Ethylene

Ethylene is a plant hormone that helps in the ripening of fruit. Additionally, ethylene is used widely in the manufacture of many products. Ethylene is an organic hydrocarbon gas that occurs naturally but may also be synthesized in several ways. Because of its usefulness in both agriculture and manufacturing, ethylene is among the most commonly produced chemicals.

Background

Ethylene, sometimes called ethene, is a colorless gas. It has a slightly sweet smell when manufactured but no scent when released naturally by plants. Ethylene is flammable in large quantities. It is composed of four hydrogen atoms and a pair of carbon atoms connected by a double bond. The double bond makes ethylene very reactive; the density of the electrons in the area of the double bond allows for easy bonding with and change by other chemicals to which it is exposed.

For centuries, people have known that certain fruits can help other fruits ripen faster. Ancient Africans and Egyptians would cut figs to help them ripen faster, and Chinese pear farmers noticed that burning incense in a room with pears made them ripen more quickly. It was not until 1901, however, that seventeen-year-old Russian scientist Dimitry Neljubow explained why this happened.

Neljubow worked in a laboratory and happened to notice that the trees nearest to the gas streetlamps were not growing in the same manner as trees located farther from the lamps. The trees close to the lamps were shorter and had thick, twisted branches. Acting on the theory that something in the lamp fuel was causing the problem, Neljubow conducted experiments with pea plants. He grew them in darkened boxes and pumped air from the laboratory, which was lit by coal lamps, into some of the boxes. The peas grown in these boxes were stunted, with shorter, thicker stems than the others. Neljubow experimented further and determined that ethylene gas produced by the combustion of the coal gas was the cause for the unusual growth.

Ethylene gas had been synthesized since the mid-1790s, when four Dutch chemists produced what was called an olefiant, or oil-making gas. The gas did not become known as ethylene until around 1852. This name was derived by appending the Greek suffix –ene, meaning "daughter of," to ethyl. Ethyl is a chemical derivative of ethane, an odorless, colorless gas. Ethylene was the first hydrocarbon, or a compound of hydrogen and oxygen, to be artificially replicated.

Overview

Neljubow was the first to identify the effect that ethylene gas had on plants. Within a decade, others realized that ethylene was released by some fruits and affected others. In 1910, chemist H. H. Cousins noticed that when bananas and oranges were shipped together, the bananas ripened more quickly than when they were shipped alone. It was subsequently determined that while oranges release very little ethylene, the mold that frequently grows on them releases enough to affect the bananas. In the 1930s, other orange growers discovered that when they used kerosene heaters to protect their crops from freezing, the oranges ripened more quickly. They soon determined that the faster ripening was not caused by the heat, as they originally had thought, but by the gas released by the heaters. In 1934, ethylene was identified as a plant hormone.

Scientists ultimately determined that fruits release ethylene and that the amount released increases if the fruit is damaged by heat, cutting, bruising, and so on. The gas can also trigger other functions of a fruiting plant, such as causing seeds to sprout, flowers to die, or leaves to change color and fall. In the 1990s, researchers identified specific plant genes responsible for these changes and determined that ethylene gas triggers a chain reaction that releases other enzymes and chemicals that tell a plant to grow or a fruit to ripen. These changes help the plant to grow and help more plants to be propagated as the fruit ripens and is consumed and its seeds are spread.

While researchers continue to investigate how plants produce and use ethylene, agriculturalists use synthesized versions of the gas to help ripen fruit. With this technique, fruit growers can control the timing of their crops. Alternatively, they can pick underripe fruit and treat it during its shipment so that it ripens closer to the time when it will be sold.

Synthesized ethylene is very useful in manufacturing, too. It is primarily used for the production of polyethylene, a component in plastic bags and wraps and coatings for pipes, wires, paper, and cardboard. About 50 percent of the ethylene produced is used to make polyethylene. Ethylene derivatives are used to make cleaners and detergents. They are used in the soft drink and textile industries and in the production of rubberlike products for tires and footwear.

Ethylene is manufactured through a steam cracking process that uses high heat (as high as 950 degrees Celsius or 1,742 degrees Fahrenheit) to break down gaseous or semiliquid hydrocarbons, such as natural gas or oil. The smaller hydrocarbons are subjected to repeated compression and distillation to "crack" the gas and produce ethylene. Ethylene can also be produced in a laboratory by treating ethanol with sulfuric acid and a solution of sodium hydroxide.

Chemically-produced ethylene is treated in many ways for use in manufacturing. It may be oxidized, or treated with oxygen; polymerized, which combines smaller molecules into larger ones; halogenated or hydrohalogenated, or treated with one of several chemicals known as halogens; alkylated, or treated with an alkyl; hydrated, or treated with water; or oligomerized, or converted to a longer chain of molecules. The ease with which ethylene can be treated makes it quite versatile.

Ethylene is used in the manufacture of solvents, antifreeze, diapers, toys, pharmaceuticals, cosmetics, automotive products, electronics, paints and inks, textiles, and other products. More than 150 million tons of ethylene are produced worldwide each year. The Middle East and the United States lead the world in ethylene production.

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