Organic Chemistry

Z isomer

of 2-butene

E isomer

of 2-butene

I

n Chapter 2, you learned that alkanes are hydrocarbons that con-

tain only carbon–carbon singlebonds. Hydrocarbons that contain a

carbon–carbon doublebond are called alkenes. Early chemists noted

that an oily substance was formed when ethene the smallest

alkene, reacted with chlorine. On the basis of this observation, alkenes were

originally called olefins(oil forming).

Alkenes play many important roles in biology. Ethene, for example, is a plant hor-

mone—a compound that controls the plant’s growth and other changes in its tissues.

Ethene affects seed germination, flower maturation, and fruit ripening.

Insects communicate by releasing pheromones—chemical substances that other in-

sects of the same species detect with their antennae. There are sex, alarm, and trail

pheromones, and many of these are alkenes. Interfering with an insect’s ability to send

or receive chemical signals is an environmentally safe way to control insect popula-

tions. For example, traps with synthetic sex attractants have been used to capture such

crop-destroying insects as the gypsy moth and the boll weevil. Many of the flavors and

fragrances produced by certain plants also belong to the alkene family.

muscalure

sex attractant of the housefly

a-farnesene

found in the waxy coating

on apple skins

limonene

from lemon and

orange oils

-phellandrene

oil of eucalyptus

multifidene

sex attractant of

brown algae

b

(H 2 C“CH 2 ),

3

Alkenes

Structure, Nomenclature, and an Introduction to Reactivity

Thermodynamics

and Kinetics

111

3-D Molecules:

Limonene; -Phellandrene;

Multifidene

b

Ethene is the hormone that

causes tomatoes to ripen.

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