[DStarfire]

I've read from a lot of places but I still don't understand...
How do I determinate the oxidation number of carbon on organic reactions?

[Schrödinger]

Just like how you do for other molecules...

Assign the shared pair of electrons from an X-Y bond to the more electronegative element say Y. Then the oxidation number of X = +1 and that of Y = -1.

If X=Y (double bond), then Y=-2, X=+2.

example :
CH3-CHO

The first carbon(aldehydic) carbon has a -1 due to one H(electronegativity of C>H) , 0 due to adjacent Carbon and +2 due to C=O. So, net oxidation state on C1 = +1

I hope you were able to understand.

[DStarfire]

Thanks for the reply!

So, following the example, the oxidation state of the other carbon will be -4?

[Schrödinger]

The second carbon, whose oxidation state you reported to be -4, will actually have oxidation state = -3.

This is because two carbons have the same electronegativity and hence, there will be no preference is allotting the shared pair of electrons. As a result neither carbon will contribute to the oxidation state of the other.

[UnintentionalChaos]

Keep in mind also, that oxidation state in organic chemistry (covalent bonds) is a relative term. For example, if you treat acetic acid with sulfur tetrafluoride, you should produce 1,1,1-trifluoroethane. By the above counting method, both acetic acid and 1,1,1-trifluoroethane have the same oxidation state. However, the electronegativity difference between carbon and fluorine is greater than the electronegativity difference between carbon and oxygen. The electron density around carbon has been reduced, which is the definition of oxidation.

Long story short, oxidation measured by whole numbers is merely an approximation.