Properties of ethene

1. Properties of ethene:

Alkenes are hydrocarbons that consist of at least one double carbon to carbon bond and
follow the general formula CnH2n. As the first organic compound of this homologous series,
ethene is a colourless gas at STP with a low boiling point of -104
o
C and a melting point of -
169
o
C. It is non-polar, therefore it is insoluble in polar solvents (e.g. water) and soluble in
non-polar solvents. Ethene, with molecular formula C2H4, is a small molecule that comprises
of a carbon joined to another carbon by a double bond and two hydrogen atoms attached to
each carbon, as shown in Figure 2. The molecule is held together by dispersion forces.












Figure 2
6
. Structure of ethene.

Other properties of ethene are show in the table below:

Further Properties
Molecular Weight 28.08 g/mol
Odor and taste Slightly sweet
Enthalpy of Formation 52.32 kJ/mol
Structure of molecule Planar
Heat of Combustion 47.183 MJ/kg
Flammability High
Heat of fusion 119.5 kJ/kg

Due to the presence of the carbon-carbon double bond, ethene is classified as an unsaturated
compound and is more reactive than the saturated compound ethane, which only has single
bonds. The double bond is seen as the active site.

Reactions:

Addition Reactions
One type of reaction that ethene undergoes is the addition reaction where the double bond is
transformed into a single bond by addition of atoms or functional groups. Substances that
may be added to ethene include; hydrogen, water, halogens such as bromine and chlorine and
their hydrogen halides.

Halogenation
Halogenation is the addition reaction between ethene and a halogen. Such examples include
chlorine, bromine and iodine. In the examples below
6
, chlorine gas and ethene react to form

2. 1,2-Dichloroethane and bromine water and ethene react to form 1,2-Dibromoethane
respectively.

AlCl3

+ 




AlCl3
C2H4 (g) + Cl2 (g)  C2H4Cl2 (g)




+ 



C2H4 (g) + Br2 (g)  C2H4Br2 (g)

Alkenes and bromine water:
Bromine water can be used as a test for alkenes as bromine water loses its orange-yellow
colour after reacting with an alkene. An example is the reaction between ethene and bromine
water, as shown in the equation above. Since water is present in bromine water, the end
product is mostly 2-bromoethanol and some 1,2-dibromoethane.

Addition Polymerisation
The formation of polyethene occurs from the addition of ethene molecules to form a long
chain. This reaction is known as addition polymerization and is denoted by the reaction
below. Polymerization can also occur with other monomers such as vinyl chloride and
styrene. However, in the below example, ethene is the monomer and polyethene is the
polymer.

Catalyst
n(CH2 == CH2)  … …

Catalyst
Ethene  Polyethene ((CH2 – CH2)n)
Where n is the number of ethene molecules.



Oxidation Reactions
As an alkene, ethene has a characteristic reaction with cold dilute potassium manganate (VII)
solution and is reduced by the manganate (VII) ions which are a strong oxidizing agent. The
reaction that occurs depends on the pH of the potassium manganate solution. In acidic
conditions, the solution which is originally purple due to the manganate (VII) ions, react with
ethene to form a colourless solution containing manganese (II) ions.

3. 5C2H4 (g) + 2H2O (l) + 2MnO4
-
(aq) + 6H
+
(aq)  5CH2OH-CH2OH (aq) + 2Mn
2+
(aq)

In alkaline conditions, a dark green solution results from the presence of manganate (VI) ions
with a dark brown manganese (VI) dioxide precipitate.

3C2H4 (g) + 4H2O (l) + 2MnO4
-
(aq)  3CH2OH-CH2OH (aq) + 2MnO2

(aq) + 2OH
-
(aq)

The above reaction would also occur in neutral conditions.

Combustion Reactions
As ethene is highly flammable, it would readily combust in the presence of oxygen to form
carbon dioxide and water, denoted by the equation below.

C2H4 (g) + 3O2 (g)  2CO2 (g) + 2H2O (g)