Alkenes - Ethene, A Typical Example

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Molecular formula: C₂H₄

Structure:

Common name: EthyleneEthene is an unsaturated hydrocarbon with a double bond.

Preparation of ethene

Ethene is prepared by cracking of higher alkanes as follows:

Preparation by cracking of hexane

Ethene is prepared by thermal cracking of hexane.

Preparation of ethene by cracking -Laboratory method

Principle

Thermal cracking of kerosene.

Procedure

• Sand soaked with kerosene is taken in hard glass test tube.

• The test tube is clamped horizontally to a clamp stand

• Broken pumice is placed at the other end of the test tube.

• The apparatus is assembled as shown in the figure.

• The burner is lighted and the test tube is heated strongly. Pumice and sand are heated alternately.

• Thermal cracking of kerosene vapours takes place in the pores of pumice to liberate ethene gas.

Laboratory Preparation of Ethene

Mode of collection

Ethene gas formed is collected by the downward displacement of water.

Properties of ethene

Physical properties

Ethene is a colourless gas with a sweetish odour. It is insoluble in water and soluble in organic solvents. It causes unconsciousness when inhaled.

Chemical properties

Combustion

Ethene burns in air with a sooty flame to give carbon dioxide,water vapour and heat. A sooty flame is produced because ethene is an unsaturated compound having a higher percentage of carbon than hydrogen (unlike saturated compounds).

Addition reactions

Ethene being an unsaturated compound, undergoes addition reactions to form saturated compounds.

a. With chlorine -Chlorination

Reaction takes place to form dichloro ethane.

b. With bromine - Bromination

Reaction takes place to form dibromo ethane

Dibromo ethane

c. With hydrogen - Hydrogenation

Reaction takes place to form ethane.

Ethane

d. With Hydrochloric acid

Reaction takes place to form mono chloroethane.

e. Polymerisation

Ethene undergoes addition polymerisation with itself at 200^oC and 2000 atmospheres of pressure to form polythene.

f. Reaction with water - Hydration

Reaction takes place to form ethyl alcohol in the presence of phosphoric acid, a catalyst.

Uses

• In the manufacture of ethyl alcohol and ethylene glycol which are used as important starting materials in the manufacture of synthetic fibres.

• Manufacture of plastics like polythene.

• In artificial ripening of fruits like bananas, apples and oranges.