Ethylene

Ethylene (or IUPAC name ethene) is the simplest alkene hydrocarbon, consisting of two carbon atoms and four hydrogens. There is a double bond between the two carbons. Because it contains a double bond, ethylene is called an unsaturated hydrocarbon or an olefin.

The molecule cannot twist around the double bond, and all six atoms lie in the same plane. The angle made by two carbon-hydrogen bonds in the molecule is 117°, very close to the 120° that would be predicted from ideal sp² hybridization.

1 Nomenclature
2 Chemistry
3 Production
4 Uses
5 Ethylene as a plant hormone
- 5.1 Location, Characteristics and Occasions for Synthesis Induction
- 5.2 Effects
6 External links

Nomenclature

From 1795 on, ethylene was referred to as the olefiant gas (oil-making gas), because it combined with chlorine to produce the oil of the Dutch chemists (1,2-dichloroethane), first synthesized in 1795 by a collaboration of four Dutch chemists.

In the mid-19th century, the suffix -ene (a Greek root added to the end of female names meaning "daughter of") was widely used to refer to a molecule or part thereof that contained one fewer hydrogen atoms than the word being modified. Thus, ethylene (C₂H₄) was the "daughter of ethyl" (C₂H₅). The name ethylene was used in this sense as early as 1852.

In 1866, the German chemist Augustus von Hofmann proposed a system of hydrocarbon nomenclature in which the suffixes -ane, -ene, -ine, -one, and -une were used to denote the hydrocarbons with 0, 2, 4, 6, and 8 fewer hydrogens than their parent alkane. In this system, ethylene became ethene. Hofmann's system eventually became the basis for the Geneva nomenclature approved by the International Congress of Chemists in 1892, which remains at the core of the IUPAC nomenclature. However, by that time, the name ethylene was deeply entrenched, and it remains in wide use today, especially in the chemical industry.

Chemistry

The double bond is a region of slightly higher electron density, and most of ethylene's chemistry involves other molecules reacting with and adding across its double bond. Ethylene can react with bromine, chlorine, and other halogens, to produce halogenated hydrocarbons. It can also react with water to produce ethanol but the rate at which this happens is very slow unless a suitable catalyst, such as phosphoric or sulfuric acid, is used. Under high pressure, and in the presence of a catalytic metal (platinum, rhodium, nickel), hydrogen will react with ethylene, saturating it.

Production

Ethylene is produced in the petrochemical industry via steam cracking. In this process, gaseous or light liquid hydrocarbons are briefly heated to 750–950 °C, causing numerous free radical reactions to take place. Generally, in the course of these reactions, large hydrocarbons break down in to smaller ones and saturated hydrocarbons become unsaturated.

The result of this process is a complex mixture of hydrocarbons in which ethylene is one of the principal components. The mixture is separated by repeated compression and distillation.

Another process is catalytic cracking where it is used in oil refineries to crack large hydrocarbon molecules into smaller ones. Use of zeolite as a catalyst allows the cracking to be achieved at a lower temperature. It is an important way of separating alkenes from alkanes using a fractionating column.

Uses

Chemistry - Ethylene is used primarily as an intermediate in the manufacture of other chemicals, especially plastics. Ethylene may be polymerized directly to produce polyethylene (also called polyethene or polythene), the world's most widely used plastic. Ethylene can be chlorinated to produce 1,2-dichloroethane, a precursor to the plastic polyvinyl chloride, or combined with benzene to produce ethylbenzene, which is used in the manufacture of polystyrene, another important plastic.

Smaller amounts of ethylene are oxidized to produce chemicals including ethylene oxide, ethanol, and polyvinyl acetate.

Ethylene is also a widely used refrigerant in commercial low temperature systems due to the low boiling point.

Ethylene was once used as an inhaled anesthetic, but it has long since been replaced in this role by nonflammable gases.

Ethylene as a plant hormone

Ethylene functions as a hormone in plants. It stimulates the ripening of fruit, the opening of flowers, and the abscission of leaves. Its biosynthesis starts from methionine with 1-aminocyclopropane-1-carboxylic acid (ACC) as a key intermediate.

Ethylene was discovered when the byproducts of gas burning street lamps were shown to cause plant senescence in a greenhouse.

Location, Characteristics and Occasions for Synthesis Induction

Directly induced by high levels of auxin
Found in germinating seeds
Induced by root flooding
Induced by drought
Synthesized in nodes of stems
Synthesized in tissues of ripening fruits
Synthesized in response to shoot environmental, pest, or disease stress
Synthesized in senescent leaves and flowers
Rapidly diffuses
Inhibiting effects of ethylene on shoot growth (more specifically on stem elongation) reduced in the presence of light. Also ethylene levels are decreased by light
Released in mature cells when they do not have enough minerals and water to support both themselves and any dependent cells
Released by all cells when they are experiencing conditions which would normally cause a mature shoot cell to produce ethylene

Effects

Stimulates leaf and flower senescence
Induces leaf abscission mainly in older versus younger leaves.
Induces seed germination
Induces root hair growth – this increases the efficiency of water and mineral absorption
Stimulates epinasty – leaf petiole grows out, leaf hangs down and curls into itself
Stimulates fruit ripening
Induces the growth of adventitious roots during flooding
Usually inhibits growth - just shoot growth
Affects neighboring individuals
Disease/wounding resistance
Triple response when applied to seedlings – root ? and shoot growth inhibition and pronounced hypocotyl hook bending
Inhibits stem swelling ? (Contradictory to the finding below – contradictory sources)
Stimulates cell broadening (and lateral root growth)
Interference with auxin transport (when hormone levels are increasing)
Directly or indirectly induces auxin at high levels
Inhibits the rate of metabolism of cells in the shoot (who are not already at their lowest metabolism rates) in response to an decrease in the levels minerals and/or water
Induces flowering in pineapples