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 WebsiteThe 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