Assigning Oxidation State Numbers to Carbon

The qualitative rules repeated below may be used to create a carbon atom redox number that reflects its oxidation state.

1. If the number of hydrogen atoms bonded to a carbon increases, and/or if the number of bonds to more electronegative atoms decreases, the carbon in question has been reduced (i.e. it is in a lower oxidation state).
2. If the number of hydrogen atoms bonded to a carbon decreases, and/or if the number of bonds to more electronegative atoms increases, the carbon in question has been oxidized (i.e. it is in a higher oxidation state).
3. If there has been no change in the number of such bonds, then the carbon in question has not changed its oxidation state. In the hydrolysis reaction of a nitrile shown above, the blue colored carbon has not changed its oxidation state.

We begin by noting that elemental carbon has a zero oxidation state by definition. A carbon atom bonded only to other carbon atoms, as in the structures on the right below, is therefore assigned an oxidation number of zero. If one of the carbon substituents is replaced by a hydrogen atom this oxidation number changes by -1; whereas, if the carbon substituent is replaced by a more electronegative substituent (O, N, F, Cl, Br etc.) the oxidation number changes by +1. Further examples of this simple system are shown in the diagram.