How Do We Name Alkenes and Alkynes

Alkenes are named using the lUPAC system, but, as we shall see, some are still referred to by their common names. 

We form lUPAC names of alkenes by changing the -an- infix of the parent alkane to -en-(Section 3.5). Hence, CH2 = CH2 is named ethene, and CH3CH=CH2 is named propene. In higher alkenes, where isomers exist that differ in the location of the double bond, we use a numbering system. We number the longest carbon chain that contains the double bond in the direction that gives the carbon atoms of the double bond the lower set of numbers. We then use the number of the first carbon of the double bond to show its location. We name branched or substituted alkenes in a manner similar to the way we name alkanes (Section 3.3). We number the carbon atoms, locate the double bond, locate and name substituent groups, and name the main (parent) chain. 

Note that there is a six-carbon chain in 2-ethyl-3-methyl-l-pentene. However, because the longest chain that contains the carbon-carbon double bond has only five carbons, the parent hydrocarbon is pentane, and we name the molecule as a disubstituted 1-pentene. 

Write the lUPAC name of each unsaturated hydrocarbon  

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