Introduction Alkenes are hydrocarbons that contain a carbon–carbon double bond. A carbon– carbon double bond is both an important structural unit and an important functional group in organic chemistry. The shape of an organic molecule is influenced by the presence of this bond, and the double bond is the site of most of the chemical reactions that alkenes undergo. Some representative alkenes include isobutylene (an industrial chemical), á-pinene (a fragrant liquid obtained from pine trees), and farnesene (a naturally occurring alkene with three double bonds). These pages deal with alkenes. They describe their structure, nomenclature and bonding. Example pages for educational purposes by M. P. S. 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. Ethylene is an acceptable synonym for ethene in the IUPAC system. Propylene, isobutylene, and other common names ending in -ylene are not acceptable IUPAC names. 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 doubly 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. The common names of certain frequently encountered alkyl groups, such as isopropyl and tert-butyl, are acceptable in the IUPAC system. Three alkenyl groups—vinyl, allyl, and isopropenyl—are treated the same way. 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 chloride), often called simply vinyl, has many applications, including siding for houses, wall coverings, and PVC piping. 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 terminology 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. Example pages for educational purposes by M. P. S. Structure and bonding Figure 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 ó component and a ð component. The ó 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 ó bond contains two of the four electrons of the double bond. The ð bond contributes the other two electrons and is formed by a “sideby- side” overlap of singly occupied p orbitals of the two carbons. FIGURE 1 (a) The framework of ó bonds in ethylene showing bond distances in picometers and bond angles in degrees. All six atoms are coplanar. The carbon–carbon bond is a double bond made up of the ó component shown and the ð component illustrated in b. (b) The p orbitals of two sp2 hybridized carbons overlap to produce a ð bond. An electron pair in the ð bond is shared by the two carbons. The double bond in ethylene is stronger than the C-C single bond in ethane, but it is not twice as strong. The C=C bond energy is 605 kJ/mol (144.5 kcal/mol) in ethylene 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 C=C bond energy between its ó and ð components, but all agree that the ð bond is weaker than the ó bond. There are two different types of carbon–carbon bonds in propene, CH3CH=CH2. The double bond is of the ó + ð type, and the bond to the methyl group is a ó bond formed by sp3–sp2 overlap. The simplest arithmetic approach subtracts the C-C ó bond energy of ethane (368 kJ/mol; 88 kcal/mol) from the C=C bond energy of ethylene (605 kJ/mol; 144.5 kcal/mol). This gives a value of 237 kJ/mol (56.5 kcal/mol) for the ð bond energy. Example pages for educational purposes by M. P. S. Isomerism 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 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. Stereoisomeric alkenes are sometimes referred to as geometric isomers. 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. In principle, cis-2-butene and trans-2-butene may be interconverted by rotation about the C-2oeC-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 1, 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. 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 ð bond contribution to the total C=C bond strength of 605 kJ/mol (144.5 kcal/mol) in ethylene and compares closely with the value estimated from thermochemical data. FIGURE 5.2 Interconversion of cis- and trans-2-butene proceeds by cleavage of the ð component of the double bond. The red balls represent the two methyl groups. Example pages for educational purposes by M. P. S.