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Author Topic: When is OH a good leaving group?  (Read 700 times)

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saippo

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When is OH a good leaving group?
« on: December 17, 2018, 09:01:18 AM »

Now I know that OH usually isn't however there seems to be cases when it can leave? I remember asking about this years ago and I remember the professor mentioning something like OH being able to leave at the beta carbon when the alpha carbon is attached to a carbonyl group (esp in basic condition). I'm struggling to remember the name of the exact reaction but I think in sophomore organic chem, it often followed after the aldol reaction. My memory is a bit hazy.

If anyone does know what I'm talk about, can someone clear this up? I was confused by the explanation then and it still confuses me now.


Thanks in advance
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hypervalent_iodine

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Re: When is OH a good leaving group?
« Reply #1 on: December 17, 2018, 10:04:55 AM »

I suspect you might be talking about the dehydration that you can get following Aldol condensation upon heating under acidic conditions (the aldol reaction itself is done under basic conditions). In this case, the OH becomes a decent leaving group when it is protonated to give water.
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saippo

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Re: When is OH a good leaving group?
« Reply #2 on: December 18, 2018, 08:20:17 AM »

That might have been it. I know that's usually how OH is converted into a good leaving group however in those examples, the OH just left without being protonated.
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Babcock_Hall

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Re: When is OH a good leaving group?
« Reply #3 on: December 18, 2018, 09:11:44 AM »

I wonder whether you might be thinking of the E1cb mechanism, which includes aldol condensations.
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nakhimov

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Re: When is OH a good leaving group?
« Reply #4 on: December 19, 2018, 09:41:15 AM »

The conversion of OH groups to oxonium ions [ROH2]+ is when it's a good LG.

You could also modify the OH functionality to make it an even better LG, since in general alcohols are pretty poor LGs.
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spirochete

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Re: When is OH a good leaving group?
« Reply #5 on: January 01, 2019, 02:29:16 PM »

Good leaving groups are weak bases. So hydroxide is somewhere in the middle in terms of leaving group ability. It is much more basic than halides and tosylates, but less basic than the amide ion, hydride ion and carbanions. Even stronger bases can leaving groups in special circumstances: hydride ion seems to be a leaving group in https://en.wikipedia.org/wiki/Chichibabin_reaction, and carbanions seem to be leaving groups in https://en.wikipedia.org/wiki/Wolff%E2%80%93Kishner_reduction

I have noticed that strongly basic leaving groups become more reasonable when they expelled in exergonic unimolecular processes where one strong base expels another strong base as a leaving group. This usually means the collapse of a tetrahedral intermediate. It helps if the process is more exergonic, because then the transition state will be earlier (Hammond's Postulate). Processes are relatively exergonic for various reasons: regeneration of aromaticity, formation of a strong bond such as a carbonyl pi bond, or creation of a stable small gas molecule.

Strong bases never seem to be OK as a leaving group in bimolecular processes like Sn2/E2, or in endergonic steps such as carbocation formation (Sn1/E1).
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saippo

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Re: When is OH a good leaving group?
« Reply #6 on: January 14, 2019, 06:46:02 AM »

Thanks so much for the replies! They helped me clear up what I was confused about.

 I think I'm not sure why in an E1cB elimination, the LG can be poor (ex. OH)?
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rolnor

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Re: When is OH a good leaving group?
« Reply #7 on: January 14, 2019, 07:13:14 AM »

In the E1cB case you have a carbanion thats kicks out the OH. The carbanion is more basic then OH- and this is a driving force I think.
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