11-1 Naming the Alkenes is C the same as for CHAPTER 11 Alkenes and Infrared Spectroscopy hheedyl-an chain that includes both carbons of Tentbeoegabh2ndcartonsnydoatkenesare eioheteameeedhbiegatheoeat ow-inin n人人了 rding the CH. HA 1 1 CHAPTER 11 Alkenes and Infrared Spectroscopy 11-1 Naming the Alkenes Alkenes are characterized by the presence of a double bond. The general formula of an alkene is CnH2n, the same as for a cycloalkane. Common nomenclature for alkenes replaces the corresponding alkane suffix –ane with -ylene. IUPAC nomenclature replaces the alkane suffix –ane with –ene (ethene, propene, etc.). Rules for naming alkenes: Rule 1: Find the longest chain that includes both carbons of the double bond. Rule 2: Indicate the location of the double bond in the main chain by number starting at the end of the chain closest to the double bond. The two double bond carbons in cycloalkenes are numbered 1 and 2. Alkenes with the same formula but differing in the location of the double bond are called double-bond isomers. A 1-alkene is referred to as a terminal alkene; the others are called internal. Rule 3: Add substituents and their positions as prefixes to the alkene stem. If the stem is symmetric, begin from the end giving the first substituent the lowest possible number. Rule 4: Identify any cis/trans stereoisomers. These are examples of diastereomers, or stereoisomers that are not mirror images of each other. In cycloalkenes, trans isomers are stable only for the larger ring sizes