Ethylene
The ethylene , is a unsaturated Hydrocarbure. One can also find it under R1150 name.
Chemistry
Properties
Physical properties
The ethene (ethylene) is simplest of the Alcènes.
It is a gas colorless, volatile, of density close to the air with which it forms explosive mixtures. Starting from 425°C, it ignites and burns with a clear flame. (heat of combustion: 47.200 kJ/kg).
Chemical properties
It is a very reactive gas. ; Hydration The ethylene can be hydrated out of ethanol by addition of a water molecule in acid medium.H2C = CH2 + H2O => H3C - CH2OH
; Halogenation by halogenation, ethylene led to dibromoethane.
H2C = CH2 + Br2 => BrH2C - CH2Br
; Hydroxylation The ethylene is hydroxylated in 1,2-éthanediol (Glycol) in the presence of Catalyseur S.
; Polymerization The polymerization of ethylene is carried out in the presence of a catalyst by a ridicalizing process.
Production
In petrochemical industry, the ethylene is obtained:- starting from natural gas rich in ethane (the USA) by Pyrolysis or Cracking with high temperature: In this process, the proportion of ethylene in the resulting gas mixture is approximately 45%.
- starting from the Naphta coming from the split Distillation of the Pétrole (Europe) by Vapocraquage. The proportion of ethylene in the mixture obtained is approximately 25%.
Uses
The ethylene replaced the Acétylène like basic raw material in chemical industry.The products resulting from the ethylene C2H4 are: vinyl chloride, ethylbenzene, ethylene oxide, ethanol (or ethanol), polyethylene
With vinyl chloride one manufactures:
vinyl polychloride which gives: plastics,
Ethylbenzene gives two products: styrene and rubber SBR.
Styrène one transforms it by Polymérisation:
out of polystyrene which itself gives: plastics, Rubber SBR one obtains:
rubber resins Ethylene oxide ((CH2) 2O) (very explosive because assoiffé of oxygen) gives:
ethylene glycol which itself, combined with: the terephtalic acid which gives: polyester fibers,
The ethanol gives directly:
solvent Polyethylene gives directly:
plastic films The ethylene oxide ((CH2) 2O) is a very unstable product because of “the non-saturation” of its chemical structure and explodes immediately in the presence of oxygen, i.e. he seeks to fill the atomic vacuum of his structure by collecting the oxygen atoms in air. In 1957, an ethylene oxide master engine exploded in Antwerp (Belgium) making several deaths and the blast of the explosion moved a mass spectrometer (of more than one hundred tons) several tens of meters. The ethylene glycol is also useful in manufacture of antifreezes.
It is the same for propylene except that the diagram is packed more because this one contains a carbon atom moreover. The propylene resulting from the vapocraquor will give rise to certain a number of other new products.
Vegetable hormone
It is also a phytohormone (vegetable hormone) with the multiple effects.
History
The ethylene was discovered as a vegetable hormone in 1901, one noticed that the sheets of the plants located near the standard lamps (with lamp with gas) fell prematurely.In 1910, one realizes that a confined fruit matures more quickly than a fruit with the free air. One then makes a first bringing together with ethylene. In 1934 one discovers the metabolic ways of ethylene.
And in 1960, by gas chromatography, one manages to proportion ethylene emitted by the plants.
Biosynthesis
The ethylene originates in the Méthionine.
In the Cycle of Yang methionine is transformed into S-adénosyl methionine (by the SAM Synthétase ):
Methionine + ATP - > SAM + PPi + P ( SAM Synthétase )
SAM is then degraded into 5 ' méthylthioadénosine (which is re-used by the cycle of Yang) and in Acid 1-AminoCyclopropane-1-Carboxilic by ACC Syntase . Part of the ACC is then converted into ethylene (volatile) thanks to ACC Oxydase , the remainder goes conjuger with of N-Malonyl to give of N-Malonyl ACC (not bird) stored in a metabolic reserve which could be hydrolized according to the needs for the plant.
Inductive factors
The limiting factor is the production of ACC (Acid 1-AminoCyclopropane-1-Carboxilic) by ACC Synthase . This hormone is present in very small quantity in the Cytosol, in the fruits in maturation (at the time or ethylene is most abundant), it accounts for approximately 0,0001%. Its production is controlled by environmental factors like a wound, the cold, a hydrous stress, a diminition of O2 (immersion in water); like by endogenous factors: the Auxine or the Cytokinine S, but also ethylene. For that, the production of ethylene is a phenomenon Autocatalytique.
Inhibiters
The AOA (acid aminooxyacetic) and the AVG (Aminoéthoxyvinyglycine) block the operation of ACC Synthase .
An absence of oxygen (anaerobic), strong temperatures (sup. with 35°C), ions Co2+ cobalt, inhibit the operation of ACC Oxydase .
The silver nitrate AgNO3-, the money thiosulfate Ag (S2O3) 2-3 or a medium enriched in CO2, inhibit the action of ethylene downstream.
The methyl cyclopropene (1-MCP) sets in an almost irreversible way on the receivers ethylene, which then transmit a signal leading to the inactivity of the system of perception, in spite of the presence of ethylene molecules on close receivers.
Effects
The ethylene modulates many metabolisms (answers of the plants to the biotic and abiotic stresses), is implied in the stages of flowering and stimulates the maturation of many fruits. This molecule has effects if varied because it is very simple and thus not very specific.
Maturation of the fruits
The ethylene is an essential catalyst of the maturation of the fruits. For example, a lawyer does not mature on the tree but six to eight days after harvest. One then observes a peak of production of ACC, then of ethylene which starts the maturation of the fruit. The banana produces ethylene to mature, to prevent ripening the cold is not enough it is also necessary to ventilate to avoid the accumulation of ethylene. when one wants redemarer ripening it is enough to diffuse ethylene
Senescence of the bodies
The senescence of the bodies is a genetically programmed process influencing the physiological age of the alive entities. An exogenic contribution of ACC or ethylene involves a premature senescence, whereas an exogenic contribution of Cytokinine delays the process.
An increase in the production of ethylene is associated with a loss of Chlorophylle of the sheets, a degradation of proteins and ARN, a loss of pigmentation of the flowers, and other symptoms of ageing.
Abscission of the sheets
(= falls of the sheets - or other parts of the plant)
/! \ Not to confuse with the abscissic Acid .
The cells of the zones requiring a abscission answer ethylene specifically. A multitude of hydrolytic enzymes such as pectinases or polygalacturonases (which degrade the galacturonic acid) is then stimulated, lyses the cellular walls and weakens the structure of the plant. Generally an external agent such as the wind, gives the death-blow and makes fall the body.
The young sheets produce Auxine which anaesthetizes them with ethylene. After the development of the sheet, the production of auxine decreases then arrète: the cells of the Pétiole are then exposed to increasingly strong ethylene concentrations. At the end of a certain time the zones of abscission answer by the synthesis of hydrolytic enzymes.
Very strong concentrations of auxine stimulate the production of ethylene and thus the fall of the sheets.
Movements of épinastie
The roots perceive the flood by a strong reduction in the dioxygene concentraion in the medium. The Anoxie stimulates the production of SAM ( SAM Synthétase ) and involves an increase in the content of ACC because ACC Oxydase does not function: it cannot oxidize without oxygen! The ACC exédentaire of the roots ends up being found in the sheets to be transformed into ethylene. It is this ethylene which is responsible for the movements of épinastie.
Flowering
This hormone inhibits the Floraison except at certain species like the mango or the Ananas or one synchronizes the flowering of the fruits while bringing ethylene on the tree.
The ethylene can change the nature of the floral bodies. At the species Monoïque S, it is a feminizing hormone.
See too
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