Chapter 4: Alkenes and Alkynes 99 Chapter 4: Alkenes and Alkynes Problems 4.1 Write the IUPAC name of each unsaturated hydrocarbon: (a) (b) (c) 3,3-dimethyl-1-pentene 2,3-dimethyl-2-butene 3,3-dimethyl-1-butyne 4.2 Name each alkene, and, using the cis-trans system, specify its configuration: (a) (b) cis-4-methyl-2-pentene trans-2,2-dimethyl-3-hexene 4.3 Name each alkene and specify its configuration by the E,Z system: Cl (a) (b) (c) Cl Br (E)-1-chloro-2,3- (Z)-1-bromo-1- (E)-2,3,4- dimethyl-2-pentene chloro-1-propene trimethyl-3-heptene 4.4 Write the IUPAC name for each cycloalkene: (a) (b) (c) 1-isopropyl-4- 4-tert-butylcyclohexene cyclooctene methylcyclohexene 100 Chapter 4: Alkenes and Alkynes 4.5 Draw structural formulas for the other two cis-trans isomers of 2,4-heptadiene: cis,trans-2,4-heptadiene cis,cis-2,4-heptadiene 4.6 How many cis-trans isomers are possible for the following unsaturated alcohol? CH3 CH3 CH3 CH3C=CHCH2CH2C=CHCH2CH2C=CHCH2OH There are three carbon-carbon double bonds, but cis-trans isomerism is only possible for the two that are indicated by arrows. cis-trans Isomerism is not possible for the alkene on the far left of the molecule because one of the two alkene carbons is bonded to two 2 identical groups (CH3). The number of possible cis-trans isomers is therefore 2 = 4. Quick Quiz 1. Ethylene and acetylene are constitutional isomers. False. Constitutional isomers must have the same molecular formula but different atom connectivity (bonding sequence). Ethylene and acetylene are respectively C2H4 and C2H2. 2. Alkanes that are liquid at room temperature are insoluble in water and when added to water will float on water. True. Alkanes are nonpolar and therefore water-insoluble, and they have a lower density than water. 3. The bulk of the ethylene used by the chemical industry worldwide is obtained from nonrenewable resources. True. Ethylene is derived from the cracking of hydrocarbons. 4. Alkenes and alkynes are nonpolar molecules. True. Alkenes and alkynes do not contain any polar bonds, so the molecules do not have a net dipole moment. 5. The IUPAC name of CH3CH=CHCH3 is 1,2-dimethylethylene. False. The longest carbon chain is four carbons long, which leads to the name 2-butene. 6. Cyclohexane and 1-hexene are constitutional isomers. True. They have the same molecular formula but a different connectivity of atoms. 7. The IUPAC name of an alkene is derived from the name of the longest chain of carbon atoms that contains the double bond. True. The chain is also numbered such that the double bond has the lowest possible number. Chapter 4: Alkenes and Alkynes 101 8. There are two classes of unsaturated hydrocarbons, alkenes and alkynes. False. Arenes, which are compounds based on benzene, are also unsaturated hydrocarbons. 9. Both geraniol and menthol (Figure 4.2) show cis-trans isomerism. True. Geraniol can exhibit cis-trans isomerism at one of the double bonds. Menthol can show cis-trans isomerism with respect to the substituents bonded to the cyclohexane ring. 10. Terpenes are identified by their carbon skeletons, namely, one that can be divided into five-carbon isoprene units. True. All terpenes consist of isoprene building blocks. 11. 1,2-Dimethylcyclohexene shows cis-trans isomerism. False. The double bond is a part of a six-membered ring and must be cis with respect to the ring. There is no cis-trans isomerism with respect to the methyl groups on the ring because they are both bonded to sp2 carbons and are planar. 12. 2-Methyl 2-butene shows cis-trans isomerism. False. Carbon 2 is bonded to two identical substituents, two methyl groups. 13. Both ethylene and acetylene are planar molecules. True. The carbon atoms in ethylene are sp2-hybridized, and all the carbon and hydrogen atoms lie in the same plane. Acetylene has sp-hybridized carbons, and all the atoms of acetylene lie in the same plane. 14. The physical properties of alkenes are similar to those of alkanes with the same carbon skeletons. True. These properties include density, melting point, and boiling point. 15. Isoprene is the common name for 2-methyl-1,3-butadiene. True. Realize that once this compound is incorporated into a terpene, it need not bear the same order of single and double bonds. Other chemical modifications can also be performed on the terpene. However, the five-carbon skeleton of isoprene is always retained in a terpene. Chemical Connections 4A. Explain the basis for the saying “A rotten apple can spoil the barrel.” Ethylene is a natural ripening agent for apples. A rotten apple, or one that is overripe, releases ethylene and causes the other apples in the barrel to ripen. 4B. Based on the information in this chemical connection, what can you deduce about the physical properties of leaf cell membranes? Isoprene is dissolved in the leaf cell membranes before it is released into the atmosphere. Because isoprene is a nonpolar hydrocarbon, the membranes must also be relatively nonpolar. At higher temperatures, isoprene is more volatile and more easily released. 102 Chapter 4: Alkenes and Alkynes End-of-Chapter Problems Structure of Alkenes and Alkynes 4.7 Classify each of the following as either an alkane, alkene, alkyne, or arene. (a) (b) (c) (d) (e) (a) alkene (b) alkane (c) arene (d) arene (e) alkene 4.8 Give the hybridization of each of the highlighted carbons in problem 4.7. (a) sp2 (b) sp3 (c) sp2 (d) sp (e) sp2 4.9 Each of the highlighted carbons in problem 4.7 is attached to two other carbons. Give the approximate C—C—C bond angle (with the highlighted carbon being the central carbon) for each of these compounds. (a) 120° (b) 109.5° (c) 120° (d) 180° (e) 120° 4.10 How many hydrogens are attached to of each of the highlighted carbons in problem 4.7? (a) 1 (b) 2 (c) 1 (d) 0 (e) 1 4.11 Cyclohexene (see below) is stable, but cyclohexyne is not. Offer an explanation why. cyclohexene cyclohexyne There is too much “angle strain” in cyclohexyne. The alkyne carbons want to have linear geometry (bond angle = 180°), but the geometry of the ring is forcing the angle to be closer to 120°, Chapter 4: Alkenes and Alkynes 103 4.12 Each carbon atom in ethane and in ethylene is surrounded by eight valence electrons and has four bonds to it. Explain how VSEPR (Section 1.3) predicts a bond angle of 109.5° about each carbon in ethane, but an angle of 120° about each carbon in ethylene. Although each carbon atom in ethane and ethylene is surrounded by eight valence electrons and has four bonds to it, the shape and geometry about each carbon is based on the number of regions of electron density associated with it. In ethane, each carbon has four regions of electron density and a tetrahedral shape, which has bond angles of 109.5°. Whereas, each carbon in ethylene has three regions of electron density (remember that a double bond is counted as a single region of electron density), resulting in a trigonal planar arrangement (120°). 4.13 Explain the difference between saturated and unsaturated. A compound that is saturated does not contain any carbon-carbon π bonds. Compounds that are unsaturated contain one or more carbon-carbon π bonds. 4.14 Following is the structure of 1,2-propadiene (allene). In it, the plane created by H−C−H of carbon 1 is perpendicular to that created by H−C−H of carbon 3. H H 1 2 3 C C C H H 1,2-Propadiene (Allene) (a) State the orbital hybridization of each carbon in allene. Carbons 1 and 3 are sp2-hybridized, while carbon 2 is sp-hybridized. (b) Account for the molecular geometry of allene in terms of the orbital overlap model. Specifically, explain why all four hydrogen atoms are not in the same plane. Carbon 2, which is the carbon that bears two double bonds, is sp-hybridized. In sp hybridization, the two remaining p orbitals are perpendicular to each other. One of the two p orbitals forms the π bond with carbon 1 while the other p orbital forms the π bond with carbon 2. Accordingly, these two π bonds are perpendicular to each other. As a result, the trigonal plane formed by the CH2 on the left is perpendicular to the one formed by the CH2 group on the right. 104 Chapter 4: Alkenes and Alkynes Nomenclature of Alkenes and Alkynes 4.15 Nine different compounds have the molecular formula C4H6. Draw all nine. C 1-butyne 2-butyne 1,3-butadiene 1,2-butadiene cyclobutene 1-methylcyclopropene 2-methylcyclopropene 4.16 Give IUPAC names for seven of the compounds you drew in question 4.15 (there are two you won’t be able to name based on what you’ve learned so far). (See names under structures above.) 4.17 Would any of the compounds you drew in question 4.15 exist as cis-trans isomers? No. 4.18 Draw a structural formula for each compound: (a) 4-ethyl-3-methyl-1-heptene (b) 2,3-dimethylcyclopentene (c) 2-methyl-3-hexyne (d) 4-methyl-1,3-pentadiene (e) cyclopentadiene (f) trans-3,6-dimethylcyclohexene (a) (b) (c) (d) (e) (f) Chapter 4: Alkenes and Alkynes 105 4.19 Write the IUPAC name for each compound: (a) (b) (c) (d) (e) (f) (a) 2-methyl-2-pentene (b) 3-methylcyclohexene (c) 3-methyl-1-pentyne (d) 6-methyl-1,5-heptadiene (e) 1,3,5,7-cyclooctatetraene (f) 2-ethyl-1-pentene 4.20 Explain why each name is incorrect, and then write a correct name for the intended compound: (a) 2-Ethyl-1-propene 4 3 The parent chain is four carbons long.