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How to draw structures of alkenes
In this video, we will focus on drawing structures of alkenes.
Alkenes
Alkenes are unsaturated hydrocarbons with carbon-carbon double bond. A double bond is formed by the sharing of two pairs of electrons between the 2 carbon atoms. That is why, the first member of alkene has to have 2 carbons. A single carbon alkene can never exist. For alkene homologous series, the first member, which has 2 carbons, is ethene. The second member has 3 carbons, it is called propene. The fourth member has 4 carbons. It is butene.
Alkenes are unsaturated hydrocarbons
Naming of alkenes
Now, how do we draw the structures of alkenes? When we show all the bonds between the atoms in a molecule, we are writing down the full structural formula of the organic compound.
Carbon has 6 electrons, and it will use its 4 outermost electrons for bonding to achieve stable noble gas octet electronic configuration. Hydrogen has only 1 electron in its first shell, it needs 1 more electron to achieve stable noble gas duplet electronic configuration.
Dot and cross diagram of carbon and hydrogen
The first member of alkene has 2 carbons and they share 2 pairs of electrons. An alkene must have a carbon-carbon double bond and they will use this functional group for chemical reactions. The carbon will then bond with 4 other hydrogens and now, all the atoms have achieve stable noble gas electronic configuration. Carbon now has 8 outermost electrons, while hydrogen has 2 outermost electrons. When we write the full structural formula, we do not need to draw the electron shell or draw dot and cross to represent the electrons. When drawing the structure, we use a dash, which represents a bond between 2 sharing electrons in a covalent molecule.
Hence, 1 bond means the sharing of 2 electrons.
So now, we replace each of the electron pair with a bond. Here you go, this is the structure of ethene.
Dot and cross diagram and structure of ethene
Next, we have propene. Propene has 3 carbons. Similarly, this alkene has carbon-carbon double bond, the third carbon forms a single bond with one of the carbons. The carbons will then fulfil the octet rule by using its remaining electrons to form bonds with hydrogen. This is the dot and cross diagram of propene. Let’s replace each electron pair with a bond. There you go, we have achieved the structure of propene.
Dot and cross diagram and structure of propene
What about butene? Butene has 4 carbons. This is the dot and cross diagram of butene. We start by drawing the carbon-carbon double bond. Then the third and fourth carbon forms a single bond with the carbons. The carbons will then fulfil the octet rule by using its remaining electrons to form bonds with hydrogen. This is the dot and cross diagram of butene. Let’s replace each electron pair with a bond. There you go, we have achieved the structure of butene.
Dot and cross diagram of butene
Dot and cross diagram and structure of butene
If we look at each of these alkenes, you should notice carbon always form 4 bonds while hydrogen always form 1 bond. When you are familiar with structure drawing of organic compounds, you should not need to depend on dot and cross diagram to achieve the full structure formula of organic compounds to save time. Practise more, you will be more confident in writing full structural formula of organic compound.
Full structural formula of alkenes
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