October 21, 2021, 05:28:49 PM
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Topic: What mechanism causes nitroalcohol dehydration following the henry reaction?  (Read 3673 times)

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Offline Zensation

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Most literature, formal and informal, imply after the Nitroalcohol is formed during a Nitroaldol condensation of a benzaldehyde and nitroalkane, a subsequent dehydration takes place to form the Nitroalkene. Other write ups on this reaction appear use Sodium Bisulfate to preserve the Hydroxyl group, although I do not know very much about that particular variation.

Curiously I stumbled upon this particular reaction using a phase transfer catalyst.

Comprehensive Organic Reactions in Aqueous Media - 436 Page PDF
Quote
"The Henry (nitroaldol) reaction was reported under very mild reaction conditions in aqueous media using a stiochiometric amount of nitroalkane, and an aldehyde, in NaOH 0.025 M and in the presence of cetyltrimethylammonium chloride (CTACl) as cationic surfactant. Good to excellent yields of β-nitroalkanol are obtained. Under these conditions several functionalities are preserved, and side reactions such as retro-aldol reaction or dehydration of 2-nitroalcohols are avoided.
The relevant study:
http://www.organic-chemistry.org/abstracts/literature/527.shtm


This last bit of the quote (in bold) had me wondering what causes the dehydration to the nitroalkene.... In the above study, does this simply not happen, due to the very small quantity of base? I read that alcohols are amphoteric... so this is my best guess. If there's not enough base to activate the hydroxyl group, then only very small quantities would dehydrate? If this is the case that could explain why yields were not 98%+ for the particular listed product. Assuming the PTC allowed for high reaction yields, the impurity could be the nitroalkene. The small quantity of base could still allow for some dehydration to occur.

Can anyone offer some knowledge? I haven't really learned much about base-catalyzed dehydration reactions... almost all of the ones I have encountered in literature have been acid-catalyzed and run at high temperatures, but this dehydration that happens during the henry reaction seems to even go over at room temperature.

Offline Zensation

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Am I allowed to *Ignore me, I am impatient* this?

I've noticed some particular molecules seem to automatically dehydrate, while others don't. Though this doesn't make very much sense seeing as the merging of the aldehyde and the nitroalkane is pretty standard on most substrates in this type of reaction. What conditions facilitate dehydration of the molecule? I first thought temperature might be the culprit, but further looking at various papers on the topic, some reactions are run at low temperatures(below freezing) others at high temperatures (above 80c), yet yield dehydrated products.

Anyone have a clue?

Offline Benzene

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I've wondered this too, I always assumed you needed a proton source in the reaction mixture to dehydrate to the nitroalkene

Offline Zensation

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I don't think so. Many of the particular reactions occur in base only environments. I've noticed the trend in that high levels of base = dehydration, and low levels of base = reduced amounts of dehydration.

There are particular cases were sodium bisulfate is used with the aldehyde to create an abduct that is not subject to dehydration but will still undergo condensation

Offline Zensation

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I ran into some readings that seemed to imply that dehydration is favored at low temperatures in some cases if the hydroxyl group is close to an unreduced aromatic ring because the electron withdrawing properties of the double bonds. Though, all references to this that I have found have been vague at best

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