The Foundations of Chemistry

largest groups in the chain are the terminal XCH 3 (carbon 1) and the XCH 2 CH 3 (carbons 4
and 5); these are on the same sideof the double bond, so we name the compound as a deriva-
tive of cis-2-pentene. The only substituent is the methyl group at carbon 3. The full name of
the compound is 3-methyl-cis-2-pentene.

(b) There are two choices for the longest chain, and either one would have an ethyl substituent.

We could number either chain from the other end and still have the double bond starting at
carbon 3; we number from the end that gives the carbon bearing the ethyl group the lowest
possible position number, 3. Carbon 3 has two equivalent substituents, so geometric isomerism
is not possible, and we do not use the cis–transterminology. The name is 3-ethyl-3-hexene.

You should now work Exercise 20.

EXAMPLE 27-7 Naming Alkenes

Name the following alkene.

Plan

As we did in Example 27-6(b), 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.

Solution

There are two choices for the longest chain, and either one would have an ethyl substituent.

Carbon 3 has two equivalent substituents, so geometric isomerism is not possible, and we do
not use the cis-transterminology. The name is 3-ethyl-2-pentene.

You should now work Exercise 22.

The cycloalkenes are represented by the general formula CnH 2 n–2. Two cycloalkenes and
their skeletal representations are

CH 2

CH 2

CH 2

CH

H 2 C

HC

or or

CH 2

CH 2

CH

H 2 C

HC

cyclopentene cyclohexene

C C

CH 3 CH 2 CH 3

H CH 2 CH 3

4 3 2 1

5

C C

CH 3 CH 2 CH 3

H CH 2 CH 3

4 5

2 3

1

or

C C

CH 3 CH 2 CH 3

H CH 2 CH 3

C C

CH 3 CH 2 CH 2 CH 3

H CH 2 CH 3

4 3 2 1

6 5

C C

CH 3 CH 2 CH 2 CH 3

H CH 2 CH 3

2 1

4 3

6 5

or

One would not number the carbons as:

Rule 1 requires that we use the longest

chain that includes both carbons of the

double bond.

CH 3 CH 2 CH 3

H CH 2 CH 3

CC 4 5

3

2 1

27-3 Alkenes 1057