Transcript Naming the Alkenes
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. Rule 5: Use the IUPAC E,Z system when cis/trans labels are not applicable (3 or 4 different substituents attached to the double-bond carbons). Apply the sequence rules devised for R,S substituent priorities to the two groups on each double-bond carbon. If the two groups of highest priority are on opposite sides of the double bond, the molecule is an E isomer. If they are on the same side of the double bond, the molecule is a Z isomer. Rule 6: Give the hydroxy functional group precedence over the double bond in numbering a chain. Alcohols containing double bonds are named alkenols. The stem incorporating both functions is numbered to give the OH carbon the lowest possible assignment. The last “e” and “alkene” is dropped in naming alkenols. Rule 7: Substituents containing a double bond are named alkenyl. The numbering of a substituent chain containing a double bond begins at the point of attachment to the basic stem.