Organic Chemistry

Section 8.3 Relative Stabilities of Dienes 301

PROBLEM 1

Give the systematic name for each of the following compounds:

a. e.

b. f.

c. g.

d. h.

8.2 Configurational Isomers of Dienes

A diene such as 1-chloro-2,4-heptadiene has four configurational isomers because
each of the double bonds can have either the Eor the Zconfiguration. Thus, there are
E-E,Z-Z,E-Z, and Z-Eisomers. The rules for determining the Eand Zconfigurations
were given in Section 3.5. Recall that the Zisomer has the high-priority groups on the
same side.

PROBLEM 2

Draw the configurational isomers for the following compounds, and name each one:

a. 2-methyl-2,4-hexadiene b. 2,4-heptadiene c. 1,3-pentadiene

8.3 Relative Stabilities of Dienes

In Section 4.11, we saw that the relative stabilities of substituted alkenes can be deter-
mined by their relative values of for catalytic hydrogenation. Remember that
the least stable alkene has the greatest value; the least stable alkene gives off
the most heat when it is hydrogenated, because it has more energy to begin with. The
-¢H°for hydrogenation of 2,3-pentadiene (a cumulated diene) is greater than that of

-¢H°

-¢H°

CC

C CH 2 CH 3

ClCH 2

H

H

C

ClCH 2

C

H

H

(2Z,4Z)-1-chloro-2,4-heptadiene

CC

C

CH 2 CH 3

H

H

H

H

(2Z,4E)-1-chloro-2,4-heptadiene

CC

C CH 2 CH 3

H

ClCH 2 H

H

C

H

(2E,4Z)-1-chloro-2,4-heptadiene

CC

C

CH 2 CH 3

H

ClCH 2 H

H

C H

(2E,4E)-1-chloro-2,4-heptadiene

CH 3 CH CCH 2 CHCH 2 OH

CH 3 CH 3

CH 3 CH 2 CH CCH 2 CH 2 C

CH CH 2

CH

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

CH 3

CH 2 CHCH 2 C CCH 2 CH 3 HOCH 2 CH 2 CCH

CH 3

CH 3

The Zisomer has the high-priority

groups on the same side.

Molecules:

(1Z,3Z)-1,4-Difluoro-

1,3-butadiene;

(1E,3E)-1,4-Difluoro-

1,3-butadiene;

(1Z,3E)-1,4-Difluoro-

1,3-butadiene

The least stable alkene has the greatest

≤H°value.