[PoetryInMotion]

Sorry, stupid post

I₂ acts as a catalyst of course. This post may be removed.

[Dan]

I presume this question concerns the iodine-catalysed formation of cyclic ketals from acetone and a 1,2-diol?

I₂ acts as a catalyst of course.

That is not an explanation of its function, i.e. not an answer to the question. That is something equivalent to:

Q: What is the role of baking soda in bread making.
A: It is an ingredient.

The answer is unsatisfactory.

This post may be removed.

We do not remove posts. One purpose of Chemical Forums is to build a searchable database of chemistry related discussions that may be useful to people in the future. Threads are not deleted once the original poster is satisfied with the answer.

[PoetryInMotion]

We do not remove posts. One purpose of Chemical Forums is to build a searchable database of chemistry related discussions that may be useful to people in the future. Threads are not deleted once the original poster is satisfied with the answer.

Sorry for a bad start  I suppose keeping the thread the way I posted it, and just adding my new idea as a reply or an edit would have been preferable?

That is not an explanation of its function, i.e. not an answer to the question. That is something equivalent to:

Q: What is the role of baking soda in bread making.
A: It is an ingredient.

The answer is unsatisfactory.

Hmm. True. Thought the mechanism of the catalysis was trivial, and didn't look any further into it, but now you say so, I realize it was more complicated than I first thought. The best start I can come up with is something like this:



But the R-OI, which then is formed, doesn't look any familiar, so must say I have no clue how to proceed :/ Perhaps I was wrong about the catalysis?

I presume this question concerns the iodine-catalysed formation of cyclic ketals from acetone and a 1,2-diol?

That's correct, yes.

[orgopete]

… The best start I can come up with is something like this:



But the R-OI, which then is formed, doesn't look any familiar, so must say I have no clue how to proceed. Perhaps I was wrong about the catalysis?

I believe the alcohol oxygen atoms are the most basic. I suggest writing a similar reaction and write out all of the products of that reaction completely. Perhaps you may discover something you recognize as a catalyst.

[PoetryInMotion]

Thanks for your answer.

Okay, so the diol nicks the iodine forming a rather weird compound with a -O(I)H⁺. I don't know what to do about that, but I realize that the iodide ion is an excellent nucleophile, and probably will attack the carbonyle carbon, forming 2-iodopropan-2-ol. Is that a better start?

[orgopete]

If you wrote it in its neutral form, it would have looked like ROI + HI. I thought this might be recognized as a catalyst. (If you are particular, in alcohol or water, the actual catalyst is largely ROH2⁺ and not HI.)

[PoetryInMotion]

What about this?

I guess that this increases the basicity (and thus the nucleophilicity) of the diol oxygen? But what about the iodine atoms (both the ones in HI and those in the ROI groups)? Aren't they even better nucleophiles, considering the protic alcohole molecules floating around?

(If you are particular, in alcohol or water, the actual catalyst is largely ROH2⁺ and not HI.)

No water is present from start, and if the alcohole can act as a catalyst, what is the then function of the iodine? :-/ Perhaps I misunderstand you?

[orgopete]

If the reaction were written with HI, would you have been able to write a mechanism for this reaction? I was pointing up that an alcohol plus I2 can generate HI. I am guessing the RCH2OI decomposes to HI plus aldehyde. If the alcohol is in excess, it probably won't be noted. This looks like an easy way to add anhydrous HI.

You may also note that a CH3CHOH group can give a positive halo form test. You should now be able to write its mechanism also.

[PoetryInMotion]

If the reaction were written with HI, would you have been able to write a mechanism for this reaction? I was pointing up that an alcohol plus I2 can generate HI. I am guessing the RCH2OI decomposes to HI plus aldehyde. If the alcohol is in excess, it probably won't be noted. This looks like an easy way to add anhydrous HI.

You may also note that a CH3CHOH group can give a positive halo form test. You should now be able to write its mechanism also.

Thanks for your patience, but I am afraid I don't get it. I understand how we get the aldehyde and that we generate more HI. But what then? Without the hydroxy groups, I don't realize how the diole should be able to attack the acetone at all 

[orgopete]

A diol, ketone, plus acid gives a ketal. Iodine seems a masked form of acid (HI).

[Dan]

I was pointing up that an alcohol plus I2 can generate HI.

I agree with this

I am guessing the RCH2OI decomposes to HI plus aldehyde.

I have done a few of these reactions and they do not decolourise - which to me suggests that the iodine is not reduced by an alcohol (at RT).

The mechanistic explanation for this reaction is usually the standard acetal/ketal formation mechanism but with I⁺ acting as a Lewis acid instead of H⁺ (along the lines of poetryinmotion's first proposed mechanism). Whether this is a realistic model, or whether it is just catalysis by trace HI formed as shown (or both), I don't know.

I don't know if kinetic data is available for HI vs I₂ for comparison.

[PoetryInMotion]

Thank you for your answers, both of you. Two more questions:

How many times during the  does the reaction need acid catalysis? My textbook says one (when the hydroxy group formed from the carbonyl oxygen is about to leave), but wouldn't an acid also make the carbonyl carbon more attractive already in the first step by protonating the double bonded oxygen (the mechanism below)?



If the iodine act as a catalyst through I⁺, which I guess in turn will act as a lewis acid on the carbonyl oxygen, what leaving group will be formed? IO? I₂O? Are oxygen and iodine compounds of that kind really stable?