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Author: Subject: Reducing aliphatic nitro compounds to amines
DJF90
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[*] posted on 24-5-2011 at 07:24


postart - The Henry reaction is a condensation (c.f. Aldol, but where the nucleophilic component is a nitroalkane). There is no reduction involved.
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[*] posted on 24-5-2011 at 07:46


Quote: Originally posted by DJF90  
The Henry reaction is a condensation (c.f. Aldol, but where the nucleophilic component is a nitroalkane).

I haven't been annoying for a long time, so I will grab the opportunity.
The Henry reaction is not a condensation, it is an addition (nucleophilic).
The Knoevenagel reaction is the one that is a condensation (and starts with a nucleophilic addition followed by an elimination).
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DJF90
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[*] posted on 24-5-2011 at 08:31


Can you therefore explain why an aldol condensation is not called a Knoevenagel reaction..?

I can see the why the Aldol is called a condensation if you consider the product is the enone (which is actually the Knoevenagel product), but the actual product of an Aldol is the hydroxy-carbonyl compound (hence derivation of the name, Aldehydic alcohol. Seems as if people have been too loose with their definitions and caused too much confusion.

Not meaning to be a pedantic ass, just wondering if theres some kind of explanation (other than looseness of definitions).

[Edited on 24-5-2011 by DJF90]
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[*] posted on 25-5-2011 at 13:09


Quote: Originally posted by DJF90  
Can you therefore explain why an aldol condensation is not called a Knoevenagel reaction..?

You can call a condensation of a ketone/aldehyde with an aldehyde to give an enone as a Knoevenagel reaction whenever the reaction goes trough the enolate addition on the iminium ion (meaning that it is catalyzed by amines, piperidine being the one most commonly used).
For example, benzaldehyde and some methyl alkyl ketones will condense under Knoevenagel reaction conditions with secondary amine catalysis to give the corresponding enone. It is however not proper to call a base catalyzed condensation of these same reactants as a Knoevenagel reaction even when it gives the same enone as the product, because this reaction is based on an aldol reaction followed by E1cB.
In the case of nitroalkane's condensations with benzaldehydes, the reaction is clearly a Knoevenagel under amine catalysis, but there is also an aldol-like version of this same transformation (using NaOH or KOH followed by acidification). This version can not be called a Knoevenagel, because it is mechanistically and methodologically different (nucleophilic addition of nitronate on the carbonyl, which is essentially a Henry reaction, followed by acid catalyzed elimination).

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I can see the why the Aldol is called a condensation if you consider the product is the enone (which is actually the Knoevenagel product), but the actual product of an Aldol is the hydroxy-carbonyl compound (hence derivation of the name, Aldehydic alcohol. Seems as if people have been too loose with their definitions and caused too much confusion.

Whenever you have two different reactions that starting from the same reactants give the same product, there is such confusion. A Knoevenagel condensation and an aldol + E1cB reaction can both give the same product, but their mechanism and methodology is different. There are many such cases where confusion is common. For example, the Ullmann condensation, Buchwald-Hartwig cross coupling, the nucleophilic aromatic substitution and the aromatic elimination-addition reaction ("benzyne reaction") also give the same product from the same combination of reactants, but they are several different types of reactions with very different mechanisms, methodologies and scope&limitations.
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DJF90
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[*] posted on 26-5-2011 at 12:45


Thanks for the clarity, makes much more sense after breifly revising the mechanisms involved.
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[*] posted on 20-12-2014 at 22:03


Quote: Originally posted by Melgar  
Gallium is just used to expose the aluminum to the environment; you just need a very small amount of it, and often it can be reused.

The link I posted is what I've been trying to follow, but HCl was substituted for formic acid. That's also been proven to work, for example here:

http://www.erowid.org/archive/rhodium/chemistry/norpseudoeph...

The nitroalcohol was created via the Henry reaction between benzaldehyde and nitroethane.


This is not totally accurate. Gallium removes the oxide later from aluminum by alloying with it. The Ga and Al would have to be melt together with a high percentage of Ga. The electron transfer/"reduction" occurs on the surface of amalgamated Al. Salts of Hg in solution can catalytically expose the surface of Al for reaction. I wonder if Ga salts can do this. It wouldn't make sense if it couldn't. That would be much more cost efficient than elemental Ga if avoidance if Hg is in mind.
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[*] posted on 21-12-2014 at 04:43


generally metal reductions are slow ,need a lot of metal,are messy and most of the metal stays unreacted .catalytic hydrogenation is a better idea
Quote: Originally posted by Melgar  

For the zinc reduction, there were 4 moles of zinc per one mole of reactant added into the mixture (not all at once though, it was scooped in as it ran out), then stirring was turned on and HCl dripped into the mixture, very slowly.


IIRC,small amounts of Zn should be added to the mixture of nitro+HCl after regular intervals ,not the other way round :)

raney nickel is very good for reducing nitro groups(heating the nitro compound with raney nickel at 60'C will give the amine)

but as you are starting with para-flourobenzaldehyde(because you want to make 4-MAR) then raney cannot be used because raney nickel will remove all halogen groups from the benzene ring ,even flourine :o

Urushibara nickel should be immediately used after it has been made,otherwise it loses its reducing power
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