I want to talk about the concept of an oxidation state. According to IUPAC, it is “a measure of a degree of oxidation of an atom in a substance”. While inorganic chemistry has a fairly clear-cut set of rules about how to count oxidation states of metals in organometallic complexes, it is certainly not so simple in the case of organic compounds. There are vastly different ways of assigning oxidation states to carbon centres in organic molecules and things get unnecessarily confusing. I decided to take a look at some online resources… In the following video (http://www.youtube.com/watch?v=M4Q7Ba1ELIQ) the oxidation state of carbon in methane is assigned to -4, while in ethane it is assigned to -3! Of course, everyone understands that this is sheer nonsense. If we follow these guidelines, an aldehyde and a ketone will have DIFFERENT oxidation states at the carbonyl centre, which is not consistent with their fairly similar properties.
I would say that we have to go back to the drawing board and ask a question: why bother with oxidation states at all? There is only one purpose: to understand structure and reactivity better. In inorganic chemistry, Pd in the oxidation state “0” does things that Pd in the oxidation state “+2” is not known for (or known to do differently). This knowledge really helps us. Why don’t we keep things simpler in organic chemistry? Well, hybridization states and C-C bonds (including multiple ones!) screw us up. To make things more useful in this oxidation state mess, I think it is important to first consider one-carbon molecules. I will just look at the following four and unambiguously assign oxidation states based on the number of carbon-heteroatom bonds.
More elaborate carbonaceous molecules are composed of fragments that correspond to the variations of the blocks above. I admit that carbon-carbon multiple bonds make counting tricky. If a carbon atom has one pi bond and no heteroatom partners, its oxidation state is “+1” (e.g. ethylene). If a carbon is BOTH part of a pi-system and is connected to a heteroatom, the oxidation level is defined as the number of pi-bonds plus the number of heteroatoms attached to the carbon under consideration. Take the following case as an example:
The reason I really like things this way is that they enable me to think about heterocycles with some clarity. If someone knows of a better way to easily count oxidation states, let me know.
Concerning oxidation numbers in organic chemistry, please check this article by Gion Calzaferri for a discussion of the problem and one (imperfect) solution:
Lukas – thanks a lot, this is a useful read! Andrei