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468 Chapter 21 Organic Chemistry Alkenes and Alkynes 21.49 are the only structural isomers. Remember that cis-trans isomerism generates geometric isomers, not structural isomers. 21.51 Given: alkene structures Find: name Conceptual Plan: Count the number of carbon atoms in the longest continuous carbon chain that contains the multiple bond to determine the base name of the compound. Find the prefix corresponding to this number of atoms in Table 21.5 and add the ending -ene to form the base name. Consider every branch from the base chain to be a substituent. Name each substituent according to Table 21.6. Beginning with the end closest to the multiple bond, number the base chain and assign a number to each substituent. Write the name of the compound in the following format: (subst. #)-(subst. name)(base name). If there are two or more substituents, give each one a number and list them alphabetically with hyphens between words and numbers. If a compound has two or more identical substituents, designate the number of identical substituents with the prefix di- (2), tri- (3), or tetra- (4) before the substituent's name. Separate the numbers with a comma, indicating the positions of the substituents relative to each other. The prefixes are not taken into account when alphabetizing. Solution: (a) has four carbons as the longest continuous chain. The prefix for 4 is but-, and the base name is butene. There are no substituent groups. Start numbering on the left because it is closer to the double bond. Because the double bond is between positions 1 and 2, the name of the compound is 1-butene. CH₃ CH₃ (b) has five carbons as the longest continuous chain. The prefix for 5 is pent-, and the base name is pentene. The substituent groups are two methyl groups. CH₃ CH₃ If we start numbering the chain at the end closest to the double bond, the double bond is between positions numbered 2 and 3 and the methyl substituents are assigned the numbers 3 and 4. The name of the compound is 3,4-dimethyl-2-pentene. (c) has six carbons as the longest continuous chain. The CH₃ prefix for 6 is hex-, and the base name is hexene. The only substituent group is an isopropyl group. Start numbering on the left because it is closer to the double CH₃ bond. Because the double bond is between positions 1 and 2 and the isopropyl group is at position 3, the name of the compound is 3-isopropyl-1-hexene. Copyright © 2017 Pearson Education, Inc.