Isoprene (EYE-so-preen) is a clear, colorless, volatile
liquid that is both very flammable and quite explosive. It is
classified as a diene compound because its molecules contain
two (‘‘di-’’) double bonds (‘‘-ene’’). It is also a member of the
terpene family. The terpenes are a large family of organic
compounds that contain two or more isoprene units. An
example of a terpene is vitamin A, whose molecular formula
is C20H30O. Vitamin A contains four isoprene units. The
terpenes occur abundantly in nature in both plants and
animals.
Some common terpenes include geraniol, found in geraniums;
limonene, oil of orange; a-pinene, or oil of turpentine;
a-farnesene, oil of cintronella; zingiberene, oil of ginger;
farnesol, found in lily of the valley; b-selinene, oil of celery;
and caryophyllene, oil of cloves. Isoprene is also produced in
animal bodies and is said to be the most common hydrocarbon
present in the human body. By one estimate, a 70-kilogram
(150-pound) person produces about 17 milligrams of
isoprene per day. Probably the best-known source of isoprene
is natural rubber, which is a polymer consisting of long
chains of isoprene units joined to each other.
A number of methods are available for preparing isoprene
from petroleum. Perhaps the most common process is
the cracking of hydrocarbons present in the naphtha portion
of refined petroleum. Cracking is the process by which large
hydrocarbons are broken down into smaller hydrocarbons
either with heat or over a catalyst, or by some combination
of heat and catalyst. The naphtha portion of petroleum consists
of hydrocarbons with boiling points between about
50C and 200C (120F and 400F). Other methods for the
preparation of isoprene include the dehydrogenation
(removal of hydrogen) of isopentene (CH3CH(CH3)CH=CH2),
the pyrolysis (decomposition by high heat) of methylpentene
(CH2=C(CH3)CH2CH2CH3), or the dehydration (removal of
water) of methylbutenol (CH3C(CH3)(OH)CH2CH3).
Natural rubber has been known to humans for hundreds
of years. Archaeologists have found that the Indians of
South and Central America were making rubber products
as early as the eleventh century. Until the end of the nineteenth
century, natural supplies of rubber obtained from
the rubber tree, Hevea brasiliensis, were sufficient to meet
consumer demand for the product. However, with the development
of modern technology—especially the invention of
the automobile—natural supplies of the product proved to
Interesting Facts
• Isoprene and other terpenes are now known
to undergo reactions that contribute to the development
of pollutants, such as ozone and oxides
of nitrogen in the atmosphere.
• Isoprene is a key intermediary in the synthesis
of cholesterol in the human body.
• The production of isoprene by plants seems to be
associated with the process of photosynthesis
and is affected by temperature, sunlight, other gases, and other
factors.
• The polymer of isoprene is called polyisoprene. It
exists in two forms, cis- and trans-polyisoprene. The two
forms are called geometric isomers. They have the
same kind and number of atoms, but the atoms
are arranged differently in the two forms. Natural
rubber consists of transpolyisoprene, while another product found in rubber plants, gutta percha, is made of cis-polyisoprene.
be insufficient to meet growing demand. Chemical researchers
began to look for ways of producing synthetic forms of
rubber.
One approach was to attempt making synthetic rubber
with exactly the same chemical composition as that of natural
rubber, that is, a polymer of trans-polyisoprene. As early
as the 1880s, British chemist Sir William Augustus Tilden
(1842–1926) was successful in achieving this objective. Tilden
found that he could make isoprene by heating turpentine
(C10H16 ). The isoprene then polymerized easily when exposed
to light. After more than twenty years of research, however,
Tilden decided that synthetic trans-polyisoprene could never
be made economically, and he encouraged his friends to
forget about the process.
Over the years, chemists did find ways of making other
types of synthetic rubber, and some never abandoned the
effort to make synthetic trans-polyisoprene. The critical
breakthrough needed in this research occurred in about
1953 when Swiss chemist Karl Ziegler (1898–1973) and Italian
chemist Giulio Natta (1903–1979) each found a way of
polymerizing isoprene in such a way that its geometric
structure matched that of natural rubber exactly. A year
later, chemists at two of the largest rubber companies in
the world, B. F. Goodrich and Firestone, announced that they
had developed methods for making synthetic trans-polyisoprene
using essentially the methods developed earlier by
Ziegler and Natta.
In the early twenty-first century, more than 95 percent
of the isoprene produced is used to make trans-polyisoprene
synthetic rubber. The remaining 5 percent is used to make
other types of synthetic rubber and other kinds of polymers.
A small amount of the compound is used as a chemical
intermediary, a substance from which other organic chemicals
is made.
Isoprene is a dangerous fire hazard. It also poses a risk to
human health and that of other animals. It is an irritant to
skin, eyes, and the respiratory system. Upon exposure, it
produces symptoms such as redness, watering, and itching
of the eyes and itching, reddening, and blistering of the skin.
If inhaled, it can irritate the lungs and respiratory system.
Isoprene is a known carcinogen.
