Naming Alkenes and Alkynes
Because of their multiple bond, alkenes and alkynes have fewer hydrogens
per carbon than related alkanes and are therefore referred to as unsaturated.
Ethylene, for example, has the formula C2H4, and acetylene has the formula
C2H2, whereas ethane has the formula C2H6.
Alkenes are named using a series of rules similar to those for alkanes (Sec-
tion 2.3), with the sufﬁx -ene used in place of -ane to identify the family. There
are three steps.
Name the parent hydrocarbon.
Find the longest carbon chain that contains the double bond, and name the
compound using the sufﬁx -ene in place of -ane.
Number the carbon atoms in the chain.
Begin numbering 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
Write the full name.
Number the substituents on the main chain according to their position, and
list them alphabetically. Indicate the position of the double bond by giving
the number of the ﬁrst alkene carbon and placing that number directly
before the -ene sufﬁx. If more than one double bond is present, give the
position of each and use the appropriate multiplier sufﬁx -diene, -triene,
-tetraene, and so on.
We should also note that IUPAC changed its naming rules in 1993. Prior
to that time, the locant, or number locating the position of the double bond,
was placed before the parent name rather than before the -ene sufﬁx:
2-butene rather than but-2-ene, for instance. Changes always take time to
be fully accepted, so the new rules have not yet been adopted universally
and some texts have not yet been updated. We’ll use the new naming system
in this book, although you may encounter the old system elsewhere. Fortu-
nately, the difference between old and new is minor and rarely causes