In this video, we will focus on drawing structures of carboxylic acids.

Carboxylic acids are organic acids with carboxyl (-COOH) functional group.

Carboxylic acids

The first member of carboxylic acid homologous series has 1 carbon. It is called methanoic acid. The second member has 2 carbons, and we call it ethanoic acid. The third member has 3 carbons, and it is called propanoic acid. The fourth has 4 carbons, we call it butanoic acid.

How do we draw the structures of carboxylic acids? 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

Carbon has 6 electrons. 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. Oxygen 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.

All members in the carboxylic acid homologous series have carboxyl (-COOH) group. They will use this functional group for chemical reactions. 

Methanoic acid

The first member, methanoic acid, has 1 carbon. There is already 1 carbon in the carboxyl functional group, we do not need to add more carbon for methanoic acid. This carbon will share its last unpaired valence electron with hydrogen. Now, all the atoms have achieved 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. The bond highlighted green is called a double bond. 4 electrons are shared, hence forming 2 bonds. The bond highlighted yellow is called a single bond. It is made by the sharing of 2 electrons. Here you go, this is the structure of methanoic acid.

Ethanoic acid

Next, we have ethanoic acid, which has 2 carbons. There is already 1 carbon in the carboxyl functional group, we just need to add 1 more carbon for ethanoic acid. Now there are 2 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 ethanoic acid. Let’s replace each electron pair with a bond. There you go, we have achieved the structure of ethanoic acid.

Propanoic acid

What about propanoic acid, which has 3 carbons? Similarly, there is 1 carbon in the carboxyl functional group, we just need to add 2 more carbons. Now there are 3 carbons in total. 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 propanoic acid. Let’s replace each electron pair with a bond. There you go, we have achieved the structure of propanoic acid.

Butanoic acid

What about butanoic acid, which has 4 carbons? There is 1 carbon in the carboxyl functional group, we just need to add 3 more carbons. Now there are altogether 4 carbons in this acid. 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 butanoic acid. Let’s replace each electron pair with a bond. There you go, we have achieved the structure of butanoic acid.

If we look at each of these carboxylic acids, 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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