Some alkenes, called polyenes,have two or more carbon–carbon double bonds per
molecule. The suffixes -adiene, -atriene,and so on are used to indicate the number of
(CUC) double bonds in a molecule.1234 4321
CH 2 UCHXCHUCH 2 CH 3 XCHUCUCH 2
1,3-butadiene 1,2-butadiene1,3-Butadiene and similar molecules that contain alternatingsingle and double bonds are
described as having conjugated double bonds.Such compounds are of special interest
because of their polymerization reactions (Section 27-19).1056 CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties
EXAMPLE 27-6 Naming Alkenes
Give the IUPAC names of the following two alkenes.Plan
For each compound, we first find the longest chain that includes the double bond, and then
number it beginning at the end nearer the double bond (Rules 1 and 2). Then we specify the
identities and positions of substituents in the same way we did for alkanes. In (a) we specify
the geometric isomer by locating the two largest groups attached to the double-bonded carbons
and then describing their relationship using the cis–transterminology.
Solution
(a)The longest such chain contains five atoms and has a double bond beginning at atom 2; thus,
the compound is named as a derivative of 2-pentene. Now we must apply Rule 4. The twoC CCH 3 CH 2 CH 3H CH 312 34 5C CCH 3 CH 2 CH 3H CH 3(a)
C CCH 3 CH 2 CH 2 CH 3H CH 2 CH 3(b)Problem-Solving Tip:When Is the Location of a Substituent or
a Multiple Bond Not Included in an
IUPAC Name?If only one location is possible for a substituent or double bond, the number that indi-
cates the location can be left out of the compound’s name. If there is doubt, put the
number in the name. For example, “2-methyl-1-propene” is named simply methyl-
propene. There are no other locations possible for the substituent or the double bond.
The name without the numbers is the correct name. Both numbers are necessary to name
3-methyl-1-butene, however.CH CHCH 3CH 3 C CH 2CH 3CH 3 CH 2methylpropene 3-methyl-1-butene