Guide for Reading. Key Concepts How are organic compounds classified? What is a halocarbon? How may halocarbons be prepared?

Transcription

1 23.1 Introduction to Functional Groups Connecting to Your World The lights dim and the audience chatter subsides as the musicians enter the hall. All the musicians wear black, and from a distance they look nearly identical. But their differences become apparent when they pick up their instruments. As the conductor signals the beginning of the piece, a single flute is heard. The warm sounds of the stringed instruments join in, and soon each musician contributes a unique sound to the music. In a similar way, one hydro is nearly identical to another until it picks up a functional group. In this section, you will learn how functional groups determine the character of organic compounds. Functional Groups In Chapter 22, you learned about hydro chains and rings the essential components of every organic compound. Yet in most chemical reactions involving organic molecules, the saturated hydro skeletons of the molecules are chemically inert. ow, then, can there be hundreds of different kinds of organic reactions? Most organic chemistry involves substituents, which are groups attached to hydro chains. The substituents of organic molecules often contain oxygen, nitrogen, sulfur, and/or phosphorus. They are called functional groups because they are the chemically functional parts of the molecules. A functional group is a specific arrangement of atoms in an organic compound that is capable of characteristic chemical reactions. Most organic chemistry is functional-group chemistry. rganic compounds can be classified according to their functional groups. Figure 23.1 shows several consumer products that contain various functional groups. The symbol R represents any chains or rings attached to the functional group. Because the double and triple bonds of alkenes and alkynes are chemically reactive, they are also considered functional groups. Table 23.1 on the following page lists the functional groups that you will learn about in this chapter and in Chapter 24. You will find it helpful to refer to this table throughout the chapter. Figure 23.1 Many consumer products contain hydro derivatives. The hydro skeletons in these products are chemically similar. Functional groups give each product unique properties and uses. Section Resources Print Guided Reading and Study Workbook, Section 23.1 Core Teaching Resources, Section 23.1 Review Transparencies, T Guide for Reading Key Concepts ow are organic compounds classified? What is a halo? ow may halos be prepared? Vocabulary functional group halos alkyl halides aryl halides substitution reaction Reading Strategy Making and Using Tables As you read, examine the tables carefully. Make a table in your notebook that organizes the material in this section. Section 23.1 Introduction to Functional Groups 725 Technology Interactive Textbook with ChemASAP, Assessment 23.1 Go nline, Section FCUS bjectives Explain how organic compounds are classified Identify halos and the IUPAC rules for naming halos Describe how halos can be prepared. Guide for Reading Build Vocabulary Paraphrase ave students explain in their own words the meaning of the term substitution reaction. Reading Strategy Use Prior Knowledge Review Chapter 22. A thorough knowledge of hydros is needed before students learn about substituents. A review of intermolecular forces also will be helpful. 2 INSTRUCT Ask, ow do functional groups determine the character of organic compounds? (Each functional group has certain chemical properties that affect the behavior of any compound that contains the functional group. Functional Groups Discuss Stress the importance of structure in the chemistry of organic molecules. Molecular shape is dictated by bonding functional groups. Ask students to hypothesize about the structure of a molecule related to its taste or smell. (Both senses rely on the ability of chemoreceptor molecules on cell surfaces, which recognize a molecule by its shape.) Functional Groups 725

2 Section 23.1 (continued) Table 23.1 ave students study the table. Encourage them to memorize the general compound structure for each functional group. It may be helpful to list specific examples of compounds for each functional group so that students become more familiar with these groups in larger molecules. Include examples of alkenes and alkynes. Ask, Why are the double and triple bonds considered to be functional groups? (They are chemically reactive.) What do most of the functional groups in the table have in common? (Most functional groups contain oxygen.) CLASS Activity dors of Functional Groups Purpose Students will learn that compounds containing the same functional group have similar chemical and physical properties. Materials 2-propanol, propanoic or butanoic acid, butyl amine, vanillin or cinnamaldehyde, octyl acetate or ethyl butyrate Safety ave students carefully waft vapors of the compounds in order to detect their odors. Remind students that normally it is not safe to smell chemicals. An exception is being made for these known compounds. Procedure Bring samples of organic compounds to class. Allow students to observe the characteristic odors associated with alcohols (2-propanol), acids (propanoic or butanoic acid), amines (butyl amine), aldehydes (vanillin or cinnamaldehyde), and esters (the orange odor of octyl acetate and the pineapple odor of ethyl butyrate). ave students record their observations, and then make a summary table on the board. Ask students to consider what other properties molecules with the same functional groups might have in common. Figure 23.2 The IUPAC rules for naming halos are based on the name of the parent hydro. Structural formulas and space-filling models of halos. a The common name for chloromethane is methyl chloride. b The common name for chloroethene is vinyl chloride. c The common name for chlorobenzene is phenyl chloride. 726 Chapter 23 Table 23.1 Differentiated Instruction Gifted and Talented L3 ave students research examples in which the structure of a chemical plays an important role in a biological process. Suggestions for research include neurotransmitters, such as acetylcholine, and hormones, such as rganic Compounds Classified by Functional Group Compound type Compound structure Functional group alo R X (X F, Cl, Br, or I) alogen Alcohol R ydroxyl Ether R R Ether Aldehyde Ketone Carboxylic acid Ester a Cl C C Chloromethane (methyl chloride) b Cl Chloroethene (vinyl chloride) estradiol and testosterone. ave students research how environmental pollutants can interfere with the action of these natural chemicals. c Carbonyl Carbonyl Carboxyl Ester Amine R N 2 Amino Amide R C R C R R C R C R R C N R alogen Substituents Amido alos are a class of organic compounds containing covalently bonded fluorine, chlorine, bromine, or iodine. A halo is a containing compound with a halogen substituent. The IUPAC rules for naming halos are based on the name of the parent hydro. The halogen groups are added as substituents. Examples of IUPAC names for several simple halos along with their structural formulas and spacefilling models are given in Figure Common names are in parentheses. Common names of halos consist of two parts. The first part names the hydro portion of the molecule as an alkyl group, such as methyl- or ethyl-. The second part gives the halogen with an -ide ending. Cl Chlorobenzene (phenyl chloride) 726 Chapter 23

3 Table 23.2 Names of Some Common Alkyl Groups Name Alkyl group Remarks Isopropyl Isobutyl Secondary butyl (sec-butyl) Tertiary butyl (tert-butyl) Vinyl Phenyl C C C 2 C 2 C C tertiary C C primary secondary The prefix iso- is used when there is a methyl group on the second from the unsubstituted end of the longest chain. group is bonded to one other ; it is a primary. group is bonded to two other s; it is a secondary. group is bonded to three s; it is a tertiary. When used as an alkyl group in giving compounds common names, this group is called vinyl. Phenyl is derived from benzene. n the basis of their common names, halos in which a halogen is attached to a of an aliphatic chain are called alkyl halides. The number of atoms attached to the alkyl group determines whether the is primary, secondary, or tertiary. Table 23.2 lists the names of alkyl groups other than methyl, ethyl, and propyl. alos in which a halogen is attached to a of an arene ring are called aryl halides. The attractions between halo molecules are primarily the result of the weak van der Waals interactions called dispersion forces. These attractions increase with the degree of halogen substitution. Thus, more highly halogenated organic compounds have higher boiling points, as illustrated in Table 23.3 on the next page. Very few halos are found in nature, but they can be readily prepared and used for many purposes. For example, halothane (2-bromo- 2-chloro-1,1,1-trifluoroethane) is used as an anesthetic. ydrofluoros are used as refrigerants in automobile air-conditioning systems. Figure 23.3 alothane is a halo that is used as an anesthetic. alogen Substituents Table 23.2 Display the table using a transparency and overhead projector and discuss the rules for naming alkyl halides. Write examples of primary, secondary, tertiary, vinyl, and aryl halides on the board. elp students to recognize the alkyl portion of the molecule by circling it in a different color chalk. Write the structure of the original parent hydro next to the alkyl halide. After students have learned the meaning of the terms primary, secondary, and tertiary, point out the derivation of the common names: secbutylbromide and tert-butylbromide. CLASS Activity alo Structures and Names Divide the class into groups of two or three students. ave each group practice naming halos from structural formulas and writing structural formulas from their names. Students in each group should take turns devising questions and giving answers. Discuss Use examples from this section to show how a chemist can start with a simple alkane, change it to an alkyl halide, then change the halide to an alcohol. Point out that this is one way organic chemists synthesize molecules. Give an example of a halo. Section 23.1 Introduction to Functional Groups 727 Facts and Figures Anesthetics Contrary to popular belief, some anesthetics were used during the Civil War. pium was sometimes added to wine or liquor, if it was available. Chloroform and ether were not always available on the battlefield. And most surgeries, especially amputations, were done without anesthesia. Differentiated Instruction English Learners Encourage students to practice writing and then naming the structures of 1-, 2-, and 3- alkyl halides. ave them build a model of one of these structures and discuss how it is produced from the corresponding alkane. Answers to... Cl chloromethane, Functional Groups 727

4 Section 23.1 (continued) Table 23.3 ave students study the table. Point out the trend in boilingpoint temperatures. Ask, What can you infer about the cause of this pattern in boiling-point temperatures? (The more halogen substituents a molecule has, the greater will be the attraction of the molecule to other halo molecules.) Substitution Reactions TEACER Demo Testing for alos Purpose Students will observe the Beilstein test for determining the presence of halogen functionalities in organic compounds. Point out that organic chemists often perform qualitative chemical tests to identify unknown organic compounds. Safety Wear safety goggles and a laboratory apron while performing the demo. Materials copper wire, Bunsen burner, forceps, bromobenzene, benzoic acid Procedure Prepare a piece of copper wire with a small loop in one end. eat the loop in a Bunsen burner flame and allow the wire to cool. Next, dip the loop into a sample of aryl halide (bromobenzene). eat the wire in the flame again. Repeat the test with a negative control, benzoic acid. Expected utcomes A green flame will be produced with bromobenzene, indicating the presence of a halogen. The flame will not be green with benzoic acid. For: Links on Substitution Reactions Visit: Web Code: cdn Chapter 23 Table 23.3 Molecular Masses and Boiling Points of Chloromethanes Molecular structure * Included for purposes of comparison. Substitution Reactions rganic reactions often proceed more slowly than inorganic reactions. This is because organic reactions commonly involve the breaking of relatively strong covalent bonds. Catalysts are often needed. Many organic reactions are complex, often producing a mixture of products. The desired product must then be separated by distillation, crystallization, or other means. A common type of organic reaction is a substitution reaction, in which an atom, or a group of atoms, replaces another atom or group of atoms. A halogen can replace a hydrogen atom on an alkane to produce a halo. The symbol X stands for a halogen in this generalized equation. R + X 2 R X + X Alkane alogen alo ydrogen halide Sunlight or another source of ultraviolet radiation usually serves as a catalyst. From the generalized equation, you can write a specific one. Even under controlled conditions, this simple halogenation reaction produces a mixture of mono-, di-, tri-, and tetrachloromethanes. Name What is a substitution reaction? Molar mass C 4 * Methane Cl C 2 Cl 2 CCl 3 CCl 4 Chloromethane (methyl chloride) Dichloromethane (methylene chloride) Trichloromethane (chloroform) Tetrachloromethane ( tetrachloride) C 4 + Cl 2 UV light Cl + Cl Boiling point ( C) Methane Chlorine Chloromethane ydrogen chloride Download a worksheet on Substitution Reactions for students to complete, and find additional teacher support from NSTA SciLinks. Facts and Figures Perc Perchloroethylene (PCE, or perc) is the solvent used in about 95 percent of drycleaning businesses in the United States. This halo has been the subject of close government and public scrutiny for more than 20 years because there is some evidence that it has adverse health effects, including cancer, in humans. Because government agencies in the United States and around the world have not agreed about the potential of environmental exposure to PCE to cause health problems, use of the compound has not been restricted. 728 Chapter 23

5 Treating benzene with a halogen in the presence of a catalyst causes the substitution of a hydrogen atom in the ring. Iron compounds are often used as catalysts for aromatic substitution reactions. For example, a rusty nail dropped in the reaction flask can act as a catalyst. Br catalyst Br 2 Br alogens on chains are readily displaced by hydroxide ions to produce an alcohol and a salt. The general reaction is as follows. R X R + X - alo ydroxide ion Alcohol alide ion Chemists usually use aqueous solutions of sodium or potassium hydroxide as the source of hydroxide ions. The chemical equations for two specific examples are shown below. I1l 2 + K1aq 2 2 1l 2 + KI1aq 2 Iodomethane Potassium Methanol Potassium (methyl iodide) hydroxide iodide C 2 Br1l 2 + Na1aq 2 2 C 2 1l 2 + NaBr1aq 2 Bromoethane Sodium Ethanol Sodium (ethyl bromide) hydroxide bromide Fluoro groups are not easily displaced. Thus, fluoros are seldom, if ever, used to make alcohols Section Assessment 1. Key Concept ow are organic compounds classified? 2. Key Concept What is a halo? 3. Key Concept ow can a halo be prepared? 4. Identify the functional group in each structure. a. b. C 2 N 2 c. C d. C 2 C 2 Br e. C 2 C 2 5. Give the structural formula for each compound. a. isopropyl chloride b. 1-iodo-2,2-dimethylpentane c. p-bromotoluene 6. Write the names of all possible dichloropropanes that could form from the chlorination of propane. andbook alogens Learn more about the chemistry of halogens on pages R32 R35 of the Elements andbook. Write a short report about the importance of halogens as functional groups in organic chemistry. Assessment 23.1 Check your understanding of the important ideas and concepts in Section withchemasap 3 ASSESS Evaluate Understanding Write the structural formulas for ethene, ethyl chloride, and ethanol on the board. Ask students to name the compounds and identify their functional groups. (double bond, halogen, hydroxyl group) Ask students to choose two of the compounds to illustrate a substitution reaction. (ydroxide ions can replace the chlorine in ethyl chloride to form ethanol.) ave students explain the terms primary, secondary, and tertiary. (They are used to characterize the atom that joins an alkyl group to other groups.) Reteach Work with students to construct a concept map showing the connection between alkanes and their derivatives: alkenes, alkynes, halos, and alcohols. Use propane as a specific example and start with the heading alkanes. Write the structural formula for the propyl derivative in each case. Elements andbook Student paragraphs should describe some of the many uses of halos. In addition to their uses as anesthetics and cleaning agents, they are used for non-stick surfaces (Teflon); pipes and other plastic items (PVC); and refrigerants (chlorofluoros and hydrofluoros). 1. according to their functional groups 2. alos are a class of organic compounds containing covalently bonded fluorine, chlorine, bromine, or iodine. 3. by reacting an alkane with a halogen, catalyzed by UV light 4. a., hydroxyl b. N 2, amino c. C, carboxyl d. Br, halogen e. C C, ether Section 23.1 Introduction to Functional Groups 729 Section 23.1 Assessment 5. a. b. c. C Cl 6. 1,1- dichloropropane; 1,2-dichloropropane; 2,2-dichloropropane; 1,3-dichloropropane I C 2 CC 2 C 2 Br If your class subscribes to the Interactive Textbook, use it to review key concepts in Section Answers to... with ChemASAP a reaction in which an atom or group of atoms replaces another atom or group of atoms Functional Groups 729