[Mary]

My class is doing a lab on Dehydration of benzopinacol to benzopinacolone, which is  pinacol rearrangment. We are supposed to combine benzopinacol (a glycol) with acetic acid and iodine. I understand why we're supposed to add acetic acid since it's an acid-catalyzed dehydration. But what does the iodine do?


Thanks!!
Mary      

[NISHANT]

IODINE HERE ACTS AS A BASE AND PULLS OUT THE HYDROGEN OF THE OXYGEN ATTACHED TO THE SAME CARBON ON WHICH BENZENE IS ATTACHED?

[AWK]

See: Hibbert H. JACS 37, 1755 (1915)
and
http://www.orgsyn.org/OrgSyn/orgsyn/orgsyn_form.asp?formgroup=basenpe_form_group&dbname=orgsyn&formmode=edit&unique_id=undefined&commit_type=undefined&indexvalue=1&form_change=true&time=8916.203

[kotoro]

i have a similar problem, but i can't use the link provided, i think theres something wrong with the website you linked to. Does the Iodine pull off a hydrogen or what?

[Mitch]

copy / paste  

[toooom]

I got the same question, and I can't find any answer anywhere, the link doesn't work and I can't acces the article, can anyone explain what's happening ?

I have considered a mechanism leading from AcOH + I2 to IH2CCOOH + HI but I can't find any information about it...
And if it is right, is  IH2CCOOH stable ?

[orgopete]

Since AWK didn't leave any further comment, I don't know what was meant by the link. I think it is simply a link to the OrgSyn prep,
http://orgsyn.org/orgsyn/pdfs/CV2P0073.pdf

Unless it is different, I didn't find any mechanistic help.

Re: mechanism
This is a good question. I don't know the mechanism either, but in thinking about it, this is what I propose. Chlorine will react with water to give hypochlorite. Bromine is similar. Hypochlorite can react with alcohols to give a ROCl bond. Hypochlorite would be a better leaving group than hydroxide, although if there is an alpha-hydrogen, it will result in an oxidation with formation of chloride.

Therefore, I will guess that iodine reacts with the OH to give a ROI bond. This will be a better leaving group than hydroxide (which of course is unlikely as it is under acidic conditions). However, this may also be a source of HI, the other product of a reaction with the ROH group. The HI can give a regular acid catalyzed pinacol rearrangement. If that is the case, iodine would be a simple and easy source for a catalytic amount of HI. HI could even protonate the ROI oxygen to make it an even better and neutral leaving group. The actual mechanism could be the effect of one or more of the proposed reactions or another completely different reaction.

[nox]

I got the same question, and I can't find any answer anywhere, the link doesn't work and I can't acces the article, can anyone explain what's happening ?

I have considered a mechanism leading from AcOH + I2 to IH2CCOOH + HI but I can't find any information about it...
And if it is right, is  IH2CCOOH stable ?

you necro'd a SIX year old topic, seriously?

[toooom]

Yes, I'm serious, chemichal questions don't get old, it's like wine, it gets better with time if it has not been consumed

I apologize if I  don't use the right terms (I'm french, the chemical vocabulary is kind of different).
What you proposes is really different of what I guessed, which is close to what we call the "halloform reaction" (of iodine on methyled ketone).
I explain why I thought of it :
I made nmr analysis of three different products from a substitued benzopinacol rearrangement, and in each one, I had a signal I couldn't identify in the 1H spectra, at 2.15 ppm and none at the 13C spectra.
The iodomethane match with it (same signal in 1H and a signal in 13C at -21ppm which is out of my nmr borders).
Moreover, the solid I extracted was yellow pale, which is the color of iodomethane (but which can be due to a small amount iodine too...).
So I guess all those reactions can happen, but do you see one more probable than other, with the analysis I have ?

By the way, thanks for the link, I've been looking for it...

[Tayeb]

To answer this question, I would think about the mechanism of the rxn. Lone pairs on one -OH group are making a carbonyl by pushing one of the phenyl on the neighboring carbon which has another -OH as a leaving group. Since the other -OH group on the neighboring group is a bad leaving group, we use acetic acid to make it a better one by forming -OH₂⁺. As everybody knows, I₂ is a good electrophile which could be attacked by -OH to form -OHI⁺ (and I^-) as an even better leaving group. Therefore, I₂ is acting as a lewis acid here!!!

[Dan]

Please don't necro 8 year old topics.