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 opposite 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-methylpropene 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 carbon–carbon double bond is restricted, but this is an understatement. Conventional laboratory sources of heat do not provide enough thermal energy for rotation about the double 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 rotation 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 estimated by manipulation of thermochemical data on page 171. Make molecular models of cis-and trans-2-butene to verify 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 similar 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 cinnamon, 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 stereochemistry has been developed based on an atomic number criterion for ranking substituents 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 configuration 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 component 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 complicated 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 hydrogen. 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 configuration 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 developed by R. S. Cahn and Sir Christopher Ingold (England) and Vladimir Prelog (Switzerland) in the context of a different aspect of organic stereochemistry; they will appear again in Chapter 7. The physical properties of selected 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 chlorine (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. Precedence is determined at the first point of difference: 3. Work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding further along the chain: 4. When working outward from the point of attachment, 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 example, 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 transCH3CHœCHCl 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 stabilities 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 Figure 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