《有机化学》课程教学资源（教材文献，英文版）CHAPTER 5 STRUCTURE AND PREPARATION OF ALKENES：ELIMINATION REACTIONS

CHAPTER 5 STRUCTURE AND PREPARATION OF ALKENES ELIMINATION REACTIONS lkenes are hydrocarbons that contain a carbon-carbon double bond. A car bon-carbon double bond is both an important structural unit and an important functional group in organic chemistry. The shape of an organic molecule is infl enced by the presence of this bond, and the double bond is the site of most of the chem- ical reactions that alkenes undergo Some representative alkenes include isobutylene(an industrial chemical), a-pinene(a fragrant liquid obtained from pine trees), and farnese (a naturally occurring alkene with three double bonds). CH3 CH3)2C=CH, Isobutylene Farnesene (used in the production (a major constituent (present in the waxy coating of synthetic rubber) turpentine) found on apple skins) his chapter is the first of two dealing with alkenes; it describes their structure, bonding, and preparation. Chapter 6 discusses their chemical reaction 5.1 ALKENE NOMENCLATURE We give alkenes IUPAC names by replacing the -ane ending of the corresponding alkane with -ene. The two simplest alkenes are ethene and propene. Both are also well known y their common names ethylene and propylene 167 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

167 CHAPTER 5 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS Alkenes are hydrocarbons that contain a carbon–carbon double bond. A car￾bon–carbon double bond is both an important structural unit and an important functional group in organic chemistry. The shape of an organic molecule is influ￾enced by the presence of this bond, and the double bond is the site of most of the chem￾ical reactions that alkenes undergo. Some representative alkenes include isobutylene (an industrial chemical), -pinene (a fragrant liquid obtained from pine trees), and farnesene (a naturally occurring alkene with three double bonds). This chapter is the first of two dealing with alkenes; it describes their structure, bonding, and preparation. Chapter 6 discusses their chemical reactions. 5.1 ALKENE NOMENCLATURE We give alkenes IUPAC names by replacing the -ane ending of the corresponding alkane with -ene. The two simplest alkenes are ethene and propene. Both are also well known by their common names ethylene and propylene. Isobutylene (used in the production of synthetic rubber) (CH3)2C CH2
-Pinene (a major constituent of turpentine) CH3 H CH3 CH3 Farnesene (present in the waxy coating found on apple skins) Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions CH-CH CH3 CH=CH2 IUPAC ethene IUPAC name: propene Ethylene is an acceptable synonym for ethene in the IUPAC system. Propylene, isobuty lene, and other common names ending in -ylene are not acceptable IUPAC names thylene was known to chemists in the eigh- Ethylene is the cornerstone of the world's mam- teenth century and isolated in pure form in moth petrochemical industry and is produced in vast 1795. An early name for ethylene was gaz olefi. quantities. In a typical year the amount of ethylene ant(French for oil-forming gas"), a term suggested produced in the United States(5X"lb)exceeds to describe the fact that an oily liquid product is the combined weight of all of its people. In one formed when two gases--ethylene and chlorine -re- process, ethane from natural gas is heated to bring act with each other bout its dissociation into ethylene and hydrogen CHaCH CH2=CH2 H2 Chlorine 1. 2-Dichloroethane Ethane (bp:83° Ethylene Hydrogen This reaction is known as dehydrogenation and is si- The term gaz olefiant was the forerunner of the gen- multaneously both a source of ethylene and one of eral term olefin, formerly used as the name of the the methods by which hydrogen is prepared on an in- class of compounds we now call alkenes dustrial scale. Most of the hydrogen so generated is Ethylene occurs naturally in small amounts as a subsequently used to reduce nitrogen to ammonia plant hormone. Hormones are substances that act as for the preparation of fertilizer. messengers and play regulatory roles in biological processes. Ethylene is involved in the ripening of Similarly, dehydrogenation of propane gives many fruits, in which it is formed in a complex series propene of steps from a compound containing a cyclopropane CH3CH2 CH3-CH3 CH=CH2+ H rIng -CH2-CH2+ other products Propene is the second most important petrochemical and is produced on a scale about half that of ethylene cyclopropane- Almost any hydrocarbon can serve as a starting material for production of ethylene and propene. Cracking of petroleum(Section 2. 13)gives ethylene Even minute amounts of ethylene can stimulate and propene by processes involving cleavage of ripening, and the rate of ripening increases with the carbon-carbon bonds of higher molecular weight concentration of ethylene This property is used to hydrocarbons advantage, for example, in the marketing of ba- The major uses of ethylene and propene are as green by being stored with adequate ventilation to ene and polypropylene plastics, fibers, and films. mit the amount of ethylene present, and then in- These and other applications will be described in duced to ripen at their destination by passing ethyl- Chapter 6. ene over the fruit.* "For a review, see"Ethylene--An Unusual Plant Hormone"in the April 1992 issue of the Journal of Chemical Education (pp. 315-318). Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

168 CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions Ethylene is an acceptable synonym for ethene in the IUPAC system. Propylene, isobuty￾lene, and other common names ending in -ylene are not acceptable IUPAC names. CH2œCH2 IUPAC name: ethene Common name: ethylene CH3CHœCH2 IUPAC name: propene Common name: propylene ETHYLENE Ethylene was known to chemists in the eigh￾teenth century and isolated in pure form in 1795. An early name for ethylene was gaz oléfi- ant (French for “oil-forming gas”), a term suggested to describe the fact that an oily liquid product is formed when two gases—ethylene and chlorine—re￾act with each other. The term gaz oléfiant was the forerunner of the gen￾eral term olefin, formerly used as the name of the class of compounds we now call alkenes. Ethylene occurs naturally in small amounts as a plant hormone. Hormones are substances that act as messengers and play regulatory roles in biological processes. Ethylene is involved in the ripening of many fruits, in which it is formed in a complex series of steps from a compound containing a cyclopropane ring: Even minute amounts of ethylene can stimulate ripening, and the rate of ripening increases with the concentration of ethylene. This property is used to advantage, for example, in the marketing of ba￾nanas. Bananas are picked green in the tropics, kept green by being stored with adequate ventilation to limit the amount of ethylene present, and then in￾duced to ripen at their destination by passing ethyl￾ene over the fruit.* several NH3 steps CO2 1-Amino￾cyclopropane￾carboxylic acid CH2 CH2 Ethylene other products CH2œCH2 Ethylene (bp: 104°C) Cl2 Chlorine (bp: 34°C) ClCH2CH2Cl 1,2-Dichloroethane (bp: 83°C) Ethylene is the cornerstone of the world’s mam￾moth petrochemical industry and is produced in vast quantities. In a typical year the amount of ethylene produced in the United States (5 1010lb) exceeds the combined weight of all of its people. In one process, ethane from natural gas is heated to bring about its dissociation into ethylene and hydrogen: This reaction is known as dehydrogenation and is si￾multaneously both a source of ethylene and one of the methods by which hydrogen is prepared on an in￾dustrial scale. Most of the hydrogen so generated is subsequently used to reduce nitrogen to ammonia for the preparation of fertilizer. Similarly, dehydrogenation of propane gives propene: Propene is the second most important petrochemical and is produced on a scale about half that of ethylene. Almost any hydrocarbon can serve as a starting material for production of ethylene and propene. Cracking of petroleum (Section 2.13) gives ethylene and propene by processes involving cleavage of carbon–carbon bonds of higher molecular weight hydrocarbons. The major uses of ethylene and propene are as starting materials for the preparation of polyethyl￾ene and polypropylene plastics, fibers, and films. These and other applications will be described in Chapter 6. CH3CH2CH3 Propane H2 Hydrogen CH3CHœCH2 Propene 750°C CH3CH3 Ethane H2 Hydrogen CH2œCH2 Ethylene 750°C *For a review, see “Ethylene—An Unusual Plant Hormone” in the April 1992 issue of the Journal of Chemical Education (pp. 315–318). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

The longest continuous chain that includes the double bond forms the base name of the alkene, and the chain is numbered in the direction that gives the doubly bonded carbons their lower numbers. The locant(or numerical position) of only one of the dou bly bonded carbons is specified in the name; it is understood that the other doubly bonded carbon must follow in sequence CH2=CHCH,CH3 CH3 CH, CH, CH=CHCH3 1-Butene 2-Hexene not 1. 2-butene) (not 4-hexene) determining the main carbon chain and the direction in which it is numbered sen Carbon-carbon double bonds take precedence over alkyl groups and halogens in CH3CHCH=CH, BrCH,CH,CH, CHCH,CH,CH CH3 CH=CH 6-Bromo-3-propyl-1-hexene longest chain that contains double bond is six carbons) Hydroxyl groups, however, outrank the double bond. Compounds that contain both a double bond and a hydroxyl group use the combined suffix -en+ -ol to signify that both functional groups are present 1-ol HOCH CH,CH CHa (not 2-methyl-2-hexen-6-o1) PROBLEM 5.1 Name each of the following using IUPAC nomenclature (a)(CH3)2C-C(CH3)2 d)CH2=CHCH2 CHCH3 (b)(CH3)3CCH=CH2 e) CH2=CHCH, CHCH SAMPLE SOLUTION (a) The longest continuous chain in this alkene contains four carbon atoms the double bond is between c-2 and c-3, and so it is named as a derivative of 2-butene C CH 2,3-Dimethyl-2-butene Identifying the alkene as a derivative of 2-butene leaves two methyl groups to be accounted for as sub ttached to the main chain this alkene is 23 dimethyl-2-butene (It is so es called tetramethylethylene but that is a com- mon name not an IUPAc We noted in Section 2.10 that the common names of certain frequently encoun- tem. Three alkenyl groups--vinyl, allyl, and isopropenyl-are treated the same at tered alky! groups, such as isopropyl and tert-butyl, are acceptable in the IUPAC sy Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

The longest continuous chain that includes the double bond forms the base name of the alkene, and the chain is numbered in the direction that gives the doubly bonded carbons their lower numbers. The locant (or numerical position) of only one of the dou￾bly bonded carbons is specified in the name; it is understood that the other doubly bonded carbon must follow in sequence. Carbon–carbon double bonds take precedence over alkyl groups and halogens in determining the main carbon chain and the direction in which it is numbered. Hydroxyl groups, however, outrank the double bond. Compounds that contain both a double bond and a hydroxyl group use the combined suffix -en -ol to signify that both functional groups are present. PROBLEM 5.1 Name each of the following using IUPAC nomenclature: (a) (CH3)2CœC(CH3)2 (d) (b) (CH3)3CCHœCH2 (e) (c) (CH3)2CœCHCH2CH2CH3 SAMPLE SOLUTION (a) The longest continuous chain in this alkene contains four carbon atoms. The double bond is between C-2 and C-3, and so it is named as a derivative of 2-butene. Identifying the alkene as a derivative of 2-butene leaves two methyl groups to be accounted for as substituents attached to the main chain. This alkene is 2,3- dimethyl-2-butene. (It is sometimes called tetramethylethylene, but that is a com￾mon name, not an IUPAC name.) We noted in Section 2.10 that the common names of certain frequently encoun￾tered alkyl groups, such as isopropyl and tert-butyl, are acceptable in the IUPAC sys￾tem. Three alkenyl groups—vinyl, allyl, and isopropenyl—are treated the same way. C CH3 CH3 H3C H3C 1 2 3 4 C 2,3-Dimethyl-2-butene CH2œCHCH2CHCH3 W OH CH2œCHCH2CHCH3 W Cl C CH3 HOCH2CH2CH2 CH3 H 123 4 5 6 C 5-Methyl-4-hexen-1-ol (not 2-methyl-2-hexen-6-ol) 4 32 1 CH3CHCHœCH2 W CH3 3-Methyl-1-butene (not 2-methyl-3-butene) 6543 2 1 W CHœCH2 BrCH2CH2CH2CHCH2CH2CH3 6-Bromo-3-propyl-1-hexene (longest chain that contains double bond is six carbons) 1 23 4 CH2œCHCH2CH3 1-Butene (not 1,2-butene) 6 5 4 3 21 CH3CH2CH2CHœCHCH3 2-Hexene (not 4-hexene) 5.1 Alkene Nomenclature 169 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions CH=CH— as in CH,=CHCl ion of poly(vinyl CH,=CHCH,- as in CH,-CHCH,OH hloride). Poly(vinyl chlo- Allyl Allyl alcohol de), often called simpl CH,=C—asin CH=CCI including siding for house all coverings, and PvC pip Isopropenyl chloride When a CH2 group is doubly bonded to a ring, the prefix methylene is added to the name of the ring. Cycloalkenes and their derivatives are named by adapting cycloalkane terminal gy to the principles of alkene nomenclature CH 1-Methvleyclohexene 3-chlorocvclohe (not l-chloro-2-cycloheptene No locants are needed in the absence of substituents: it is understood that the double bond connects C-I and C-2 Substituted cycloalkenes are numbered beginning with the double bond, proceeding through it, and continuing in sequence around the ring. The direction of numbering is chosen so as to give the lower of two possible locants to the substituent PROBLEM 5.2 Write structural formulas or build molecular models and give the UPAC names of all the monochloro-substituted derivatives of cyclopentene 5.2 STRUCTURE AND BONDING IN ALKENES The structure of ethylene and the orbital hybridization model for the double bond were presented in Section 1. 17. To review, Figure 5. 1 depicts the planar structure of ethylene, its bond distances, and its bond angles. Each of the carbon atoms is sp-hybridized, and the double bond possesses a o component and a T component. The o component results when an sp- orbital of one carbon, oriented so that its axis lies along the internuclear axis, overlaps with a similarly disposed sp- orbital of the other carbon. Each sp- orbital contains one electron, and the resulting g bond contains two of the four electrons of the double bond. The T bond contributes the other two electrons and is formed by a"side- y-side"overlap of singly occupied p orbitals of the two carbons Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

When a CH2 group is doubly bonded to a ring, the prefix methylene is added to the name of the ring. Cycloalkenes and their derivatives are named by adapting cycloalkane terminol￾ogy to the principles of alkene nomenclature. No locants are needed in the absence of substituents; it is understood that the double bond connects C-1 and C-2. Substituted cycloalkenes are numbered beginning with the double bond, proceeding through it, and continuing in sequence around the ring. The direction of numbering is chosen so as to give the lower of two possible locants to the substituent. PROBLEM 5.2 Write structural formulas or build molecular models and give the IUPAC names of all the monochloro-substituted derivatives of cyclopentene. 5.2 STRUCTURE AND BONDING IN ALKENES The structure of ethylene and the orbital hybridization model for the double bond were presented in Section 1.17. To review, Figure 5.1 depicts the planar structure of ethylene, its bond distances, and its bond angles. Each of the carbon atoms is sp2 -hybridized, and the double bond possesses a component and a  component. The component results when an sp2 orbital of one carbon, oriented so that its axis lies along the internuclear axis, overlaps with a similarly disposed sp2 orbital of the other carbon. Each sp2 orbital contains one electron, and the resulting bond contains two of the four electrons of the double bond. The  bond contributes the other two electrons and is formed by a “side￾by-side” overlap of singly occupied p orbitals of the two carbons. Cyclopentene Cl 1 2 3 4 6 5 7 3-Chlorocycloheptene (not 1-chloro-2-cycloheptene) 1 CH3 2 3 4 5 6 1-Methylcyclohexene Methylenecyclohexane CH2 CH2œCH± Vinyl as in CH2œCHCl Vinyl chloride CH2œCHCH2± Allyl as in CH2œCHCH2OH Allyl alcohol CH2œC± as in W CH3 Isopropenyl W CH3 CH2œCCl Isopropenyl chloride 170 CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions Vinyl chloride is an industrial chemical produced in large amounts (1010 lb/year in the United States) and is used in the preparation of poly(vinyl chloride). Poly(vinyl chlo￾ride), often called simply vinyl, has many applications, including siding for houses, wall coverings, and PVC pip￾ing. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

5.2 Structure and Bonding in Alkenes IGURE 5.1(a)The 117.2° framework of o bonds in eth √134 six atoms are coplanar. The arbon -carbon bond is a double bond made up of the 110 pm T component illustrated in b (b) The p orbitals of two hybridized carbons overlap to produce a t bond. An electron pair in the t bond is shared by the two carbons The double bond in ethylene is stronger than the C-C single bond in ethane, but The simplest arithmetic ap. it is not twice as strong. The C=C bond energy is 605 kJ/mol (144.5 kcal/mol) in eth- proach subtracts the C-c ylene versus 368 kJ/mol(88 kcal/mol)for the C-C bond in ethane Chemists do not bond energy of ethane (368 agree on exactly how to apportion the total C-C bond energy between its o and T com- C-c bond energy of ethyl ponents, but all agree that the T bond is weaker than the o bond There are two different types of carbon-carbon bonds in propene, CH3 CH=CH2. kcal/mol). This gives a value The double bond is of the o+ type, and the bond to the methyl group is a o bond of/mol (56 5 kcal/mol) formed by sp'-sp- overlap H sp hybridized carbon H C=C C-C bond length= 150 pm C=C bond length= 134 pm H H PROBLEM 5.3 We can use bond-line formulas to represent alkenes in much the same way that we use them to represent alkanes. Consider the following alkene (a) what is the molecular formula of this alkene? (b)What is its IUPAC name (c)How many carbon atoms are sp-hybridized in this alkene? How many are sp hyb (d)How many o bonds are of the sp2-sp' type? How many are of the sp -sp3 type? SAMPLE SOLUTION (a) Recall when writing bond-line formulas for hydrocar bons that a carbon occurs at each end and at each bend in a carbon chain the appropriate number of hydrogens are attached so that each carbon has four bonds. thus the compound shown is CH3 CH=C(CH, CH3)2 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

The double bond in ethylene is stronger than the C±C single bond in ethane, but it is not twice as strong. The CœC bond energy is 605 kJ/mol (144.5 kcal/mol) in eth￾ylene versus 368 kJ/mol (88 kcal/mol) for the C±C bond in ethane. Chemists do not agree on exactly how to apportion the total CœC bond energy between its and  com￾ponents, but all agree that the  bond is weaker than the bond. There are two different types of carbon–carbon bonds in propene, CH3CHœCH2. The double bond is of the  type, and the bond to the methyl group is a bond formed by sp3 –sp2 overlap. PROBLEM 5.3 We can use bond-line formulas to represent alkenes in much the same way that we use them to represent alkanes. Consider the following alkene: (a) What is the molecular formula of this alkene? (b) What is its IUPAC name? (c) How many carbon atoms are sp2 -hybridized in this alkene? How many are sp3 - hybridized? (d) How many  bonds are of the sp2 –sp3 type? How many are of the sp3 –sp3 type? SAMPLE SOLUTION (a) Recall when writing bond-line formulas for hydrocar￾bons that a carbon occurs at each end and at each bend in a carbon chain. The appropriate number of hydrogens are attached so that each carbon has four bonds. Thus the compound shown is CH3CH2CHœC(CH2CH3)2 H H H C H H H C±C bond length  150 pm CœC bond length  134 pm sp3 hybridized carbon C C sp2 hybridized carbon 5.2 Structure and Bonding in Alkenes 171 FIGURE 5.1 (a) The framework of  bonds in eth￾ylene showing bond dis￾tances in picometers and bond angles in degrees. All six atoms are coplanar. The carbon–carbon bond is a double bond made up of the  component shown and the  component illustrated in b. (b) The p orbitals of two sp2 hybridized carbons overlap to produce a  bond. An electron pair in the  bond is shared by the two carbons. The simplest arithmetic ap￾proach subtracts the C±C bond energy of ethane (368 kJ/mol; 88 kcal/mol) from the CœC bond energy of ethyl￾ene (605 kJ/mol; 144.5 kcal/mol). This gives a value of 237 kJ/mol (56.5 kcal/mol) for the  bond energy. 117.2 134 pm 110 pm 121.4 (a) (b) Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER FIVE Structure and Preparation of Alkenes Elimination Reactions The general molecular formula for an alkene is C,H2n. Ethylene is C2H4; propene is C3H6. Counting the carbons and hydrogens of the compound shown(C8H16 reveals that it, too, corresponds to CnH2n 5.3 ISOMERISM IN ALKENES Although ethylene is the only two-carbon alkene, and propene the only three-carbon alkene there are four isomeric alkenes of molecular formula Cah CH, CH3 H CH3 H3 CH3 CH3 Make molecular models of cisand trans-2-butene to v ify that they are different. 1-Butene 2-Methylpropene cis-2-Butene rans-2-Butene 1-Butene has an unbranched carbon chain with a double bond between c-1 and c-2. it is a constitutional isomer of the other three. Similarly, 2-methylpropene, with a branched carbon chain. is a constitutional isomer of the other three The pair of isomers designated cis-and trans-2-butene have the same constitution both have an unbranched carbon chain with a double bond connecting C-2 and C-3. They differ from each other, however, in that the cis isomer has both of its methyl groups on the same side of the double bond, but the methyl groups in the trans isomer are on oppo- site sides of the double bond recall from section 3. 12 that isomers that have the same constitution but differ in the arrangement of their atoms in space are classified as Stereoisomeric alkenes stereoisomers. cis-2-Butene and trans-2-butene are stereoisomers and the terms"cis and"trans"specify the configuration of the double bond Cis-trans stereoisomerism in alkenes is not possible when one of the doubly bonded carbons bears two identical substituents. Thus, neither 1-butene nor 2-methyl propene can have stereoisomers CH, CH3 CH3 Identical CH3 1-Butene 2-Methylpropene (no stereoisomers possible) (no stereoisomers possible) PROBLEM 5.4 How many alkenes have the molecular formula CsH1o? Write their The activation energy for structures and give their IUPAC names. Specify the configuration of stereoisomers as cis or trans as appropriate 250 kJ/mol(about 60 In principle, cis-2-butene and trans-2-butene may be interconverted by rotation cal/mo). This quantity may about the C-2=C-3 double bond. However, unlike rotation about the C-2-C-3 singl be taken as a measure of the bond in butane, which is quite fast, interconversion of the stereoisomeric 2-butenes does otal C-C bond strength of not occur under normal circumstances. It is sometimes said that rotation about a car bon--carbon double bond is restricted, but this is an understatement Conventional labo- in ethylene and compares osely with the value est ratory sources of heat do not provide enough thermal energy for rotation about the dou ated by manipulation of ble bond in alkenes to take place. As shown in Figure 5.2, rotation about a double bond thermochemical data on requires the p orbitals of C-2 and C-3 to be twisted from their stable parallel alignment- page 171 in effect, the T component of the double bond must be broken at the transition state Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

The general molecular formula for an alkene is CnH2n. Ethylene is C2H4 ; propene is C3H6. Counting the carbons and hydrogens of the compound shown (C8H16) reveals that it, too, corresponds to CnH2n. 5.3 ISOMERISM IN ALKENES Although ethylene is the only two-carbon alkene, and propene the only three-carbon alkene, there are four isomeric alkenes of molecular formula C4H8: 1-Butene has an unbranched carbon chain with a double bond between C-1 and C-2. It is a constitutional isomer of the other three. Similarly, 2-methylpropene, with a branched carbon chain, is a constitutional isomer of the other three. The pair of isomers designated cis- and trans-2-butene have the same constitution; both have an unbranched carbon chain with a double bond connecting C-2 and C-3. They differ from each other, however, in that the cis isomer has both of its methyl groups on the same side of the double bond, but the methyl groups in the trans isomer are on oppo￾site sides of the double bond. Recall from Section 3.12 that isomers that have the same constitution but differ in the arrangement of their atoms in space are classified as stereoisomers. cis-2-Butene and trans-2-butene are stereoisomers, and the terms “cis” and “trans” specify the configuration of the double bond. Cis–trans stereoisomerism in alkenes is not possible when one of the doubly bonded carbons bears two identical substituents. Thus, neither 1-butene nor 2-methyl￾propene can have stereoisomers. PROBLEM 5.4 How many alkenes have the molecular formula C5H10? Write their structures and give their IUPAC names. Specify the configuration of stereoisomers as cis or trans as appropriate. In principle, cis-2-butene and trans-2-butene may be interconverted by rotation about the C-2œC-3 double bond. However, unlike rotation about the C-2±C-3 single bond in butane, which is quite fast, interconversion of the stereoisomeric 2-butenes does not occur under normal circumstances. It is sometimes said that rotation about a car￾bon–carbon double bond is restricted, but this is an understatement. Conventional labo￾ratory sources of heat do not provide enough thermal energy for rotation about the dou￾ble bond in alkenes to take place. As shown in Figure 5.2, rotation about a double bond requires the p orbitals of C-2 and C-3 to be twisted from their stable parallel alignment— in effect, the  component of the double bond must be broken at the transition state. Identical C H H CH2CH3 H C 1-Butene (no stereoisomers possible) Identical CH3 CH3 C H H C 2-Methylpropene (no stereoisomers possible) Identical C H H CH2CH3 H C 1-Butene CH3 CH3 C H H C 2-Methylpropene cis-2-Butene CH3 H CH3 C H C trans-2-Butene H CH3 CH3 C H C 172 CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions Stereoisomeric alkenes are sometimes referred to as geometric isomers. The activation energy for ro￾tation about a typical carbon–carbon double bond is very high—on the order of 250 kJ/mol (about 60 kcal/mol). This quantity may be taken as a measure of the  bond contribution to the total CœC bond strength of 605 kJ/mol (144.5 kcal/mol) in ethylene and compares closely with the value esti￾mated by manipulation of thermochemical data on page 171. Make molecular models of cis-and trans-2-butene to ver￾ify that they are different. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

5.4 Naming Stereoisomeric Alkenes by the E-Z Notational System trans-2-Butene p orbitals perpendicular. Worst geometry for I bond formation Optimal ry for rt bond formation π bond fo IGURE 5.2 Interconversion of cis-and trans-2-butene proceeds by cleavage of the com- ponent of the double bond. The red balls represent the two methyl groups. 5.4 NAMING STEREOISOMERIC ALKENES BY THE E-Z NOTATIONAL SYSTEM When the groups on either end of a double bond are the same or are structurally simi- lar to each other, it is a simple matter to describe the configuration of the double as cis or trans. Oleic acid, for example, a material that can be obtained from olive oil has a cis double bond. Cinnamaldehyde, responsible for the characteristic odor of cin n. has a trans double bond. CH3(CH2)CH CH(CH2)CO2H C6H5 H H H Oleic acid Cinnamaldehyde PROBLEM 5.5 Female houseflies attract males by sending a chemical signal known as a pheromone. The substance emitted by the female housefly that attracts the male has been identified as cis-9-tricosene, C23Ha6. Write a structural formula, including stereochemistry, for this compound The terms"cis" and"trans are ambiguous, however, when it is not obvious which substituent on one carbon is "similar"or "analogous to a reference substituent on the other. Fortunately, a completely unambiguous system for specifying double bond stereo- chemistry has been developed based on an atomic number criterion for ranking sub stituents on the doubly bonded carbons. When atoms of higher atomic number are on the same side of the double bond, we say that the double bond has the Z configuration, where Z stands for the German word zusammen, meaning "together. When atoms of higher atomic number are on opposite sides of the double bond, we say that the config- uration is E. The symbol E stands for the German word entgegen, meaning"opposite Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

5.4 NAMING STEREOISOMERIC ALKENES BY THE E–Z NOTATIONAL SYSTEM When the groups on either end of a double bond are the same or are structurally simi￾lar to each other, it is a simple matter to describe the configuration of the double bond as cis or trans. Oleic acid, for example, a material that can be obtained from olive oil, has a cis double bond. Cinnamaldehyde, responsible for the characteristic odor of cin￾namon, has a trans double bond. PROBLEM 5.5 Female houseflies attract males by sending a chemical signal known as a pheromone. The substance emitted by the female housefly that attracts the male has been identified as cis-9-tricosene, C23H46. Write a structural formula, including stereochemistry, for this compound. The terms “cis” and “trans” are ambiguous, however, when it is not obvious which substituent on one carbon is “similar” or “analogous” to a reference substituent on the other. Fortunately, a completely unambiguous system for specifying double bond stereo￾chemistry has been developed based on an atomic number criterion for ranking sub￾stituents on the doubly bonded carbons. When atoms of higher atomic number are on the same side of the double bond, we say that the double bond has the Z configuration, where Z stands for the German word zusammen, meaning “together.” When atoms of higher atomic number are on opposite sides of the double bond, we say that the config￾uration is E. The symbol E stands for the German word entgegen, meaning “opposite.” C6H5 H CH O C H C Oleic acid Cinnamaldehyde CH3(CH2)6CH2 CH2(CH2)6CO2H C H H C 5.4 Naming Stereoisomeric Alkenes by the E–Z Notational System 173 trans-2-Butene p orbitals aligned: Optimal geometry for π bond formation cis-2-Butene p orbitals aligned: Optimal geometry for π bond formation p orbitals perpendicular: Worst geometry for π bond formation FIGURE 5.2 Interconversion of cis- and trans-2-butene proceeds by cleavage of the  com￾ponent of the double bond. The red balls represent the two methyl groups. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions Br"Higher Lower Lower Lower Z configuration E configuration Higher ranked substituents(CI and Br) Higher ranked substituents(CI and Br) are on opposite sides of double bond he substituent groups on the double bonds of most alkenes are, of course, more com- ped by R. 5. Cahn and Sir plicated than in this example. The rules for ranking substituents, especially alkyl groups, are described in Table 5.1 and Vladimir Prelog (Switzer and)in the context of a dif- ferent aspect of organic PROBLEM 5.6 Determine the configuration of each of the following alkene stereochemistry,they will ap.Z or E as appropriate hapter 7 a)H2C、 CH2OH (c)H3c CH2CH2OH (b)H3C CH2 CH2F CH2CH2 CH2 CH3 CH3 CH2 SAMPLE SOLUTION (a)One of the doubly bonded carbons bears a meth group and a hydrogen. According to the rules of Table 5.1, methyl outranks hydro- gen. The other carbon atom of the double bond bears a methyl and a-CH2OH group. The -CH2OH group is of higher priority than methyl s HaO CH2OH+ Higher CO, H, H Lower(H)、H CH3← s Lower C(H,H, H) Higher ranked substituents are on the same side of the double bond; the config uration is Z a table on the inside back cover(right page)lists some of the more frequently encountered atoms and groups in order of increasing precedence. You should not attempt to memorize this table, but should be able to derive the relative placement of one group versus another 5.5 PHYSICAL PROPERTIES OF ALKENES Alkenes resemble alkanes in most of their physical properties. The lower molecular weight alkenes through CAHs are gases at room temperature and atmospheric pressur The dipole moments of most alkenes are quite small. Among the Isomers I-butene, cis-2-butene, and 2-methylpropene have dipole moments in the 0.3-0.5 D range; trans-2-butene has no dipole moment. Nevertheless, we can learn some things about alkenes by looking at the effect of substituents on dipole moments. Experimental measurements of dipole moments give size, but not direction. We mally deduce the overall direction by examining the directions of individual bond Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

The substituent groups on the double bonds of most alkenes are, of course, more com￾plicated than in this example. The rules for ranking substituents, especially alkyl groups, are described in Table 5.1. PROBLEM 5.6 Determine the configuration of each of the following alkenes as Z or E as appropriate: (a) (c) (b) (d) SAMPLE SOLUTION (a) One of the doubly bonded carbons bears a methyl group and a hydrogen. According to the rules of Table 5.1, methyl outranks hydro￾gen. The other carbon atom of the double bond bears a methyl and a ±CH2OH group. The ±CH2OH group is of higher priority than methyl. Higher ranked substituents are on the same side of the double bond; the config￾uration is Z. A table on the inside back cover (right page) lists some of the more frequently encountered atoms and groups in order of increasing precedence. You should not attempt to memorize this table, but should be able to derive the relative placement of one group versus another. 5.5 PHYSICAL PROPERTIES OF ALKENES Alkenes resemble alkanes in most of their physical properties. The lower molecular weight alkenes through C4H8 are gases at room temperature and atmospheric pressure. The dipole moments of most alkenes are quite small. Among the C4H8 isomers, 1-butene, cis-2-butene, and 2-methylpropene have dipole moments in the 0.3–0.5 D range; trans-2-butene has no dipole moment. Nevertheless, we can learn some things about alkenes by looking at the effect of substituents on dipole moments. Experimental measurements of dipole moments give size, but not direction. We normally deduce the overall direction by examining the directions of individual bond Higher Lower Higher Lower ±C(O,H,H) ±C(H,H,H) (C) (H) H3C H CH2OH CH3 C C CH3CH2 H CH3 C C H3C H CH2CH2F CH2CH2CH2CH3 C C H3C H CH2CH2OH C(CH3)3 C C H3C H CH2OH CH3 C C Cl Br F Higher Lower Higher Lower C H C Z configuration Higher ranked substituents (Cl and Br) are on same side of double bond Cl F Br Higher Lower Lower Higher C H C E configuration Higher ranked substituents (Cl and Br) are on opposite sides of double bond 174 CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions The priority rules were devel￾oped by R. S. Cahn and Sir Christopher Ingold (England) and Vladimir Prelog (Switzer￾land) in the context of a dif￾ferent aspect of organic stereochemistry; they will ap￾pear again in Chapter 7. The physical properties of se￾lected alkenes are collected in Appendix 1. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

5.5 Physical Properties of Alkenes TABLE 5.1 Cahn-Ingold-Prelog Priority Rules 1. Higher atomic number takes precedence over The compound lower Bromine(atomic number 35)outranks chlor- ine(atomic number 17). Methyl(C, atomic number 6) Higher Br Higher outranks hydrogen (atomic number 1) Lower Lower has the Z configuration. Higher ranked atoms(Br and C of CHa)are on the same side of the double bond 2. When two atoms directly attached to the double The compound bond are identical, compare the atoms attached with these two on the basis of their atomic numbers pre-. Higher Br CH Lower cadence is determined at the first point of difference Ethyl[一c(c,H,H】] outranks methyl[c(HHH) Lower CH2CH Hiaher has the E configuration Similarly, tert-butyl outranks isopropyl, and isopropyl outranks ethyl C(CH3)3>—CH(CH3)2>—CH2CH C(C, CC)>-C(C, C, H)>-C(C, H, H) 3. Work outward from the point of attachment, com- The compound paring all the atoms attached to a particular atom before proceeding further along the chain Higher Br CH2CH2OH Lower -CH(CH3)2[C(CC, H) outranks CH2CH2OH [C(C, H, H)I Lower CH(CH3)2 Higher has the E configuration 4. When working outward from the point of attach The compound ment, always evaluate substituent atoms one by one, never as a group. Since oxygen has a higher atomic Higher B CH2OH Higher -CH,OH [C(O, H, H outranks Lowe C(CH3)3 Lower C(CH)3[-c(C, C, o) has the Z configuration 5. An atom that is multiply bonded to another atom The compound is considered to be replicated as a substituent on that atom: Higher CH2OH Lower Lower CH=O Higher s treated as if it were CO,, H) group-CH=o [-C(o, O, H)] outranks-CH2OH has the E configuration C(O,H, H) Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

5.5 Physical Properties of Alkenes 175 TABLE 5.1 Cahn–Ingold–Prelog Priority Rules Rule 1. Higher atomic number takes precedence over lower. Bromine (atomic number 35) outranks chlor￾ine (atomic number 17). Methyl (C, atomic number 6) outranks hydrogen (atomic number 1). 2. When two atoms directly attached to the double bond are identical, compare the atoms attached with these two on the basis of their atomic numbers. Pre￾cedence is determined at the first point of difference: 3. Work outward from the point of attachment, com￾paring all the atoms attached to a particular atom before proceeding further along the chain: 4. When working outward from the point of attach￾ment, always evaluate substituent atoms one by one, never as a group. Since oxygen has a higher atomic number than carbon, Example The compound has the Z configuration. Higher ranked atoms (Br and C of CH3) are on the same side of the double bond. The compound The compound The compound has the Z configuration. has the E configuration. Similarly, tert-butyl outranks isopropyl, and isopropyl outranks ethyl: Higher Lower Higher Lower Br Cl CH3 H C C Higher Lower Lower Higher Br Cl CH3 CH2CH3 C C has the E configuration. Higher Lower Lower Higher Br Cl CH2CH2OH CH(CH3)2 C C Higher Lower Higher Lower Br Cl CH2OH C(CH3)3 C C Ethyl [±C(C,H,H)] The group ±CHœO [±C(O,O,H)] outranks ±CH2OH [±C(O,H,H)] ±CH(CH3)2 [±C(C,C,H)] ±CH2CH2OH [±C(C,H,H)] outranks methyl [±C(H,H,H)] outranks ±CH2OH [±C(O,H,H)] ±C(CH3)3 [±C(C,C,C)] outranks 5. An atom that is multiply bonded to another atom is considered to be replicated as a substituent on that atom: The compound has the E configuration. Higher Lower Lower Higher Br Cl CH2OH CH C C O ±CH is treated as if it were ±C(O,O,H) X O ±C(CH3)3  ±CH(CH3)2  ±CH2CH3 ±C(C,C,C)  ±C(C,C,H)  ±C(C,H,H) Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions dipoles. With alkenes the basic question concerns the alkyl groups attached to C-C Does an alkyl group donate electrons to or withdraw electrons from a double bond? This question can be approached by comparing the effect of an alkyl group, methyl for exam- H C=C trans-1-Chloropropene 0.3D 17D Ethylene, of course, has no dipole moment. Replacing one of its hydrogens by chlorine gives chloroethene, which has a dipole moment of 1.4 D. The effect is much smaller when one of the hydrogens is replaced by methyl; CH3CH=CH2 has a dipole moment of only 0.3 D. Now place CH3 and Cl trans to each other on the double bond. If methyl releases electrons better than H, then the dipole moment of trans-CH3 CH=CHCI should be larger than that of CH,=CHCl, because the effects of CH3 and CI reinforce each other. If methyl is electron attracting, the opposite should occur, and the dipole moment of trans-CH3CH-CHCI will be smaller than 1. 4 D. In fact, the dipole moment of trans- CH3CH=CHCI is larger than that of CH2=CHCl, indicating that a methyl group is an electron-donating substituent on the double bond. A methyl group releases electrons to a double bond in much the same way that it hby an inductive effe and by hyperconjugation(Figure 5.3). Other alkyl groups behave similarly and, as we go along, we'll see several ways in which the electron-releasing effects of alkyl substituents influence the properties of alkenes. The first is described in the following section. 5.6 RELATIVE STABILITIES OF ALKENES Earlier(Sections 2.15, 3.12)we saw how to use heats of combustion to compare the sta- bilities of isomeric alkanes. We can do the same thing with isomeric alkenes, Consider the heats of combustion of the four isomeric alkenes of molecular formula C4H. All C4H8+602→4CO2+4H2O When the heats of combustion of the isomers are plotted on a common scale as in Fig ure 5.4, we see that the isomer of highest energy(the least stable one)is 1-butene, CH2= CH3. The isomer of lowest energy (most stable) is 2-methylpropene (CH3)2C=CH carbon and are stabilized by electron-donating substituesdtive than sp-hybridized FIGURE donate ituent than hydrogen. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

dipoles. With alkenes the basic question concerns the alkyl groups attached to CœC. Does an alkyl group donate electrons to or withdraw electrons from a double bond? This question can be approached by comparing the effect of an alkyl group, methyl for exam￾ple, with other substituents. Ethylene, of course, has no dipole moment. Replacing one of its hydrogens by chlorine gives chloroethene, which has a dipole moment of 1.4 D. The effect is much smaller when one of the hydrogens is replaced by methyl; CH3CHœCH2 has a dipole moment of only 0.3 D. Now place CH3 and Cl trans to each other on the double bond. If methyl releases electrons better than H, then the dipole moment of trans-CH3CHœCHCl should be larger than that of CH2œCHCl, because the effects of CH3 and Cl reinforce each other. If methyl is electron attracting, the opposite should occur, and the dipole moment of trans-CH3CHœCHCl will be smaller than 1.4 D. In fact, the dipole moment of trans￾CH3CHœCHCl is larger than that of CH2œCHCl, indicating that a methyl group is an electron-donating substituent on the double bond. A methyl group releases electrons to a double bond in much the same way that it releases electrons to the positively charged carbon of a carbocation—by an inductive effect and by hyperconjugation (Figure 5.3). Other alkyl groups behave similarly and, as we go along, we’ll see several ways in which the electron-releasing effects of alkyl substituents influence the properties of alkenes. The first is described in the following section. 5.6 RELATIVE STABILITIES OF ALKENES Earlier (Sections 2.15, 3.12) we saw how to use heats of combustion to compare the sta￾bilities of isomeric alkanes. We can do the same thing with isomeric alkenes. Consider the heats of combustion of the four isomeric alkenes of molecular formula C4H8. All undergo combustion according to the equation C4H8 6O2 ±£ 4CO2 4H2O When the heats of combustion of the isomers are plotted on a common scale as in Fig￾ure 5.4, we see that the isomer of highest energy (the least stable one) is 1-butene, CH2œCHCH2CH3. The isomer of lowest energy (most stable) is 2-methylpropene (CH3)2CœCH2. C H H H H C Ethylene   0 D H Cl C H H C Chloroethene   1.4 D H H H CH3 C C Propene   0.3 D Cl H H CH3 C C trans-1-Chloropropene   1.7 D 176 CHAPTER FIVE Structure and Preparation of Alkenes: Elimination Reactions sp2-hybridized carbons of an alkene are more electronegative than sp3-hybridized carbon and are stabilized by electron-donating substituents. C C H CH3 Methyl group is a better electron-donating substituent than hydrogen. FIGURE 5.3 Alkyl groups donate electrons to sp2 - hybridized carbons of an alkene. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website