Ethylene

Ethylene is the simplest form of organic compounds and is also known as an alkene. It contains carbon-carbon double bonds. \(H_{2}C=CH_{2}\) is the formula of ethylene and the IUPAC name of ethylene is ethane. Ethylene is colourless and is also flammable. It also has a sweet taste and odour.

The main source of ethylene is natural gas and petroleum. Ethylene also acts as a hormone in a plant which is also a natural source. Ethylene hormone inhibits the growth and promotes the leaf fall. It also helps fruits to ripen. Ethylene has a very important role in the chemical industry as it is used as an organic chemical in industry.

Formation of Ethylene

Ethylene is produced by heating natural gas and mainly available as ethene and propene. If petroleum is heated at about \(\800^{\cdot }Cto900^{\cdot }C\)  ethylene is produced. The melting point of ethylene is -169.4 degree Celcius and the boiling point is -169.4 degree Celcius.

The use of ethylene falls into mainly two categories –i) monomer ii) as a starting material for other two-carbon compounds. The repetitive joining of small molecules of ethylene gives polyethylene. It is a polymer formed by the polymerization process. Polyethylene has so many uses as it is useful in the production of packaging films, wire coatings, squeeze bottles.

Now if the process of polymerization is carried out at high pressure and high temperature the product is called low-density polyethylene. If the process of polymerization is carried out in the presence of Ziegler- Natta catalytic conditions then it is called high-density polyethylene. High-density and low-density polyethylene have different properties. Ethylene is also useful in the formation of alpha-olefins.

Ziegler-Natta Catalytic Polymerization of Ethylene

First the catalytic is pumped under high pressure into a reaction vessel. The polymerization process takes place under the influence of Ziegler Natta catalytic. After the polymerization process completes unreacted ethylene monomer, catalyst, and solvent exit in the other end of the reactor. After that unreacted ethylene is separated from the mixture and is returned to the reactor.

The catalyst is neutralized by the alcohol wash. The excess solvent is recovered from the mixture and recycled and then a dryer dehydrates the wet polyethylene and gives it’s a powder form. It is mainly useful as industrial alcohol. Ethylene oxide is also in use as antifreeze and polyester fibres and films. Ethylene and benzene combine to form ethylbenzene and it is dehydrogenated to styrene for use in the production of plastics and synthetic rubber.

Applications of Ethylene

1)t is a simple organic molecule present in the form of gas.

2)Ethylene generally presents in ripening fruit, flower, stem, root, tubers, seed, leaf, etc.

3)Scientist Gane first found that a gas evolved from a ripe apple and can also affect the ripening of green apple. Crocker recognized it as a plant hormone.

4)The main functions of ethylene are fruit ripening, leaf abscission, suppress stem and root elongation, stimulate flower and leaf abscission.

5)Ethylene and Auxin suppress lateral bud formation.

6)It helps to stimulate latex flow in the rubber tree.

7)If we give a low concentration of ethylene it increases the sennoside.

8)Ethylene also helps in flower opening and flower petal simulation.

9)Ethylene is a gaseous hormone that results in transverse or isodiametric growth but it retards longitudinal growth.

10)It is a hydrocarbon that forms through the breakdown of methionine.

11)Ethylene has very low solubility in water and does not accumulate within the cell but diffuses out of the cell and escapes out of the plant.

12)Ethylene is produced by almost all the organs but maximum production occurs in ripening fruits and senescent leaves.

13)It also affects stem diameter and height. When the stems of the trees are subjected to wind it causes lateral stress and results in greater ethylene production occurs. Hence the tree trunks become more sturdy.

Plant Growth and Development

Ethylene promotes senescence which means loss of a cell’s power of division and growth. Ethylene helps in the ripening of fruit. It helps in increasing the length of petiole and internode of leaves as the leaf could come out of deep water and perform photosynthesis. Ethylene also helps in seed dormancy and also promotes seed germination in groundnut seeds specifically.

It also helps in the sprouting of potato tubers. One of the most important functions of ethylene as it is useful as an artificial solution named ethaphone which is in use as the ripening of fruit artificially. Ethaphone causes thinning of cotton fibre and also the thinning of the skin of walnut. Ethaphone also promotes the formation of female flowers in cucumber. Ethylene helps in flowering in pineapple.

i) Apo geotropism

It decreases the sensitivity to gravity. It promotes Apo geotropism in roots.

ii) Apical dominance

Ethylene promotes apical dominance and prolongs dormancy of lateral buds.

iii) Abscission

Abscission of various parts such as leaves, flowers, fruits is stimulated by ethylene which induces the formation of hydrolases.

iv) Sex expression

 Like auxin and cytokinins, ethylene has a feminizing effect on sex expression. The genetically male plants of cannabis can be induced to produce female flowers in the presence of ethylene. The number of female flowers is enhanced in monoecious plants like cucumber. It also helps in the production of female flowers which are genetically male.

It seems that the formation of abscisic acid in the leaves under is mediated through ethylene. Also, it increases the absorption surface of the plant roots.

Ethylene Signalling Pathway

In the signalling pathway, the role of the signalling molecule or ligand is mainly played by the ethylene and the receptor is going to be ethylene response-1(ETR-1). It is a copper ion-based receptor which used to do proper function in presence of copper. The transporter of copper ion is RAN-1. The location of the receptor is ER membrane and the receiver is present at ER lumen. The regular pathways are ETR-1, EFR1, CTR1, EIN2, EIN3. It also follows a two-component signalling pathway. Receptors are negative regulators because the binding of ethylene inactivates the receptor. Response to ethylene Arabidopsis is mediated by a family of five receptors such as ETR1, ETR2, EIN4, ERS1, ERS2.

FAQs about Ethylene

Q.1 Is there any possibility that ethylene polymerizes under mild conditions in the absence of a catalyst?

Answer: Ethylene is polymerized under pressure using a catalyst such as Ziegler-Natta catalyst or initiators. So there is a chance ethylene can be polymerized under the mild condition such as high or low pressure.

Q.2: Describe the possibilities of the banana not ripening evenly.

Answer: The main reason for bananas not ripening evenly is the insufficient amount of ethylene. Maybe there is a possibility of an air leak which reduces the level of ethylene. Maybe the fruit is harvested which is not yet reached the mature stage. If a room is full of fruits with different characteristics then there is a chance of the banana not ripening evenly. If the fruit has been held for a longer time then the ripening results will vary inevitably.

Q.3: Write two disadvantages of ethylene.

Answer: Ethylene causes the rapid death of cut flowers and it causes vegetables to go yellow. Ethylene also causes bud damage in dormant nursery stocks. It is also harmful to humans as liquid ethylene can cause headaches, fatigue, frostbite. Ethylene is flammable gas so it can cause fire and explosion hazards. Excess use of ethylene can cause early rot of the fruit so farmers should be careful.