4.23 - 4.38 The Alkenes
4.23 Meet the alkenes
Because of their double bonds, the alkenes are more reactive than the alkanes and thus they are useful for making new compounds. Alkenes can be made into polymers - such as polyethene and polystyrene - synthetic materials which have a wide variety of uses.
4.23 - 4.24 Activity 1.Find the formula
Ethene is the simplest unsaturated hydrocarbon (Alkene)
- 4.23 know that alkenes contain the functional group C=C
- 4.24 know the general formula for alkenes
4.25 Activity 2. What's unsaturated?
- 4.25 explain why alkenes are classified as unsaturated hydrocarbons
For a molecule to be saturated it must have the maximum possible number of Hydrogen atoms on it. Butene's double bond means that it does not have the maximum number of Hydrogen atoms. This means that butene is unsaturated.
Since butane has the maximum number of Hydrogen atoms, it is a saturated molecule.
Unsaturated means there is a C=C functional group present.
4.25 Activity 3. Healthy options
4.26 Activity 4. Name ene and draw ene!
- 4.26 understand how to draw the structural and displayed formulae for alkenes with up to four carbon atoms in the molecule, and name the unbranched-chain isomers
When drawing displayed formulae of the alkenes, the bond angle from H-C=C is often shown as 120o angle. Thus >C=C< can be used to represent the alkene functional group. This is shown in the displayed formula of butene:
When an alkene has more than 3 carbon atoms in its molecule, it can form isomers:
The structural formula of four alkenes are given in this section. For each alkene:
- Draw a fully displayed formula - showing the 120o bond angle
- Write the name next to the displayed formula
- Why is there not an isomer of butene called but-3-ene?
4.27 Activity 5. Alkenes with bromine
- 4.27 describe the reactions of alkenes with bromine to produce dibromoalkanes
- 4.28 describe how bromine water can be used to distinguish between an alkane and an alkene
- Bromine has a brown colour
- Dibromoethane is colourless
- The bromine solution turns colourless when an alkene is added
- The bromine and the ethene combine in an addition reaction to form Dibromoethane, which is colourless
- The bromine solution stays brown as the alkane does not react unless irradiated with ultraviolet light
- The alkane is saturated and so is unreactive