Determining Oxidation State of Carbon in Organic Compounds

Sometimes in Organic Chemistry we are required to determine oxidation state of carbon of a particular functional group.

Let's discuss how to do this.

First we need to be familiar with the definition of oxidation state and how it is determined.

I have a previous video lesson where I discussed how to determine oxidation state or oxidation number, do check it out if you are not sure!

To determine the oxidation state of carbon in an organic compound, we need to break all the bonds around carbon and decide what charge carbon would acquire if it were an ion.

The following table is useful to assign the charges.

carbon oxidation state in organic compounds determine oxidation state of carbon summary

Notes:
1. We can ignore carbon-carbon bonds as there is no difference in electronegativity and breaking of any carbon-carbon bond will not result in a charge for carbon.
2. Carbon will acquire a negative charge when C-H bond is broken as C is more electronegative than H.
3. Carbon will acquire a positive charge when C-O, C-N or C-X bond is broken as C is less electronegative than most of the other elements in organic compounds.

Let's have a few examples to illustrate this.

1. CH3CH2CH3

carbon oxidation state in organic compounds CH3CH2CH3

Drawing the displayed structure will help us visualise the breaking of the bonds better.

carbon oxidation state in organic compounds CH3CH2CH3 working

From the diagram we can see that the first carbon will acquire a -3 charge when all its bonds are broken, so its oxidation state will be -3.

The second carbon will acquire a -2 charge when all its bonds are broken, so its oxidation state will be -2.

2. CH3CH(OH)CH2OH

carbon oxidation state in organic compounds CH3CHOHCH2OH

We can determine that the oxidation state of the secondary alcohol carbon is zero, and that of primary alcohol carbon is -1.

carbon oxidation state in organic compounds CH3CHOHCH2OH working

3. CH3COCOOH

carbon oxidation state in organic compounds CH3COCOOH

We can determine that the oxidation state of ketone carbon is +2 and that of carboxylic acid carbon is +3.

carbon oxidation state in organic compounds CH3COCOOH working

For the detailed step-by-step discussion on how to determine oxidation state of carbon in organic compounds, check out this video!

Topic: Introduction to Organic Chemistry, Organic Chemistry, A Level Chemistry, Singapore

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