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4.23 - 4.38 The Alkenes

 4.23 Meet the alkenes

Limonene is an alkene which can be found in the skin of citrus fruits

The Alkenes are another Homologous series of Hydrocarbons. They differ from the Alkanes because they contain in their molecules at least one double bond between two of the carbon atoms.

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.

This video shows the steam distillation of limonene from orange peel. Steam distillation is used to avoid the limonene becoming too hot.


 4.23 - 4.24 Activity 1.Find the formula

Ethene is the simplest unsaturated hydrocarbon (Alkene)

 Students should:

  • 4.23 know that alkenes contain the functional group C=C
  • 4.24 know the general formula for alkenes

Use the images and your knowledge of displayed formula and naming of organic compounds to complete a copy of the table below  

No. Carbon atoms Name Formula
No. Carbon atoms Name Formula
1 not possible 
nGeneral formulaCnH2n

 4.25 Activity 2. What's unsaturated?

Saturated | Butane
Unsaturated | Butene

 Students should:

  • 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

Unsaturated fats contain the C=C functional group and are considered to be healthy options

Which is which ? 

The images show ball and stick models of two fatty acids. One of the molecules shown here is linoleic acid . The other is lauric acid. linoleic acid is unsaturated. Lauric acid is saturated.  

a saturated and an unsaturated fatty acid

Enter your text here ...

 Linoleic acid 

Lauric acid 

 4.26 Activity 4. Name ene and draw ene!

 Students should:

  • 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:

  1. Draw a fully displayed formula - showing the 120o bond angle
  2. Write the name next to the displayed formula
  3. Why is there not an isomer of butene called but-3-ene?

Enter your text here ...

 4.27 Activity 5. Alkenes with bromine

 Students should:

  • 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
  • what is the colour of a solution of bromine?
  • what is the colour of a dibromoethane?
  • what is the colour change produced when an alkene is added to a solution of bromine?
  • explain why this colour change happens
  • what happens when an alkane is added to a solution of bromine?
  • Explain your answer
  • 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