10.3 – Alkenes
10.3.1 – Describe, using equations, the reactions of alkenes with hydrogen and halogens
Alkenes undergo addition reactions. This is when two substances react to form a single substance. Alkenes are more reactive that alkanes, as the double bond can be more easily converted into to a single bond. Extra atoms or groups of atoms can be added to the molecule.
Chlorine and ethene will react at room temperature. The addition of chlorine happens across the double bond, with each chlorine atoms attaching to the carbons. This happens with bromine and iodine as well, all the halogens.
Alkenes will also react with hydrogen, called hydrogenation. A catalyst is used, such as Nickel, and the product is a saturated molecule.
Platinum and palladium can also act as catalysts in this reaction.
10.3.2 – Describe, using equations, the reactions of symmetrical alkenes with hydrogen halides and water
The alkenes may react with hydrogen halides and water to produce a halogenoalkane, an addition reaction. The halogen will bond to the carbon atoms that formed the double bond.
If the double bond was located elsewhere on the molecule, then the hydrogen and chlorine atoms would bond to those carbon atoms instead.
Another addition reaction is the reaction of alkenes with water, called hydration. This is used to convert ethene and steam into ethanol. A catalyst, phosphoric acid, is used to provide hydrogen ions for the reaction. It is performed at 300°C and 70atm.
The reaction is exothermic, but is conducted at moderately high temperatures and pressures with a catalyst to maximise reaction rate and yield.
10.3.3 – Distinguish between alkanes and alkenes using bromine water
Both alkanes and alkenes are colourless. In an alkene, adding bromine will not change the colour – the bromine water undergoes decolourisation. Therefore, the test for an alkene is to add bromine water and look for a clear solution.
On the other hand, when bromine water to an alkane, there is a colour change – the solution will become yellow/orange. The two substances will not react unless there is strong UV light present.
10.3.4 – Outline the polymerisation of alkenes
Polymers of alkenes are long, chain-like molecules that consist of many monomers joined together.
Polyethene is formed in an addition polymerisation reaction. Each repeating unit, or monomer, in the chain contains has a double bond that is converted to a single bond when they join.
Since ethene is non-polar, there are only weak Van der Waals forces between the polymers. However, by adding other elements to the molecule, the properties of the polymers can be changed. For example, adding a chlorine atom makes PVC (polyvinyl chloride), which has a dipole and leads to a higher melting point and lower flammability. It is used as insulation on electrical wiring, soft drink bottles, fabric coatings and water pipes.
Polystyrene formation:
Polypropene is used in carpeting, bottles and luggage.
10.3.5 – Outline the economic importance of the reactions of alkenes
Large amounts of alkenes can be produced by cracking crude oil into smaller molecules. Alkenes form the starting materials for synthetic polymers that are useful to society. Ethene can be converted into ethanol or can be converted into plastics. Vegetable oils can form margarine. Polyethene is used to make plastic bags, bottles and toys. Polystyrene is used in Styrofoam insulation, cups and packing materials. Polychloroethene is used in plastic wrap, plumbing and garden hoses.