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Introduction to Alkenes
Alkenes, like alkanes, are hydrocarbons. However, alkenes are unsaturated hydrocarbons: they contain a carbon-carbon double bond (C = C).
Alkenes are used:
- In the production of alkanols
- In the production of polymers
Structure of alkenes
Alkenes can be drawn by first drawing the corresponding alkane. For example, to draw an alkene containing six carbon atoms, hexane is first drawn.
H H H H H H | | | | | | H - C - C - C - C - C - C - H | | | | | | H H H H H H
The carbon-carbon double bond is then added between the two relevant carbon atoms. For example, if the double bond was between carbon-2 and carbon-31, the double bond would be drawn as shown below.
H H H H H H | | | | | | H - C - C - C - C = C - C - H | | | | | | H H H H H H
Remembering that carbon must form four bonds in its organic compounds, any 'extra' bonds and their hydrogens are removed. What is left is the structural formula of the alkene.
H H H H H H | | | | | | H - C - C - C - C = C - C - H | | | | H H H H
An alkene's name is formed using the following rule: The letters ene are added to a prefix which depends on the number of carbon atoms in the longest carbon chain of the alkene.2
Between the prefix and the ene, -n- is added, where n is the first carbon atom, counting from the end of the carbon chain, that has a double bond to another carbon atom. Incidentally, the side of the carbon chain on which the counting should be started is that which gives the lower value of n. Therefore, the alkene shown above is hex-2-ene.
Producing Alkenes from Long-chain Alkanes
Alkanes can be catalytically cracked to form alkenes. A catalyst is used to increase the speed of the reaction; but is not itself used up during this reaction: it can be recovered at the end of the reaction. In this reaction, aluminium oxide is the catalyst. On a laboratory scale, this reaction can be performed as follows:
- 2cm3 of mineral wool should be pushed to the bottom of a test tube
- 2cm3 of hexane should then be poured onto this wool
- The tube should be clamped in a horizontal position about 50cm higher than the table
- Aluminium oxide granules should be piled half-way along inside the test tube.
- A delivery tube should then be set up with one end inside the test tube and the other in a trough of water
- A Bunsen burner should be used to heat the tube, directly under the aluminium oxide granules
- Four test tubes of gas should be collected
- The first tube of gas is air that expanded and was forced out the test tube. It should, therefore, be discarded
- The other three test tubes contain ethene (an alkene)
Reactions of Alkenes
Alkenes have three main types of reaction:
- Addition reactions
- Oxidation reactions
- Addition polymerisation
In addition reactions, two reactants form a single product. The alkene's double bond breaks open and becomes a single bond leaving two 'spare' bonds where atoms can be added. Two groups are then added to these spaces.
The following diagram shows how ethene's double bond can break open to allow other groups to be added across it. The groups that would be added are shown by X and Y.
H H H H H H \ / | | | | C = C → H - C - C - H → H - C - C - H / \ | | | | H H X Y
Oxidation has three possible meanings, none of which are mutually exclusive:
- The gain of oxygen3
- The loss of hydrogen
- The loss of electrons
It should be noted that the opposite of oxidation is reduction which has the exact opposite definition. For example, reduction can be defined as The gain of electrons.
Ethene can be oxidised to ethane-1,2-diol4
This reaction takes places with acidified potassium permanganate (KMnO4).
H H H H \ / | | C = C → H - C - C - H / \ | | H H O O | | H H
Alkene molecules at high pressure or using a special catalyst can form very long chain molecules: polymers.
In the formation of polymers, the alkene's double bond breaks open to form a single bond. This creates two free bonds for each alkene molecule present. Due to many other identical alkenes being present, they all bond to each other forming a very long chain molecule.
For example, shown below is ethene becoming polyethene (often referred to as polythene). Another similar molecule would join to either end of the 'unit' of the polymer shown.
H H H H \ / | | C = C → - C - C - / \ | | H H H H
The Test for Alkenes
When any alkene is shaken with a few drops of bromine water (Br2 (aq)) the solution changes colour from orange to colourless.