. Knowing that carbon always forms four bonds in organic compounds, explain how to determine the number of hydrogen atoms bonded to each carbon in a line-angle formula. 3. Because butane can vaporize readily, it is used in the formulations of gasolines in cold climates during winter. Draw the condensed structural formulas, the line-angle formulas, and the space-filling formulas for the two isomers of butane (C 4 10 ). Make models of each isomer. You re The Chemist The following small-scale activities allow you to develop your own procedures and analyze the results. 1. Analyze It! Gasoline contains isomers of hexane. Draw the line-angle formulas and name the five isomers of C 6 14. Make a model of one isomer and convert that model into the other four. 2. Design It! Gasoline also contains small amounts of the six isomers of pentene. Two of the isomers are cis and trans configurations of the same structural isomer. Experiment with your models to make these isomers. Use two toothpicks to represent a double bond. Draw structural formulas for the cis and trans isomers, and line-angle formulas for the others. 2,4-dimethylpentane 2,2,3-trimethylbutane 708 Chapter 22 You re the Chemist 1. hexane 2-methylpentane 2. C 2 C 2 1-pentene C 2 cis-2-pentene trans-2-pentene C 2 3-methylpentane 2,2-dimethylbutane 2,3-dimethylbutane 3-methyl-2-butene C 3-methyl-1-butene C 2 2-methyl-1-butene 708 Chapter 22