[Prototype]

Dear all,
While I understand that oxidation of phenols with chromic acid usually results in the formation of 1,4-diketones (quinones), I don't quite understand the logic behind it, or the reaction mechanism behind it. Seems like the books have omitted it as well.

Help would be much appreciated. Thanks!

[Archer]

Are you sure you have the nomenclature right, 1-bromophenol?

[Prototype]

Sorry, I meant 2-bromophenol. Typed it wrongly 

[Archer]

Are you familiar with the mechanism by which chromic acid oxidises 2° alcohols to ketones and 1° alcohols to acids?

Could you draw, for example, the mechanism for oxidation of cyclohexanol to cyclohexanone?

The key part of your reaction is that you are introducing an extra oxygen onto the aromatic ring. Where does this oxygen come from?

2° alcohols Ketones there is simply loss of electrons (i.e. oxidation) it is helpful if you draw the mechanism to see.

but

aldehydes  carboxylic acids,  there is addition of an extra oxygen as well as loss of electrons. Where does this extra oxygen come from? is it from the chromic acid or another source? Again draw the mechanism for yourself.

Now see if you can apply similar principals of that mechanism to the formation of the quinone. If electrons are pulled out of the aromatic system during oxidation by the Cr⁶⁺what would you expect to happen to the aromatic ring (*hint*, look at electrophillic aromatic substitution mechanisms)

[Prototype]

I'm not familiar with the the way that happens I'm afraid.

The rationale I used was that the Br's own electronegativity would enable it to pull itself off the phenol, and the resulting carbocation would have a resonance leading to a positive charge in the para-position, which would probably be favourable over the ortho or meta positions. That would then form the second OH group on the ring, and then both OH's are oxidized into ketones.

[Archer]

Br^- is not a particularly good leaving group when it is attached to an aromatic ring, it certainly doesn't do it on it's own.

Can you draw a sensible mechanism for how H₂CrO₄ would pull a bromine anion from an aromatic ring?

Look up the mechanism for H₂CrO₄ oxidation of alcohols and see how the reaction occurs.

You can't expect to understand the mechanism and logic of how one type of compound is oxidised with this reagent if you don't know the mechanism of the most common oxidation of alcohols to carbonyls and acids.

Also I am not sure that we are both singing from the same sheet here.

Is this the 1,4-diketone that you are expecting from the reaction of 2-bromophenol and H₂CrO₄?

[opsomath]

It's entirely possible that you would just get 1,2-benzoquinone.

[Archer]

It's entirely possible that you would just get 1,2-benzoquinone.

I was comming to that , one can rationalise the formation 2-bromo-1,4-benzoquinone or 1,2-benzoquinone mechanistically but not 1,4-benzoquinone.

In order that Prototype can rationalise either of these two outcomes they must first understand the action of H₂CrO₄ on alcohols and phenols.

[Prototype]

The end product is given to be 1,4-benzoquinone.

I looked at the oxidation mechanism with chromic acid, so I think I've gotten that down as it would explain the first carbonyl.

For the second group, would it be mistaken for me to think that it is aromatic substitution where a H in the benzene ring is replaced with a OH from chromic acid, which then undergoes oxidation into a ketone?

Feels like its more likely for the Br to be substituted with an OH, but I'm not sure how the 1,4 positioning came about.

Sorry for the trouble, my degree isn't in chemistry (I barely had any past my first year) so some of these concepts are a tad alien to me.

[Archer]

Please can you show your working for the first carbonyl?

Also if you can give the reaction scheme and reference where it came from (or provide your analytical data if this was an experiment) then we will be better equipped to work out what is happening.

[Prototype]

Thats for the general oxidation of a 2ndary alcohol to a ketone. The only things I need to figure out now are how exactly Br is removed, and an extra carbonyl added.

I read about it in a lecture slide as I was browsing some teaching material, and all it showed was that 2-bromophenol converts into 1,4-benzoquinone upon addition of chromic acid. It had no explanations, and the slides before/after certainly didn't touch upon this, so it got me curious how such a rather significant change in structure could occur.

[Archer]

I can't help but feel that maybe there was a mistake on the slide.

Nevertheless the only feasible mechanism I can come up with starts with keto-enol tautomerism to the following molecule.



This would be an extremely minor intermediate tautomer but it is the only way I can rationalise your proposed product.

Nucleophillic attact in the 4- position by water would eliminate HBr to give this

 

Oxidation would give 1,4-benzoquinone.

I don't like this mechanism but it is the best I can do (perhaps someone else can provide a more likely mechanism than this). The only way I could believe that it occurs is to see the formation of 1,4-benzoquinone from 2-bromophenol experimentally.

Phenol does oxidise to 1,4-benzoquinone with Chromic acid. Now that you know the mechanism for alcohol oxidation can you determine how this proceeds?

[clarkstill]

I don't quite understand where your initial premise - that CrO3 and 2-bromophenol leads to 1,4-benzoquinone - comes from!  A reaxys search reveals only 2 examples of this reactivity (e.g. http://pubs.acs.org/doi/pdf/10.1021/ja01375a032) and they seem to be a special case since the phenols are actually naphthols, so there is no ortho position available for oxidation...