Naming Alkenes and Alkynes

Alkenes are named using a series of rules similar to those for alkanes (Section 3.4), with the suffix -ene used instead of -ane to identify the family. There are three steps  

Name the parent hydrocarbon. Find the longest carhon chain containing the double bond, and name the compound accordingly, using the suffix -ene in place of -ane. 

Number the carbon atoms in the chain. Begin at the end nearer the double bond, or if the double bond is equidistant from the two ends, begin at the end nearer the first branch point. This rule ensures that the double-bond carbons receive the lowest possible numbers  

Write the full name. Number the substituents according to their positions in the chain, and list them alphabetically. Indicate the position of the double bond by giving the number of tbe first alkene carbon and placing that number directly before the -ene suffix. If more than one double bond is present, indicate the position of each and use one of the suffixes -diene, -triene, and so on. 

We might also note that lUPAC changed their naming recommendations in 1993. Prior to that time, the locant, or number locating the position of the double bond, was placed at the beginning of the name rather than before the -ene suffix 2-hutene rather than but-2-ene, for instance. Changes always need 

Ball-and-stick and spacefilling models for ethylene and acetylene. 

The names of alkenes and alkynes are derived from the corresponding alkanes. 

Select the longest continuous carbon-carbon chain that contains a double or triple bond. 

Name this parent compound as you would an alkane but change the -am ending to -em for an alkene or to -ym for an all e thus, propane changes to propene or propyne  

Number the carbon chain of the parent compound starting with the end nearer to the double or triple bond. Use the smaller of the two numbers on the double- or triple-bonded carbon atoms to indicate the position of the double or triple bond. Place this number in front of the alkene or alkyne name 2-butene means that the carbon-carbon double bond is between C-2 and C-3. 

Alkenes and alkynes are named in the same way as alkanes with the following exceptions  

The longest continuous carbon chain containing the double bond has six carbon atoms. The base name is therefore hexene. SOLUTION CH3 I H3C—C=C—CH2—CH3 H2C I CH3 

One of the exceptions for naming alkenes is to number the chain so that the double bond has the lowest number. In this case, the double bond is equidistant from the ends and is assigned the number 3. CH3 I H3C—C=C—CH2—CH3 3 4 5 6 H2C 2 CH3 

Name the compoxmd by assigning numbers to each methyl group and to the double bond. Separate numbers from names using hyphens. 3,4-dimethyl-3-hexene 

The longest continuous carbon chain containing the triple bond is five carbons long therefore the base name is pentyne. CH3 I H3C—CH I H3C—HC—CH—C=CH I CH3 

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Naming Alkenes and Alkynes Using the I.U.P.A.C. Nomenclature System... 

The system for naming alkenes and alkynes is similar to the one we have used for alkanes. The following mles are useful. 

The Beginnings of Organic 19.8 Naming Alkenes and Alkynes 19.15 Naming Alcohols... 

Study the following examples of named alkenes and alkynes ... 

The lUPAC rules for naming alkenes and alkynes are similar to those for alkanes (Sec. 2.3), but a few rules must be added for naming and locating the multiple bonds. 

The lUPAC names for alkenes and alkynes are similar to those of alkanes. The lUPAC name of the simplest alkyne is ethyne, although it is often called by the conunon name, acetylene (see Table 17.4). When naming alkenes and alkynes, the longest carbon chain must contain the double or triple bond. 

Naming Alkenes and Alkynes (20.5) Example 20.5 For Practice 20.5 Exercises 53-56... 

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