52 Naming Alkenes
Alkenes are named as if they were alkanes, but the ”-ane” suffix is changed to ”-ene”. If the
alkene contains only one double bond and that double bond is terminal (the double bond
is at one end of the molecule or another) then it is not necessary to place any number in
front of the name.
butane:C 4 H 10 (CH 3 CH 2 CH 2 CH 3 )butene:C 4 H 8 (CH 2 =CHCH 2 CH 3 )If the double bond is not terminal (if it is on a carbon somewhere in the center of the
chain) then the carbons should be numbered in such a way as to give the first of the two
double-bonded carbons the lowest possible number, and that number should precede the
”ene” suffix with a dash, as shown below.
correct:pent-2-ene (CH 3 CH=CHCH 2 CH 3 )incorrect:pent-3-ene (CH 3 CH 2 CH=CHCH 3 )The second one is incorrect because flipping the formula horizontally results in a lower
number for the alkene.If there is more than one double bond in an alkene, all of the bonds should be numbered
in the name of the molecule - even terminal double bonds. The numbers should go from
lowest to highest, and be separated from one another by a comma. The IUPAC numerical
prefixes are used to indicate the number of double bonds.
octa-2,4-diene:CH 3 CH=CHCH=CHCH 2 CH 2 CH 3deca-1,5-diene:CH 2 =CHCH 2 CH 2 CH=CHCH 2 CH 2 CH 2 CH 3Note that the numbering of ”2-4” above yields a molecule with two double bonds separated
by just one single bond. Double bonds in such a condition are called ”conjugated”, and
they represent an enhanced stability of conformation, so they are energetically favored as
reactants in many situations and combinations.
52.1 EZ Notation
Earlier in stereochemistry, we discussed cis/trans notation where cis- means same side and
trans- means opposite side. Alkenes can present a unique problem, however in that the