Oxidation State of Organic Molecules.
The most reduced form of carbon is CH4, the most oxidized is CO2. Thus the oxidation state of a one-carbon fragment is unambiguous and defined by the number of C-H bonds that have been replaced by C-X bonds, where X = any electronegative element (see periodic table on previous page). Replacing C-H bonds by C-Metal bonds is not a redox process. A C=O double bond is equivalent to two C-O single bonds (C(OH)2).
When there are C-C bonds involved, the situation is more complicated. Now, in addition to replacement of C-H by C-X, oxidations can also involve the removal of H2 by introduction of C-C and C-X double and triple bonds. To go left and right in the scheme below requires reduction and oxidation, respectively. Compounds in the same column are at the same oxidation state, and can be interconverted by nucleophilic substitutions (C-X by C-Y), electrophilic substitutions (C-H by C-M), additions or eliminations (of HX or MX fragments) and rearrangements.
The oxidation level of any carbon fragment can then be defined as the sum of the number of C-X bonds and number of π bonds to carbon (C=C, C≡C, C=O, C=N, C≡N). Two molecules with the same number of carbons and the same sum are at the same oxidation state, and can, in principle, be interconverted without any redox chemistry, or by an equal number of reductions and oxidations.
Another way to think about the oxidation state of organic molecules is to count the number of H- additions/substitutions needed to convert the molecule to the saturated hydrocarbon (alkane). H+ can be added as needed to neutralize the negative charge
