In the alkenes, there is at least one double bond between two carbon atoms. This means that they are unsaturated and are more reactive than the alkanes. The simplest alkene is ethane (also known as ethylene),
Figure 9.14: The (a) structural, (b) condensed structural and (c) molecular structure represen-
tations of ethene
As with the alkanes, the elkenes also form a homologous series. They have the general formula
The second alkene in the series would therefore be C3H6. This molecule is known as propene. Note that if an alkene has two double bonds, it is called a diene and if it has three double bonds it is called a triene.
The alkenes have a variety of uses. Ethylene for example is a hormone in plants that stimulates the ripening of fruits and the opening of flowers. Propene is an important compound in the petrochemicals industry. It is used as a monomer to make polypropylene and is also used as a fuel gas for other industrial processes.
Similar rules will apply in naming the alkenes, as for the alkanes.
Question: Give the IUPAC name for the following compound:
Answer
Step 1 : Identify the functional group
The compound is an alkene and will have the suffix -ene.
Step 2 : Find the longest carbon chain
There are four carbon atoms in the longest chain and so the prefix for this compound will be ’but’.
Step 3 : Number the carbon atoms
Remember that when there is a double or triple bond, the carbon atoms must be numbered so that the double or triple bond is at the lowest numbered carbon. In this case, it doesn’t matter whether we number the carbons from the left to right, or from the right to left. The double bond will still fall between C2 and C3. The position of the bond will come just before the suffix in the compound’s name.
Step 4 : Look for any branched groups, name them and give their position on the carbon chain
There are no branched groups in this molecule.
Step 5 : Name the compound
The name of this compound is but-2-ene.
The properties of the alkenes are very similar to those of the alkanes, except that the alkenes are more reactive because they are unsaturated. As with the alkanes, compounds that have four or less carbon atoms are gases at room temperature, while those with five or more carbon atoms are liquids.
Alkenes can undergo addition reactions because they are unsaturated. They readily react with hydrogen, water and the halogens. The double bond is broken and a single, saturated bond is formed. A new group is then added to one or both of the carbon atoms that previously made up the double bond. The following are some examples:
alkenes has carbon compound more than alkanes, so that the combustion process need more oxygen. In perfectly combustion, alkenes produced carbon dioxide gases and water vapor.
CH2=CH2 + H2 CH3-CH3
Polymerization is the formation of complex molecule from a simple molecule. The simple molecule is called monomer and the complex ones is called polymer.
nCH2=CH2 -CH2-CH2-CH2-CH2- (-CH2-CH2-)n
A catalyst such as platinum is normally needed for these reactions
CH2 = CH2 + H2 → CH3 − CH3
