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 -104oC and a melting point of -169oC. 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 PropertiesMolecular Weight 28.08 g/molOdor and taste Slightly sweetEnthalpy of Formation 52.32 kJ/molStructure of molecule PlanarHeat of Combustion 47.183 MJ/kgFlammability HighHeat 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 ReactionsOne 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.

HalogenationHalogenation is the addition reaction between ethene and a halogen. Such examples include chlorine, bromine and iodine. In the examples below6, chlorine gas and ethene react to form

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 PolymerisationThe 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 ReactionsAs 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.

5C2H4 (g) + 2H2O (l) + 2MnO4-

(aq) + 6H+ (aq) 5CH2OH-CH2OH (aq) + 2Mn2+

(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 ReactionsAs 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)