Organic Chemistry

Section 8.1 Nomenclature of Alkenes with More than One Functional Group 299

8.1 Nomenclature of Alkenes with More

than One Functional Group

To arrive at the systematic name of a diene, we first identify the longest continuous
chain that contains both double bonds by its alkane name and then replace the “ne”
ending with “diene.” The chain is numbered in the direction that gives the double
bonds the lowest possible numbers. The numbers indicating the locations of the dou-
ble bonds are cited either before the name of the parent compound or before the suffix.
Substituents are cited in alphabetical order. Propadiene, the smallest member of the
class of compounds known as allenes, is frequently called allene.

To name an alkene in which the second functional group is not another double
bond—and is named with a functional group suffix—choose the longest continuous
chain containing both functional groups, and cite both designations at the end of the
name. The “ene” ending is cited first, with the terminal “e” omitted in order to avoid
two adjacent vowels. The location of the first cited functional group is usually given
before the name of the parent chain. The location of the second functional group is
cited immediately before its suffix.
If the functional groups are a double bondand a triple bond, the chain is numbered
in the direction that yields the lowest number in the name of the compound. Thus, the
lower number is given to the alkene suffix in the compound on the left and to the
alkyne suffix in the compound on the right.

CH 2 C CH 2

propadiene

allene

systematic:

common:

CH 2 C CH CH 2 Br

1 2 3 4

4

6

5 3

2

1

2-methyl-1,3-butadiene

or

2-methylbuta-1,3-diene

isoprene

5-bromo-1,3-cyclohexadiene

or

5-bromocyclohexa-1,3-diene

CH 3 CH CHCH 2 C CH 2

6 5 4 3 2 1

2-methyl-1,4-hexadiene

or

2-methylhexa-1,4-diene

CH 3

CH 3 C CHCH CCH 2 CH 3

1 2 3 4 5 6 7

5-ethyl-2-methyl-2,4-heptadiene

or

5-ethyl-2-methylhepta-2,4-diene

CH 2 CH 3

CH 3

CH 3

CH 3 CH CH CH CHCH 3 CH 2 CH CH 2 CH CH 2

a conjugated diene

CH 3 CH C CH CH 3

a cumulated diene

an allene

an isolated diene

double bonds are

separated by one

single bond

double bonds are

separated by more

than one single bond

double bonds

are adjacent