In this video, we will focus on drawing structures of alcohols.
Alcohols are colourless liquid with hydroxyl (-OH) functional group.
The first member of alcohol homologous series has 1 carbon. It is called methanol. The second member has 2 carbons, and we call it ethanol. The third member has 3 carbons, and it is called propanol. The fourth has 4 carbons, we call it butanol.
Now, how do we draw the structures of alcohols? When we show all the bonds between the atoms in a molecule, we are writing down the full structural formula of the organic compound.
The Periodic Table
This is carbon in the periodic table. It has 6 electrons, and it will use its 4 outermost electrons for bonding to achieve stable noble gas octet electronic configuration. This is hydrogen in the periodic table. It has only 1 electron in its first shell, it needs 1 more electron to achieve stable noble gas duplet electronic configuration. This is oxygen in the periodic table. It has 8 electrons, and it will share 2 of its outermost electrons with other non-metal atoms to achieve stable noble gas octet electronic configuration.
Hydroxyl Functional Group
All members in the alcohol homologous series have hydroxyl (-OH) group. They will use this functional group for chemical reactions.
Methanol
The first member, methanol, has 1 carbon. Carbon will share 1 of its valence electrons with oxygen, then the remaining 3 electrons with hydrogen. Now, all the atoms have achieve stable noble gas electronic configuration. Carbon and oxygen now have 8 outermost electrons while hydrogen has 2.
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 methanol.
Ethanol
Next, we have ethanol, which has 2 carbons. One of the carbons will share its electron with oxygen, and each carbon will share 1 electron with each other. 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 ethanol. Let’s replace each electron pair with a bond. There you go, we have achieved the structure of ethanol.
Propanol
What about propanol, which has 3 carbons? One of the carbons will share its electron with oxygen, and carbons will share 1 electron with each other. 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 propanol. Let’s replace each electron pair with a bond. There you go, we have achieved the structure of propanol.
Butanol
What about butanol, which has 4 carbons? Similarly, one of the carbons will share its electron with oxygen, and carbons will share 1 electron with each other. 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 butanol. Let’s replace each electron pair with a bond. There you go, we have achieved the structure of butanol.
If we look at each of these alcohols, you should notice carbon always form 4 bonds, oxygen always form 2 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 structural formula of organic compounds to save time. Practise more, you will be more confident in writing full structural formula of organic compound.
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