Organic Chemistry

Section 26.6 Terpenes 015

In the case of cyclic compounds, the linkage of the head of one isoprene unit to the tail
of another is followed by an additional linkage to form the ring. The second linkage is
not necessarily head-to-tail, but is whatever is necessary to form a stable five- or six-
membered ring.

In Section 26.8, we will see that the compound actually used in the biosynthesis of
terpenes is not isoprene, but isopentenyl pyrophosphate, a compound that has the same
carbon skeleton as isoprene. We will also look at the mechanism by which isopentenyl
pyrophosphate units are joined together in a head-to-tail fashion.
Terpenes are classified according to the number of carbons they contain (Table 26.3).
Monoterpenes are composed of two isoprene units, so they have 10 carbons.
Sesquiterpenes, with 15 carbons, are composed of three isoprene units. Many fra-
grances and flavorings found in plants are monoterpenes and sesquiterpenes. These
compounds are known as essential oils.

head

tail

O O

carvone

spearmint oil

a monoterpene

head

tail

-farnesene

a sesquiterpene found in the

waxy coating on apple skins

head

tail

Table 26.3 Classification of Terpenes

Carbon atoms Classification Carbon atoms Classification

10 monoterpenes 25 sesterterpenes

15 sesquiterpenes 30 triterpenes

20 diterpenes 40 tetraterpenes

Triterpenes(six isoprene units) and tetraterpenes(eight isoprene units) have im-
portant biological roles. For example,squalene, a triterpene, is a precursor of steroid
molecules (Section 26.9).

Carotenoidsare tetraterpenes. Lycopene, the compound responsible for the red col-
oring of tomatoes and watermelon, and the compound that causes carrots
and apricots to be orange, are examples of carotenoids. b-Caroteneis also the coloring

b-carotene,

squalene