22.3. Lesson 22.3 Isomers. Overview

Transcription

1 22.3 Lesson 22.3 Isomers Objectives Explain how the properties of constitutional isomers differ Identify two types of stereoisomers. Lesson Links Ch. 22 Core TR: Section 3 Review Ch. 22 Small Scale Lab 37: Molecular Structure of Hydrocarbons 22.3 Lesson Overview (PowerPoint file) 22.3 Chemistry Tutorial: Identifying Asymmetric Carbon Atoms Chapter 22 Online Student Edition 22.3 Kinetic Art: Asymmetric Carbons Study WB Chapter 22 Lesson 3 Overview/Materials Overview In this lesson you will cover the way in which different arrangements of atoms within isomers lead to differing physical and chemical properties. You will also teach students to identify cis-trans isomers and enantiomers. Classroom Materials Standard 2 Block 1 Build Background: folding chair Constitutional Isomers: ball-and-stick molecular model kit Stereoisomers: ball-and-stick molecular model kit with at least 5 differentcolored balls Quick Lab: ball-and-stick molecular model kit Standard There are no items. page 1 of 6

2 Chemistry & You Engage Have students read the Chemistry & You feature on p Explain that retinal is a molecule in the rod and cone cells of the retina. Retinal straightens when it is excited by light energy. Chapter 22 Online Student Edition Ask What information does the threedimensional structure of a molecule reveal? ( how its atoms are arranged in space) Build Background Engage Bring a folding chair to class and show it to students. Ask What will happen to the structure of this chair if I open it? (It will change.) Ask Will opening the chair change the kinds of atoms in the chair? (no) Point out that opening or folding the chair is an analogy for isomerization. Tell students to keep this analogy in mind as they proceed with the lesson. Materials: folding chair Constitutional Isomers Explain Point out that the number of possible constitutional isomers for an alkane increases dramatically with increasing numbers of carbon atoms. Explain that 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. Point out that the existence of isomers is a major reason that so many different organic compounds exist. Figure 22.8 Figure 22.9 page 2 of 6

3 Constitutional Isomers Explore Use a class activity to have students use models to identify isomers. Have students use ball-and-stick molecular model kits to form the isomers in Figures and Provide students with enough pieces to create butane and trans-2-pentene, and have them practice rearranging the structures to form each molecule's isomers. Then have students use the model kits to construct as many isomers of pentane (C5H12) and hexane (C6H14) has they can. Have them draw the structural formula for each isomer. Ask How does the number of pentane isomers compare with the number of hexane isomers? (There are more hexane isomers.) Materials: ball-and-stick molecular model kit Quick Lab Purpose After completing this activity, students will be able to build ball-and-stick models of heptane isomers and name them. Materials ball-and-stick molecular model kit Prep Time 5 minutes Class Time 25 minutes Expected Outcome Students will model and name nine page 3 of 6

4 constitutional isomers of heptane. For Enrichment Students can model, draw, and name the 18 constitutional isomers of octane. Stereoisomers Explain Point out that the number of possible isomers for an alkene can be greater than for an alkane. Draw the complete structural formulas of 1-butene, cis-2-butene, and trans-2-butene on the board and show the three possible isomers for butene (C4H8). Introduce the term cis-trans isomers and explain how cis-2-butene and trans-2-butene are different molecules due to the lack of free rotation about the carboncarbon double bond. Stereoisomers Explore Use a class activity to allow students to observe three-dimensional models of enantiomers Kinetic Art: Asymmetric Carbons Have students use ball-and-stick molecular model kits to create models of both stereoisomers of CHFClBr. Be sure they have four different-colored "atoms" attached to the carbon center. Students should manipulate the models to prove to themselves that it is not possible to superimpose one model onto the other. Students can also observe the models with 22.3 Kinetic Art: Asymmetric Carbons on PearsonChem.com to compare the different arrangements. Materials: ball-and-stick molecular model kit with at least 5 different-colored balls page 4 of 6

5 Stereoisomers Extend Tell students that mirror images exist in nature. For example, 99% of the shells of some snail species are right-handed spirals and 1% are left-handed spirals. The rightand left-handed spirals are mirror images. Other species have shells that are normally left-handed. Have students create posters with pictures of this, and other examples, of nature's asymmetry. Flatfish (eyes on one side of the head) and fiddler crabs ( one large claw and one small claw) also demonstrate asymmetry. Assess and Remediate Evaluate Have students verbally explain what the term isomer means and then describe the differences among constitutional isomers, cis-trans isomers, and enantiomers in 60 seconds or less. Ask students to draw an example of each type of isomer. Chapter 22 Online Student Edition Then, have students complete the 22.3 Lesson Check. Remediate Use ball-and-stick models to visually explain the requirements for classifying two molecules with the same molecular formula as constitutional isomers, cis-trans isomers, or enantiomers. Differentiated Instruction Struggling Students Create ball-and-stick models of various constitutional isomers and cis-trans isomers. Have students practice identifying the type of isomer and in the case of a cis-trans isomer, the configuration. Have them use Chemistry Tutorial: Identifying Asymmetric Carbon Atoms on PearsonChem.com to practice identifying enantiomers Chemistry Tutorial: Identifying Asymmetric Carbon Atoms Ch. 22 Core TR: Section 3 Review Study WB Chapter 22 Lesson 3 page 5 of 6

6 Special Needs Students Demonstrate that one of your hands is the mirror image of the other when placed palms together. Then, have students place both hands palm down and overlap them. Point out that in this position their hands are not mirror images of each other, so they are not superimposable on each other. Tell students to keep this activity in mind as they discuss asymmetric carbons. Advanced Students Have students research the origin of the International Union of Pure and Applied Chemistry (IUPAC). Tell then to find out when, where, and why the first international congress was held. Findings should be presented in a written report. Focus on ELL and Language Write the term asymmetric carbon on the board, and pronounce it for the class. Remind students that in biology, animals that exhibit symmetry have body parts that are mirror images of each other if the animal is divided in half by an imaginary plane. Explain that the prefix a- means "without," so an asymmetric carbon is a carbon with four different molecules attached to it. Study WB Chapter 22 Lesson 3 Ch. 22 Core TR: Section 3 Review My Notes Homework page 6 of 6