Aluminium reduction of nitroalkanes to alkylamines

[Stanley]

Hello.

I'm having trouble finding a balanced equation for the reduction of nitroalkanes using (amalgamated) aluminium in IPA as the reducing agent. I've searched the net, The Hive, Rhodium's page and still haven't found what I'm looking for.

Can anyone please supply me with the balanced equation?

Thanks!


Stanley

[Bandil]

If there's any water present, it might look like this:

3H2O + 3RNO2 + 6Al => 3RNH2 + 3Al2O3

[java]

Bandil,...If this is true,

3H2O + 3RNO2 + 6Al => 3RNH2 + 3Al2O3


....If that's the case then nitromethane can easily be reduced to methylamine?.........java

[Nick_J]

H2O + RNO2 + 2Al --> RNH2 + Al2O3

You'd need a different solvent (ie non-aqueous) if trying to make MeNH2 since MeNO2 is only soluble to the extent of 10.5g/100mL in water @ 20*C, but I see no reason why the water in the equation above could not be replaced by ethanol or something, to form the respective alkoxide of aluminium:

6 ROH + R'NO2 + 2 Al --> 2 Al(OR)3 + 2 H2O + R'NH2

If you tried to use water as your solvent it should probably work, but you'd end up with an annoyingly dilute MeNH2 solution, which would require a little more work in order to make something useful.

[Bandil]

How about just using

this

(https://www.thevespiary.org/rhodium/Rhodium/chemistry/nitro2amine.zn-formate.html) method with formic acid for reducing the nitromethane, instead of the usual Al[hg], Fe or Sn reductions of nitromethane? It has proved very successfull for many nitro reductions in past cases and reacts much faster than the Fe reduction for instance(10 hrs or so). Do it with a relux condensor on top, to prevent the formed gasses to escape too much. After completion, the whole mess could be basified and distilled directly through a NaOH trap, to catch the methylamine(and the methanol, but i doubt that would do any harm).

I think it's worth a try anyway!

Regards
Bandil

[Stanley]

Right... I was thinking IPA might be a good solvent for the next step anyways, that's pretty much why I'm reluctant to adding extra water in the reaction mixture (because it will affect the equilibrium in the next step).

I see no problem neither, using an alcohol (I'm considering IPA because it's what I have the most of), the only variable here should be the rate of the procuction of H2, since 2 R-OH =alkali=> 2 R-O- + H2 is a slower reaction than when using water. I don't have a problem with that however.

Just how much MeNH2 can one expect will escape through the condenser when performing this reaction, provided one has enough IPA in the mixture to dissolve all the MeNH2 that is produced?

Bandil: I don't want to use HCOOH/Zn since I'm not sure it will fit as a reducing agent in the upcoming reaction... It's a neet reducing thingy though

[Rhodium]

Check this novel addition to my page:

Al/Hg Reduction of Nitroalcohols Promoted by Ultrasound

(https://www.thevespiary.org/rhodium/Rhodium/chemistry/al-hg.ultrasound.html)

As the title implies, it's about the reduction of nitro alcohols, but as the hydroxylic function does not participate in the reaction, everything said in it applies to reductions of any nitroalkane, and most of it applies to any Al/Hg reduction where the product is an amine.

One interesting factlet is that the reduction of nitroalkanes to amines with Al/Hg apparently is a distinct two-step reduction, R-NO₂ R-NHOH R-NH₂:  "In all cases the presence of the intermediate hydroxylamines were detected; however, conditions were such that complete reduction to the desired amino alcohols was the predominant reaction."

Also, their description of a "bench-top" Al/Hg rxn (one not making use of their novel ultrasound technique) gives a very interesting description on how to follow the reaction on TLC, as well as describing the procedure they used to achieve nitro alcohol reductions in 47-77% yield. The yields of amines from reduction of nitroalkanes (or reductive amination) are commonly higher as aminoalcohols are harder to purify and isolate, due to their considerable water solubility both as salts and free bases.  

"Benchtop (BT) Reduction of Nitroalkanols

Food grade aluminum foil (2.5 mmol) was cut into strips (6x50 mm) and spirally wound about a glass stirring rod (6 mm dia.) to prepare coils. The coils were soaked in diethyl ether to remove machining oils and amalgamated individually by immersion (20 sec) in an agitated solution of aqueous HgCl₂ (2%) using forceps. After amalgamation the prepared coil was washed by immersion in agitated diethyl ether (5 sec) and immediately added to a THF (5 mL) solution of nitroalkanol (0.5 mmol) and water (7.5 mmol). The reaction is stirred under nitrogen until TLC (CHCl₃/MeOH, 9:1) indicates consumption of starting material (high R_f) and intermediate hydroxylamine (lower R_f) to yield amine (lowest R_f).

Once the reaction was complete (12-24 h), the grey suspension was filtered through a pad (1 cm) of Celite in a fritted glass funnel (30 mm, 60 mL, medium porosity) and the filter cake was washed with THF (3x10 mL) and methanol (10 mL). The filtrate was concentrated to a colorless to pale yellow oil and flash chromatographed to afford the amine."

Note the use of the filtering aid Celite (diatomaceous earth) in a fritted glass funnel (or as second best choice: a Büchner funnel with coarse filter paper) to remove the Al sludge after the reaction, something which increases the filtration speed to 100-200 mL/min or more, compared to the very slow filtrations one may encounter when using ordinary filter paper without any Celite.

Also note that they add 15 equivalents of water for every nitro group to be reduced, this as proton (H⁺) source for the NO₂ NH₂ reaction, this proving that an excess of water does no harm, as well as the following passage fom the text, which indicates that the faster the evolution of hydrogen through the side-reaction between aluminum and water [ 6 H₂O + 2 Al -> 3 H₂ + 2 Al(OH)₃ ] the larger an excess of Aluminum is needed to ensure complete conversion of the starting material in the main reaction: "Since the concomitant reduction of water, the proton source for both BT and US cases, is also accelerated, the ultrasound cases required twice the equivalents of amalgam for complete consumption of starting material."

[armageddon]

OK, I don't know if this helps, but using google I found a paper yesterday which deals with the reductive amination of nitro compounds of the form 'R-NO₂' to their corresponding amines (Rode C. V., Vaidya M. J., Chaudhari R. V., 2001. Hydrogenation of nitrobenzene to aminophenol in four-phase reactor: reaction kinetic and mass transfer effect.Chem. Eng. Sci. 56 1299-1304).

(see also

http://www.tech.chem.ethz.ch/hungerb/research/reaction/hydrogenation.html

for details)

It seems to proceed via the hydroxylamine.


 Here it is:


1. R-NO₂ + H₂ -> R-NO + H₂O (-135 kJ/mol)

2. R-NO + H₂ -> R-NHOH (-155kJ/mol)

3. R-NHOH + H₂ -> R-NH₂ + H₂O (-260kJ/mol)

alternatively, the following reaction may occur, leading back to step 2):

3b) 2 R-NHOH -> R-NO + R-NH₂ + H₂O (-55kJ/mol)



As you can see the reaction is kinda exothermic.



They also postulated the following formula:

   R_H2 = wk'[H₂][RNO₂]
           1+K_B[RNO₂]

where
k'[(m³*kg-1*s-1)(m³*kg-1*mol-1*s-1)] = reaction rate constant
K_B [l*mol-1] = Adsorption equilibrium constants
R_A [kmol*m³-1*s-1] = Overall rate of hydrogenation reaction
w [kg/m³] = catalyst loading

Don't ask me anything about it, I just typed it for those of you who like theoretical chemistry/mathematics and for the sake of completeness, I don't understand ANYTHING besides that you could use it for calculating everything related to this reaction...


Oh, not to forget the catalytic hydrogenation (happening at the same time if, let's say, a nitropropene is being reduced):

Hydrogen and alkene are being adsorbed by the catalyst, one proton (H+) is being transferred to the carbon with the double bond (which one? a bond is always BETWEEN TWO carbons, I thought.. ) and the p-bond is being replaced by two s-bonds (at least I think it works that way, the paper says nothing about it)


...but water? It MUST bee some other reaction that converts the ketone to its amine, obviously the above reaction is favoured by removing water from the RXN...


(Sorry Rhod, I know this is perhaps off-topic, but just didn't find the right thread, but it seemed to bee worth posting it... if it will be moved to a thread more related to nitropropene reductions, that's just fine)