[discodermolide]

Oh no, not another scheme

[Pranav]

I know its something really basic but would someone be so kind to reply to my post #13? 

[Dan]

I know its something really basic but would someone be so kind to reply to my post #13? 

Bromination of ketones will be in any textbook and is probably quite easy to find on Google - if you still have questions, please start a new thread because it will be too confusing to discuss it here.

[Rutherford]

I think I got it - the structure of P is incorrect, and O is not (quite) 1,3-dibromoacetone.

Hint: Br⁺ is a Lewis acid

Structure A is OK though.

Was Br added to the carbonyl group then? But then there aren't two isomers made.

The 1,3-dibromoacetone is probably an intermediate, and may even be N - is it stated in the question that N and O are isomers?

Remember the solvent is MeOH, and there is a Lewis acid (Br₂). What do you get if you mix a ketone (like 1,3-dibromoacetone), excess alcohol and a Lewis acid?

Sorry, I skiped this info:N and O are isomers; ωH = 3.8%; ωC = 22.9%.
This would mean C:H=1:2 as there are 3C, there are 6H, so what actually happens here? I don't know the answer to you question  .

[Dan]

Sorry, I skiped this info:N and O are isomers; ωH = 3.8%; ωC = 22.9%.
This would mean C:H=1:2 as there are 3C, there are 6H, so what actually happens here? I don't know the answer to you question

This elemental analysis is consistent with my idea.
I expect that 1,3-dibromoacetone is produced in the reaction along with HBr - but another reaction can then occur.

Hint: What happens if you mix acetone, ethylene glycol and an acid catalyst?

Apply that reactivity pattern to a mixture of 1,3-dibromoacetone in methanol with an acid catalyst (HBr).

[Rutherford]

I got the ketal, but what would be its isomer? The ketal and the compound obtained from it are attached, but I don't see a way to obtain A from it.

[discodermolide]

I'm lost now, would it be worth starting a new thread?

[Rutherford]

This was concerning the first scheme. I just want this finished and I will open a new thread for the next scheme, which I again can't finish on my own  .

[Dan]

I got the ketal, but what would be its isomer?

I'm not sure about that either. Maybe 1,1-dibromoacetone dimethyl ketal?

The ketal and the compound obtained from it are attached, but I don't see a way to obtain A from it.

Heat it with 20% HCl(aq) - hydrolysis and decarboxylation. Hint: look up acetal/ketal stability.

[Rutherford]

Right, in acids the ketal is decomposed to the ketone.  What happens in the third scheme then? After reduction of P I got the attached compound. During hydrolysis, if all methyl groups leave the molecule, I will have 4 -OH. What will react with NH₂OH?

[sjb]

Right, in acids the ketal is decomposed to the ketone.  What happens in the third scheme then? After reduction of P I got the attached compound. During hydrolysis, if all methyl groups leave the molecule, I will have 4 -OH. What will react with NH₂OH?

Are all these hydroxyl groups stable?

[Rutherford]

Probably not the two attached to the same atom, so a water molecule gets released. In which reaction does this happen?

[Dan]

Probably not the two attached to the same atom, so a water molecule gets released. In which reaction does this happen?

You already said:

Right, in acids the ketal is decomposed to the ketone

So you should expect aqueous acid to hydrolyse your ketal to a ketone.

[Rutherford]

It's the step S T. I think it's ok now. Thanks for the help.

[Dan]

Let's be clear, P is this:



Question: are you saying that reduction of P with LiAlH₄ gives this?



In that case you are mistaken, ketals are stable to LiAlH₄