[Nomeru]

I'm currently in the third quarter of organic chemistry. So far, we've learned that Na2Cr2O7 will react with secondary alcohols to form ketones, and likely form carboxylic acids from primary alcohols, though my book and lecture did not confirm this.

More recently, we learned it will also a benzylic carbon so long as it is not quatenary. (other conditions: H3O, Heat). I'm curious about how it can do this.

Ar=Benzene. Start: Ar-CH-(CH3)2, End: Ar-COOH. If you react Ar-CH-(CH3)2 with Na2Cr2O7, H3O, Heat, you get Ar-COOH (and likely CH3-CH3, though that's never been specified). I expect it to go something like this. Ar-CH-(CH3)2 -(1)-> [Ar-C(+)-(CH3)2 <--> Ar(+)=C-(CH3)2] -(2)->Ar-C(OH)(CH3)2 -(3)-> Ar-C(=O)CH3 + CH3 -(4)->Ar-COO- + H3O + CH3-CH3 -(5)-> Ar-COOH + H2O. This is the best sort of mechanism I can come up with, though it has many things I'm uncertain about. How does it break carbon-carbon bonds? From what I've learned, C-C bonds tend to be more stable than C-O, C=O, and C-H bonds. I'm certain there must be some kind of resonance with the benzene to make this work, but am unsure as to what it does exactly. I am unsure about how Na2Cr2O7 actually reacts either. I know the structure is something like 2Na+ [O3CrOCrO3](2-) but do not know what this becomes after losing an oxygen, or two.

I'm also curious about KMnO4 - I understand it can take Ar-CH3 to Ar-COOH, but can it do all the same things as Cr2O7? my book/lecture were never clear on this. In addition to that, will other agents such as PCC do anything with a benzylic carbon/OH (Ar-CH2-OH)?

Thank you for any assistance in understanding these.

[orgopete]

Let me break this down into two parts. Chromic acid and permanganate, when heated, both can catalyze radical reaction. The benzylic carbon, provided it is not quaternary can form a radical. That radical can react further to give something that is oxidized further. Isopropylbenzene is oxidized with oxygen, hydrogen peroxide, and acid to phenol and acetone. I also just found a cyclic benzyl ether that was oxidized to a lactone with chromic anhydride in acetic acid at 70°C. In this case, I would imagine something like this occurring.

ArCH2OR  ARCH(OR)•  ArCH(OR)OCrO_n  ArCOOR

So the chromate ester can oxidize via an ionic path in addition to a radical pathway. I was curious to see whether a chromic or permanganate oxidation occurs in an inert atmosphere. If you search under oxygen free, you will get a list of air oxidation procedures. Toluene can be air oxidized to benzoic acid with a suitable catalyst. If anyone can tell us whether these benzylic oxidations occur in an inert atmosphere, this would be interesting.

So the oxidations must start with a free radical step and then may or may not have an ionic oxidation. Finally, dichromate is the dibasic anhydride of chromic acid like acetic anhydride is to acetic acid. Chromic anhydride is the dehydro form, like SO3 is to H2SO4.

[AWK]

If anyone can tell us whether these benzylic oxidations occur in an inert atmosphere, this would be interesting.

http://www.op.titech.ac.jp/lab/kakimoto/html/2010.Angew.Chem.Int.Ed.49.436.pdf

http://ethesis.helsinki.fi/julkaisut/mat/kemia/vk/lahtinen/oxidatio.pdf

[orgopete]

If anyone can tell us whether these benzylic oxidations occur in an inert atmosphere, this would be interesting.

http://www.op.titech.ac.jp/lab/kakimoto/html/2010.Angew.Chem.Int.Ed.49.436.pdf

http://ethesis.helsinki.fi/julkaisut/mat/kemia/vk/lahtinen/oxidatio.pdf

I should have been more clear. The main oxidant in the production of phenol and acetone from cumene is oxygen. If the oxidation of an alkylbenzene is carried out with permanganate or chromic acid at reflux, could oxygen also be one of the oxidants from a radical intermediate? Although phenol is not produced from the oxidations, cumene is also not one of the usual alkylbenzene compounds, nor need the process be entirely an air oxidation. One can find many references to air oxidations of toluene to benzoic acid for example. That simply left me questioning whether these reactions might also be air oxidations to some smaller or greater degree.

My single experience in carrying out a permanganate oxidation to a benzoic acid used a whopping amount of potassium permanganate. I needed to shovel it in in portions. Heating potassium permanganate in water results in an autocatalytic decomposition of permanganate to MnO2 and O2. Hence I wondered, was permanganate the only oxidant? I am not saying it was or wasn't. I was only speculating that the mechanism might be a bit more complicated than it already seems. It also shows I didn't think through my question about an inert atmosphere.