Sciencemadness Discussion Board

What determines the reduction of nitro groups to ketones over amines, or vice versa

Electra - 21-1-2014 at 16:07

I've seen some of the same reducing agents used for these sorts of reductions. What is it that determines if the nitro(NO2) group gets reduced to a ketone C=O, versus to an amine NH2? Is it the prescense or absense of an H+ donor? If a nitro group were to be reduced in the absense of H+ donors, is it even possible for it to stay existing as an N- radical or would it automatically form a ketone?


I did some searching through various literature on this and hadn't found much. The wiki page on the reduction of Nitro compounds doesn't have much information relating to this question. What really puzzles me is that I have seen different sources saying that an Iron/HCL reduction would reduce a nitro group to the ketone, while another one would say the Iron/HCL system would take it to the Amine. Then there was another source that said such a reduction has to be done under pressure, but why was not specified. Help a girl out :3



[Edited on 22-1-2014 by Electra]

zed - 21-1-2014 at 17:10

Seems like you might be referring to the reduction of Nitro-propenyl-benzenes. A special case. Nitro-propenyl-benzenes tend to be reduced to oximes. These oximes tend to resist further reduction, but they are easily hydrolysed to ketones.

In the case of ordinary aromatic or alkyl, nitro- groups, reduction to an amine is generally not too difficult to achieve.

In the case of Nitro-propenyl-benzenes, direct reduction to an amine is most easily achieved via LiAlH4. Other methods have been reported to work, but in practice they are usually found wanting.

Electra - 21-1-2014 at 17:38

Is the pathway, regardless of method, is to the Oxime (N-OH) first? I did some searching and found that the oxime can be hydrolyzed by heating in the presence of an acid. Is the key to stopping total reduction of Nitro- to Amine-, done by using a mild reducing agent?

Nicodem - 22-1-2014 at 06:15

Quote: Originally posted by Electra  
What really puzzles me is that I have seen different sources saying that an Iron/HCL reduction would reduce a nitro group to the ketone, while another one would say the Iron/HCL system would take it to the Amine. Then there was another source that said such a reduction has to be done under pressure, but why was not specified.

You are mixing up everything and trying to generalize from completely different reactions. Nitroaromatics and nitroaliphatics get reduced to different products, depending on the conditions, but not to their carbonyl counterparts: phenols (nitroaromatics) or aldehydes/ketones (nitroaliphatics).

The reaction that gives the ketone from the nitroaliphatics is called the Neff reaction and it is not a reduction. Strictly speaking it is a hydrolysis accompanied by a disproportionation and does not occur on the nitroaliphatic compound itself, but rather its nitronate form.

Meanwhile, the reduction of the nitroolefins is something completely different and not to be confused with nitroaliphatics.

In short, the mechanism of the reaction answers your questions. The theory of the reaction mechanisms is the very basic theory of synthetic chemistry. So I suggest you to learn about it, if you are interested in synthesis.

By the way, the chemical symbol for chlorine is "Cl", not "CL", so that hydrochloric acid is represented by "HCl". For some reason, a lot of members here still use the wrong symbol. I never figured why. I wish one them would explain it.

Please read the forum guidelines and post beginner's questions in the right forum section.

Why do nitro-groups reduce to carbonyls in Metal/HCl systems?

Electra - 18-2-2014 at 17:43

I've always been curious about this. How come systems like Tin/HCl and Iron/HCl reduce nitro-groups to carbonyls instead of to amines? This method is commonly employed to reduce Nitropropene to Propanone's, yet I find no literary reference for it.

The wikipedia page on

http://en.wikipedia.org/wiki/Reduction_of_nitro_compounds

Says Iron & Tin are both used turn nitro groups to amine groups.

Oddly enough this Nitro -> Propanone reduction is one I have only seen talked about amongst the hive community and rhodium archives. Are many of these individuals just massively misinformed and actually reducing their nitro to the respective amine? I can't imagine this to be the case if they go on to form the Schiff Base followed by a successful reduction...unless the Schiff Base can be formed reacting an amine with an amine, which I don't think it can?

Additionally, this type of reduction is commonly employed to reduce Nitromethane to Methylamine, which would seem to directly contradict the mechanism for reducing Nitro groups to carbonyls.

What exactly is happening here?

chemaddict - 18-2-2014 at 23:33

I suppose i will answer your question. ! no one here will. (before it goes to detritus)

First tin and hydrochloric acid will not directly reduce the phenylnitroalkenes your talking about to a ketone, a hydrolysis of the intermediate oxime would be needed,unlike iron and hydrochloric acid which will reduce directly to the phenylpropanone.
As you will find out that it is certainly not easy to reduce directly to amines in good yields without the use of toxic chemicals and we all no what they are, but i can tell you that it is possable to reduce the phenylnitroalkenes to the amine with a well prepared urishabara catalyst just don't fall for the trick of using hydrochloric acid for the reduction you will get to much hydrolysis to the ketone and a pot full of tar ,what you need to do is use 30% glacial acetic acid and good zinc dust this concentration of acid prevents the deterioration of the catalyst during the reaction.
Using an organic acid at this concentration instead of a mineral acid greatly reduces the formation of the ketone,
you would expect to get 50%+ yield if done correctly.

oops now i am going to get a smack on the ass by the resident wakapeadian.

Electra - 19-2-2014 at 05:56

That did not really answer my question at all. I don't see why this thread would go to detritus either seeing as it is a legitimate organic chemistry question. I have no interest in reducing anything to the amine, nor to the ketone, nor am I interested in a recipe as you seem to have provided for some reason... There are MORE than enough such recipes online if I wanted to follow one...Thanks though, I suppose?

What I am interested in is the chemical mechanism of why the Nitro group is partially kicked off in some of these seemingly weaker reduction systems, but not in the stronger reduction systems such as with Hydrides, Pressure Hydrogenations, Amalgams, etc...

[Edited on 19-2-2014 by Electra]

bfesser - 19-2-2014 at 06:19

I agree; I see no reason why this should go to Detritus.

fozzie.jpg - 31kB

kmno4 - 19-2-2014 at 09:44

Quote: Originally posted by Electra  
That did not really answer my question at all (...)

I looks like you do not want any answer that does not suit you.
Read one more time what Nicodem wrote.

2 cents from me: in alpha-aminoalkenes amino group strongly interacts with double bond. It may lead to "conversion" of C=C-NH2 (enamine) to C-C=NH (imine) and subsequent hydrolysis of imine to ketonic matters C-C=O.
Exact mechanism and products distribution depends on everything.
So you can reduce your NO2 to NH2 in every case, but conjugated configuration may be not stable.

I did you homework, only because you are inquiring.


Befesser - would you mind stop clowning ? As I have written lately: your contribution to this forum is none. I would even say that it is less than none. You slowly become a pest.
Or pestpedian..... better ?

Electra - 19-2-2014 at 13:43

Thanks kmno4,

Though I still do not see my quesiton answered.

I am asking most specifically on the reaction between these various metals such as Iron/Tin and HCl with the organic [nitro] compound. Why are these commonly used to take P-NO2's to P=O's, when most every other reduction system takes them to P-NH2?

I'm starting to feel as if nobody knows the actual reason or mechanism to why this happens.... and are just giving me vague answers, or maybe I am just not being clear enough... I'm not sure how to be any clearer than this.

Partially unrelated, but I know Zinc/HCl is used to take aldehydes and ketones to hydrocarbons because of zincs great deal of oxophilicity and weakening of carbon-oxygen bonds.... is there a similar mechanism at work with the Nitrogen in the Nitro group with Tin and Iron? How does the interaction between these specific metals different from other reducing agents?

[Edited on 19-2-2014 by Electra]

Edit: Is this just the Nef reaction?

[Edited on 19-2-2014 by Electra]

Nicodem - 20-2-2014 at 08:15

Quote: Originally posted by Electra  
Though I still do not see my quesiton answered.

Probably because you did not read my reply above, or maybe you did not understood it.
Quote:
I am asking most specifically on the reaction between these various metals such as Iron/Tin and HCl with the organic [nitro] compound. Why are these commonly used to take P-NO2's to P=O's, when most every other reduction system takes them to P-NH2?

Like I already wrote, you are asking a question by claiming something that is not true. Metal dissolving reductions are not commonly used to reduce organic nitro compounds to ketones. That is just your misconception. Only special cases where the reaction mechanism allows it, the nitro compounds can yield ketones upon some reduction methods. Like I already hinted, such nitro compounds that can be reduced to ketones, are the nitro olefins (and kmno4 nicely explained you about them), while the others cannot generally yield ketones (unlike you keep on suggesting without bothering to give one single reference). Obviously, their reduction has nothing to do with the metal used. Any reduction method that reduces nitro compounds with some good selectivity over electron poor C=C double bonds can potentially be applied to give ketones. Even hydrogenation can be used.
Quote:
I'm starting to feel as if nobody knows the actual reason or mechanism to why this happens.... and are just giving me vague answers, or maybe I am just not being clear enough... I'm not sure how to be any clearer than this.

The vagueness comes from your side. Just read your posts above. You are not being clear at all and you gave no references for the unusual claims.
Quote:
Edit: Is this just the Nef reaction?

Like I already wrote, the Nef reaction is not formally a reduction and requires no reducing reagents. It is a hydrolysis of the nitronic acid accompanied by disproportionation.