Ethylene

OVERVIEW

Ethylene (ETH-ih-leen) is a colorless, flammable gas with a sweet odor and taste. It is the simplest alkene. Alkenes are hydrocarbons that contain one or more double bonds. Ethylene was first prepared in 1794 by a group of Dutch chemists including J. R. Deiman, A. Paets van Troostwyk, N. Bondt, and A. Lauwerenburgh. They treated ethanol (ethyl alcohol; C₂H₅OH) with concentrated sulfuric acid (H₂SO₄) and obtained ethylene, although they were incorrect in believing that the compound also contained oxygen.

KEY FACTS

OTHER NAMES:

Ethene; bicarburetted hydrogen; olefiant gas

FORMULA:

CH₂=CH₂

ELEMENTS:

Carbon, hydrogen

COMPOUND TYPE:

Alkene; unsaturated hydrocarbon (organic)

STATE:

Gas

MOLECULAR WEIGHT:

28.05 g/mol

MELTING POINT:

−169.15°C (−272.47°F)

BOILING POINT:

−103.77°C (−154.79°F)

SOLUBILITY:

Insoluble in water; slightly soluble in ethyl alcohol, benzene, and acetone; soluble in ether

Ethylene occurs naturally in petroleum and natural gas, but only to a very small percentage. It also occurs naturally in plants where it functions as a hormone and has a number of important effects on the growth and development of plants. These effects have been used for thousands of years, although the chemical mechanism involved was not understood. For example, the ancient Chinese are said to have burned incense in closed containers in order to facilitate the ripening of pears. Although they were certainly not aware of the fact, the ripening effect was probably a result of ethylene gas released during combustion of the incense.

The hormonal effects of ethylene were first identified in 1901 by Russian chemist Dmitri N. Neljubow. For some time, scientists had observed that leaks from the gas lamps used to light city streets caused plants to grow old more rapidly than normal, often with strange changes in the structure of their leaves and stems. Neljubow was able to show that this effect was caused by ethylene in the lamp gas. Scientists have since learned a great deal about the production of ethylene in plants and the effects produced by the gas on the growth and development of those plants. For example, scientists discovered that ethylene is synthesized in germinating seeds, nodes of stems, and the tissue of ripening fruits. Its production is increased by flooding of the plant's roots; by drying of the soil; in response to environmental stress, such as attack by pests; by aging of the plant; and by inadequate amounts of minerals in the soil. They have also learned that ethylene increases the rate at which leaves and flowers age; promotes germination of seeds and the growth of root hairs; stimulates the ripening of fruit and flowers; and increases a plant's resistance to disease and physical damage.

HOW IT IS MADE

One reference on hydrocarbons lists more than 500 methods for making ethylene. From a practical standpoint, only a handful of those methods are important. The most common method of preparation involves the thermal or catalytic cracking of hydrocarbons. The term cracking refers to the process by which hydrocarbons from petroleum are broken down into simpler molecules. That process is usually accomplished by heating the petroleum for short periods of time at high temperatures (thermal cracking) or over a catalyst (catalytic cracking). In the cracking process, hydrocarbons with 10, 15, 20, or more carbon atoms are broken down to produce hydrocarbons with two, three, four, or some other small number of carbons. Ethylene is one of the usual products of cracking. It can be separated from the other products of cracking because it escapes from the reaction mixture as a gas.

Ethylene can also be produced from synthesis gas. Synthesis gas is the term used for various mixtures of gases produced when steam (with or without additional oxygen) is passed over hot coal. The steam and coal react to produce a rich mixture of hydrocarbons, a mixture that usually includes ethylene. Finally, ethylene can be produced in small quantities in the laboratory by the method first used by the Dutch chemists in 1794, namely by reacting ethanol with concentrated sulfuric acid.

COMMON USES AND POTENTIAL HAZARDS

In 2004, U.S. chemical manufacturers produced about 26.7 million metric tons (29.4 million short tons) of ethylene, making it the third most important chemical produced in the country in terms of volume. Over 90 percent of that ethylene was used for the production of other chemical compounds, the most important of which were polymers and related compounds, such as three major kinds of polyethylene (high density (HDP), low density (LDP), and linear low density polyethylene (LLDP)), polypropylene, ethylene dichloride, ethylene oxide, styrene, ethylbenzene, vinyl acetate, vinyl chloride, ethylene glycol, polystyrene, polyvinyl chloride (PVC), trichloroethylene, styrene-butadiene rubber (SBR), and ethyl alcohol.

Smaller amounts of ethylene were used in a number of other chemical and industrial applications. These included:

• As a spray to accelerate the ripening of fruit;
• As a refrigerant;
• In oxygen-ethylene torches used in welding and metal-cutting operations;
• As a specialized anesthetic.

Interesting Facts

• The highest rate of ethylene production yet measured in plants is that produced by the fading blossoms of the Vanda orchid, at 3.4 microliters per gram of flower per hour. The Vanda orchid is extensively used in the manufacture of Hawaiian leis.
• Air in rural areas typically contains about 5 parts per billion (ppb) of ethylene, while urban air generally contains about twenty times that amount.
• Ethylene is produced in plants in a complex series of reactions known as the Yang cycle (after its discoverer, S. F. Yang) that starts with the amino acid methionine.
• Production of ethylene in plants is significantly affected by a number of factors, such as temperature and presence of other gases (especially oxygen and carbon dioxide).

Ethylene poses a health hazard primarily because it is highly flammable and a serious explosive risk. It also acts as a narcotic at low concentrations, causing nausea, dizziness, headaches, and loss of muscular coordination. At higher concentrations, it acts as an anesthetic, causing loss of consciousness and insensitivity to pain and other stimuli. These effects tend to be of concern primarily to people who work directly with the gas. The amount of ethylene to which most people are exposed in their daily lives tends to be relatively low.

Words to Know

CATALYST

A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.

CRACKING

The process by which larger hydrocarbons from petroleum are broken down into simpler molecules.

POLYMER :

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

FOR FURTHER INFORMATION

"Ethene (ethylene): Properties, Production & Uses." Aus-e-tute. http://www.ausetute.com.au/ethene.html (accessed on December 27, 2005).

"Ethylene." Laboratory of Postharvest Physiology and Technology, Seoul National University. http://plaza.snu.ac.kr/∼postharv/p3ethylene.pdf (accessed on December 27, 2005).

"Ethylene." National Safety Council. http://www.nsc.org/library/chemical/Ethylene.htm (accessed on December 27, 2005).

See AlsoEthyl Alcohol; Ethylene Glycol; Ethylene Oxide; Ethylbenzene; Polyethylene; Polypropylene; Polystyrene; Polyvinyl Chloride

ethylene (ethene) A colourless gaseous hydrocarbon, C₂H₄, that occurs naturally in plants and acts as a growth substance in a variety of physiological roles. It is produced in response to stresses, such as water shortage, and acts as an effector for auxins: auxins stimulate tissues to produce ethylene, which diffuses rapidly to trigger responses in surrounding cells. The best known effect is the stimulation of fruit ripening: fruits such as bananas, apples, and avocados naturally produce ethylene during the later stages of ripening, and ethylene gas is used to promote the ripening of fruits, such as bananas, that are picked and shipped ‘green’. Ethylene generally suppresses flowering, except in members of the pineapple family (Bromeliaceae) – hence flowering of pineapples may be synchronized by releasing ethylene into the growing crop. Studies have shown varied and often contradictory effects of ethylene on vegetative growth. For example, in rice it acts with gibberellins to promote stem elongation, while in peas ethylene inhibits root and shoot elongation. Seed germination, bud opening, and root initiation may also be promoted by ethylene.

eth·yl·ene / ˈe[unvoicedth]əˌlēn/ • n. Chem. a flammable hydrocarbon gas, C₂H₄, of the alkene series, occurring in natural gas, coal gas, and crude oil and given off by ripening fruit. It is used in chemical synthesis, esp. in the manufacture of polyethylene.

ethylene (ethene), a gas of the formula CH₂=CH₂, produced by fruit as a hormone to speed ripening of climacteric fruits. This explains why some fruits ripen faster if they are stored in a plastic bag. It is used commercially in very small amounts to speed fruit ripening after harvesting.

ethylene See ETHENE.