The Foundations of Chemistry

Summary of IUPAC Rules for Naming Alkenes and Cycloalkenes

1.Locate the C atoms in the longestC chain that contains the double bond.Use the

base name prefix with the ending -ene.

2.Number the C atoms of this chain sequentially beginning at the end nearer the

double bond.Insert the number describing the position of the double bond (indi-

cated by its firstcarbon location) before the base name. (This is necessary only

for chains of four or more C atoms, because only one position is possible for a

double bond in a chain of two or three carbon atoms.)

3.In naming alkenes, the double bond takes positional precedence over substituents

on the carbon chain. The double bond is assigned the lowest possible number.

4.To name compounds with possible geometric isomers, consider the two largest

groups within the carbon chain that contains the double bond—these are indi-

cated as part of the base name. The isomer in which the largest groups at each

end of the CUC are located on opposite sides is called trans.If the largest groups

are on the same side, the molecule is referred to as cis.Insert the prefix cis- or

trans- just before the number of the double bond to indicate whether the largest

groups are on the same or opposite sides, respectively, of the double bond.

5.For cycloalkenes, the double bond is assumed to be between C atoms 1 and 2,

so no position number is needed to describe it.

Some illustrations of this naming system follow.

The following two names illustrate the application of Rule 3.

4-Methyl-2-pentene can exist as either of two isomers. Their names illustrate the appli-
cation of Rule 4.

C C

CH 3 CH(CH 3 ) 2

H H

4-methyl-cis-2-pentene

C C

CH 3

CH(CH 3 ) 2

H

H

4-methyl-trans-2-pentene

1

2 3

45 1

2 3

45

CH

CH 3

CH 2 CH CH CH 3

CH 3

CH 3 C CH 2 CH 3

2-methyl-1-butene 4-methyl-2-pentene

4 3 2 1 152 3 4

CH 2 CH 2 CH 3 CH CH 2 CH 3 CH 2 CH CH 2

ethene

(ethylene)

systematic:

trivial:

propene

(propylene)

1-butene

(butylene)

CH 3 CH CHCH 3 CH 3

CH 3

C CH 2

2-butene

methylpropene

(isobutylene)

4 3 2 1

12 34

The prefix trans- means “across” or

“on the other side of.” As a reminder

of this terminology, think of words

such as “transatlantic.”

27-3 Alkenes 1055

There are two isomers of 2-butene,

cis-2-butene and trans-2-butene.