167 CHAPTER 5 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS A lkenes are hydrocarbons that contain a carboncarbon double bond. A car- boncarbon double bond is both an important structural unit and an important functional group in organic chemistry. The shape of an organic molecule is infl u- 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 fi rst of two dealing with alkenes; it describes their structure, bonding, and preparation. Chapter 6 discusses their chemical reactions. 5.1ALKENE 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)2CCH2 ?-Pinene (a major constituent of turpentine) CH3 H CH3CH3 Farnesene (present in the waxy coating found on apple skins) BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website 168CHAPTER FIVEStructure 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. CH2CH2 IUPAC name: ethene Common name: ethylene CH3CHCH2 IUPAC name: propene Common name: propylene ETHYLENE E thylene was known to chemists in the eigh- teenth century and isolated in pure form in 1795. An early name for ethylene was gaz olfi - ant (French for “oil-forming gas”), a term suggested to describe the fact that an oily liquid product is formed when two gasesethylene and chlorinere- act with each other. The term gaz olfi ant was the forerunner of the gen- eral term olefi n, 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 steps NH3 CO2? ? 1-Amino- cyclopropane- carboxylic acid CH2CH2 Ethylene ?other products ?CH2CH2 Ethylene (bp: ?104C) Cl2 Chlorine (bp: ?34C) ClCH2CH2Cl 1,2-Dichloroethane (bp: 83C) Ethylene is the cornerstone of the worlds 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 carboncarbon 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, fi bers, and fi lms. These and other applications will be described in Chapter 6. ?CH3CH2CH3 Propane H2 Hydrogen CH3CHCH2 Propene 750C ?CH3CH3 Ethane H2 Hydrogen CH2CH2 Ethylene 750C *For a review, see “EthyleneAn Unusual Plant Hormone” in the April 1992 issue of the Journal of Chemical Education (pp. 315318). BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE 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 specifi ed in the name; it is understood that the other doubly bonded carbon must follow in sequence. Carboncarbon double bonds take precedence