[MangoPlant]

Let's suppose I have a mixture of benzene and pyrene (http://en.wikipedia.org/wiki/Pyrene) in a beaker. Would it be possible to oxidize only the benzene without oxidizing the pyrene?

My reasoning: It should be possible as oxidation of the pyrene requires the breakage of a larger aromatic system whereas benzene has a smaller aromatic system. Oxidizing a larger aromatic system should be thermodynamicly unfavorable relative to the oxidation of a smaller aromatic system (benzene) and thus one should be able to oxidize benzene without oxidizing pyrene.

The trouble I'm having is selecting an oxidizing reagent that is gentle enough to not oxidize pyrene but strong enough to oxidize benzene. Does anyone have any suggestions on where to start? I originally thought KMnO₄ and hydrogen peroxide may work, but now I'm not so sure.

[Corribus]

Yes, provided, as you mention, you can find the appropriate oxidant.  You'll just have to look up the redox potentials of these two molecules and find an oxidant that is sufficiently powerful to oxidize the one but not the other.  You could do it electrochemically, but of course this wouldn't really be a permanent solution.

[MangoPlant]

Regarding the selection of an oxidizing agent, are there any known collections of reduction potentials of organic molecules and oxidizing agents? Or would I have to use some software to get theoretical calculations?

I'm having trouble selecting such an oxidizing agent.

[spirochete]

My reasoning: It should be possible as oxidation of the pyrene requires the breakage of a larger aromatic system whereas benzene has a smaller aromatic system. Oxidizing a larger aromatic system should be thermodynamicly unfavorable relative to the oxidation of a smaller aromatic system (benzene) and thus one should be able to oxidize benzene without oxidizing pyrene.

I would question whether this statement is true at all. Oxidizing one of the pi bonds in pyrene to make a smaller, but still aromatic system should be far easier than completely disrupting aromaticity in benze. Larger aromatic systems do not benefit in a linear fashion from their increased size.

[orgopete]

My reasoning: It should be possible as oxidation of the pyrene requires the breakage of a larger aromatic system whereas benzene has a smaller aromatic system. Oxidizing a larger aromatic system should be thermodynamicly unfavorable relative to the oxidation of a smaller aromatic system (benzene) and thus one should be able to oxidize benzene without oxidizing pyrene.

I would question whether this statement is true at all. Oxidizing one of the pi bonds in pyrene to make a smaller, but still aromatic system should be far easier than completely disrupting aromaticity in benze. Larger aromatic systems do not benefit in a linear fashion from their increased size.

That was my feeling also. I'll surmise that a walk to the library would reveal many reactions of pyrene will take place in which benzene is inert. (If not the library, look in Wikipedia.)

[MangoPlant]

I guess you're right.

[Corribus]

I'm sorry, I totally misread the original post.  You won't be able to oxidize benzene without oxidizing pyrene - I thought you wanted to oxidize pyrene without oxidizing benzene.  The polarization half wave oxidation potential (basically, ionization potential) of benzene is 2.30 V vs SCE (in MeCN); the corresponding oxidation potential for pyrene is 1.16 V vs SCE. (Pysh and Yang, J. Am. Chem. Soc., 1963, 85 (14), pp 2124–2130).  It takes more energy to remove an electron from benzene than pyrene.

I appreciate the other posters reading your post more carefully than I did; I apologize for confusing you.

[zsinger]

Benzene is INCREDIBLY hard to break aromaticity with.  The gained stabilization energy with the conjugated system is probably about 50 J .