isoprene or 2-methyl-1,3-butadiene (ī´səprēn, byōō´tədī´ēn), colorless liquid organic compound. It is a hydrocarbon, and is insoluble in water but soluble in many organic solvents; it boils at 34°C. The isoprene molecule contains two double bonds. It is readily polymerized by the use of special catalysts; large numbers of isoprene molecules join together to form a single large, threadlike polyisoprene molecule. Isoprene polymers also occur naturally. The natural rubber caoutchouc is cis-1,4-polyisoprene, and trans-1,4-polyisoprene is present in the natural rubbers balata and gutta-percha. (The cis and trans polyisoprenes are structural isomers.)

isoprene A colourless liquid diene, CH₂:C(CH₃)CH:CH₂. The systematic name is 2-methylbuta-1,3-diene. It is the structural unit in terpenes and natural rubber, and is used in making synthetic rubbers.

isoprene (2-methyl butadiene) A five-carbon compound that forms the structural basis of many biologically important compounds, e.g. the terpenes.

isoprene (2-methyl butadiene) A 5-carbon compound that forms the structural basis of many biologically important compounds (e.g. the terpenes)

Isoprene

OVERVIEW

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.

KEY FACTS

OTHER NAMES:

2-methyl-1,3-butadiene

FORMULA:

CH₂=CH(CH₃)CH=CH₂

ELEMENTS:

Carbon, hydrogen

COMPOUND TYPE:

Alkadiene; unsaturated hydrocarbon (organic)

STATE:

Liquid

MOLECULAR WEIGHT:

68.12 g/mol

MELTING POINT:

−145.9°C (−230.6°F)

BOILING POINT:

34.0°C (93.2°F)

SOLUBILITY:

Insoluble in water; miscible with ethyl alcohol, acetone, ether, and benzene

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; β-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.

HOW IT IS MADE

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 50°C and 200°C (120°F and 400°F). 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).

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 trans-polyisoprene, while another product found in rubber plants, gutta percha, is made of cis-polyisoprene.

COMMON USES AND POTENTIAL HAZARDS

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 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.

Words to Know

CARCINOGEN

A chemical that causes cancer in humans or other animals.

MISCIBLE

Able to be mixed; especially applies to the mixing of one liquid with another.

PHOTOSYNTHESIS

The process by which green plants and some other organisms use the energy in sunlight to convert carbon dioxide and water into carbohydrates and oxygen.

POLYMER

A compound consisting of very large molecules made of one or two small repeated units called monomers.

VOLATILE

Able to turn to vapor easily at a relatively low temperature.

FOR FURTHER INFORMATION

"Hazardous Substance Fact Sheet: Isoprene." New Jersey Department of Health and Senior Services. http://www.state.nj.us/health/eoh/rtkweb/1069.pdf (accessed on December 29, 2005).

"Isoprene." Shell Chemicals. http://www.shellchemicals.com/isoprene/1,1098,1116,00.html (accessed on December 29, 2005).

"Material Safety Data Sheet: Isoprene MSDS." ScienceLab.com. http://www.sciencelab.com/xMSDS-Isoprene-9924409 (accessed on December 29, 2005).

"United States Synthetic Rubber Program, 1939–1945." National Historic Chemical Landmarks, American Chemical Society. http://acswebcontent.acs.org/landmarks/landmarks/rbb/rbb_begin.html (accessed on December 29, 2005).

Weissermel, Klaus, and Hans-Jürgen Arpe. Industrial Organic Chemistry. Weinheim, Germany: Wiley-VCH, 2003, 117-122.

See AlsoPoly(Styrene-Butadiene-Styrene)