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Author: Subject: Troubleshooting Zn/HCl reduction of nitrostyrenes
horribilis
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[*] posted on 12-11-2019 at 18:48
Troubleshooting Zn/HCl reduction of nitrostyrenes


Quote:
50 ml IPA (91% w/w), 12 ml water, 75 ml HCl (31% w/w, technical grade) were combined in a beaker and chilled to 0 C. Bright yellow beta-nitro-3,45-trimethoxystyrene was ground with mortar and pestle until finely powdered. Dull gray zinc powder, 400 mesh, was ground with mortar and pestle until absent of clumps.

The beaker was placed on salt water bath and 2.40 g nitrostyrene was added in one portion and stirred until nearly dissolved, although some small particles of nitrostyrene would not dissolve. In 10 equal portions spaced 5-6 minutes apart, 13.0 g of zinc was added to the stirred contents of the beaker. Reaction temperature was <10 C at all times despite exothermic activity and effervescence.

The reaction was left to stir for an additional 7-8 hours with reaction temperature <10 C. By hours 2-3 of the 7-8 hours of stirring there was no indication of any unreacted yellow nitrostyrene. By the final hours of stirring there was almost no trace of undissolved zinc powder. The unfiltered contents of the beaker, clear and gray in color, were made distinctly basic with 70 g NaOH in 70 ml water while maintaining a reaction temperature <20 C. A large amount of white salts formed in the aqueous layer and the alcoholic layer took on an amber color.

The mixture was stirred for several minutes and then the alcoholic layer decanted. The aqueous layer was extracted/decanted twice more with IPA, the final extraction/decanting being clear in color. The alcoholic extracts were left to sit in a freezer overnight, then dried over MgSO4 and filtered to yield a crystal clear light yellow solution. This was then made acidic with alcoholic H2SO4 to pH = 6 to 7, with the crystal clear light yellow solution turning more turbid and amber colored as the crystals precipitated. The sulfate salt of the amine was freeze precipitated overnight, vacuum filtered, and washed with acetone to yield 0.60 g (~20% yield) of the white and shiny sulfate salt. The filtrate was chilled to -15 C but no additional crystals were precipitated.


This procedure was run on the basis of persona's work, which claims yields of ~75%. See http://www.sciencemadness.org/talk/files.php?pid=603835&... Persona's work was, in turn, based on Leminger's work, see https://erowid.org/archive/rhodium/chemistry/leminger.html


To be short and to the point, I'm not sure what went wrong. The nitrostyrene is recrystallized once and bright yellow in color, free of any degradation or polymerization. The zinc is advertised as 99.9% purity. While the zinc was not activated (i.e. not washed with dilute HCl to make it bright and shiny) I don't see how this would matter as it reacts with a very large excess of HCl in situ. Temperature control was reasonable at all points of the reaction and zinc addition was as prescribed.

Does anyone with a more thorough understanding of this reaction have any input? Would it be beneficial to run it as Leminger describes it, forming an IPA/HCl solution and then adding nitrostyrene and HCl in alternating batches, to maintain constant ratios? I have noticed one paper (DOI: 10.1055/s-0034-1379481) goes so far as to add the HCl in portions, as well.

I have also read of some using an even greater excess of NaOH to fully re-dissolve the zinc salts. But I assume this is for ease of workup: filtration and extraction with separatory funnel rather than decanting. Zinc salts would not somehow complex with the freebased amine, correct? I'm also confused by some write-ups mentioning the freebased amine floated on top of the milky white solution formed subsequent to addition of base. In this trial, it was observed as an amber alcoholic layer clearly separated from a lower aqueous layer containing zinc salts. The amine was dissolved in the amber layer, not floating atop it.

[Edited on 13-11-2019 by horribilis]

[Edited on 13-11-2019 by horribilis]
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Metacelsus
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[*] posted on 12-11-2019 at 20:03


Some years ago I tried that same procedure exactly as written. It didn't work (although the starting material was certainly consumed, and the zinc dissolved). I expect something went wrong with the workup.

So you're not the only one to have trouble with this.

[Edited on 2019-11-13 by Metacelsus]




As below, so above.
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[*] posted on 13-11-2019 at 09:35


Quote: Originally posted by horribilis  
Quote:
50 ml IPA (91% w/w), 12 ml water, 75 ml HCl (31% w/w, technical grade) were combined in a beaker and chilled to 0 C. Bright yellow beta-nitro-3,45-trimethoxystyrene was ground with mortar and pestle until finely powdered. Dull gray zinc powder, 400 mesh, was ground with mortar and pestle until absent of clumps.

The beaker was placed on salt water bath and 2.40 g nitrostyrene was added in one portion and stirred until nearly dissolved, although some small particles of nitrostyrene would not dissolve. In 10 equal portions spaced 5-6 minutes apart, 13.0 g of zinc was added to the stirred contents of the beaker. Reaction temperature was <10 C at all times despite exothermic activity and effervescence.

The reaction was left to stir for an additional 7-8 hours with reaction temperature <10 C. By hours 2-3 of the 7-8 hours of stirring there was no indication of any unreacted yellow nitrostyrene. By the final hours of stirring there was almost no trace of undissolved zinc powder. The unfiltered contents of the beaker, clear and gray in color, were made distinctly basic with 70 g NaOH in 70 ml water while maintaining a reaction temperature <20 C. A large amount of white salts formed in the aqueous layer and the alcoholic layer took on an amber color.

The mixture was stirred for several minutes and then the alcoholic layer decanted. The aqueous layer was extracted/decanted twice more with IPA, the final extraction/decanting being clear in color. The alcoholic extracts were left to sit in a freezer overnight, then dried over MgSO4 and filtered to yield a crystal clear light yellow solution. This was then made acidic with alcoholic H2SO4 to pH = 6 to 7, with the crystal clear light yellow solution turning more turbid and amber colored as the crystals precipitated. The sulfate salt of the amine was freeze precipitated overnight, vacuum filtered, and washed with acetone to yield 0.60 g (~20% yield) of the white and shiny sulfate salt. The filtrate was chilled to -15 C but no additional crystals were precipitated.


This procedure was run on the basis of persona's work, which claims yields of ~75%. See http://www.sciencemadness.org/talk/files.php?pid=603835&... Persona's work was, in turn, based on Leminger's work, see https://erowid.org/archive/rhodium/chemistry/leminger.html


To be short and to the point, I'm not sure what went wrong. The nitrostyrene is recrystallized once and bright yellow in color, free of any degradation or polymerization. The zinc is advertised as 99.9% purity. While the zinc was not activated (i.e. not washed with dilute HCl to make it bright and shiny) I don't see how this would matter as it reacts with a very large excess of HCl in situ. Temperature control was reasonable at all points of the reaction and zinc addition was as prescribed.

Does anyone with a more thorough understanding of this reaction have any input? Would it be beneficial to run it as Leminger describes it, forming an IPA/HCl solution and then adding nitrostyrene and HCl in alternating batches, to maintain constant ratios? I have noticed one paper (DOI: 10.1055/s-0034-1379481) goes so far as to add the HCl in portions, as well.

I have also read of some using an even greater excess of NaOH to fully re-dissolve the zinc salts. But I assume this is for ease of workup: filtration and extraction with separatory funnel rather than decanting. Zinc salts would not somehow complex with the freebased amine, correct? I'm also confused by some write-ups mentioning the freebased amine floated on top of the milky white solution formed subsequent to addition of base. In this trial, it was observed as an amber alcoholic layer clearly separated from a lower aqueous layer containing zinc salts. The amine was dissolved in the amber layer, not floating atop it.

[Edited on 13-11-2019 by horribilis]

[Edited on 13-11-2019 by horribilis]


Did you try the other work up I brought here:

http://www.sciencemadness.org/talk/viewthread.php?tid=80555&...

from Research Gate publications, telling about reduction of nitrostyrenes with Zn and HCl at 0ºC?









[Edited on 13-11-2019 by Chemi Pharma]
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[*] posted on 13-11-2019 at 15:18


Quote: Originally posted by Chemi Pharma  

Did you try the other work up I brought here:

http://www.sciencemadness.org/talk/viewthread.php?tid=80555&...

from Research Gate publications, telling about reduction of nitrostyrenes with Zn and HCl at 0ºC?


Yes that is DOI: 10.1055/s-0034-1379481 which I referenced in my post. Do you have firsthand experience with this reaction? Does it actually work and produce acceptable yields?

The more I read, especially on HyperLab, the more I understand that this reaction is difficult. Some hypothesize that rapidly added zinc is consumed by HCl rather than reacting slowly to reduce the nitrostyrene. Some claim that zinc salts may complex with the amine, while many more claim that it does not. Some pictures exist, with the basic reaction mixture looking like my description (amber alcoholic layer on top of aqueous layer full of zinc salts). Other pictures show a milky white solution with no apparent resolution of layers, but a yellow amine floating atop.

It is all very confusing.
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[*] posted on 13-11-2019 at 15:33


Quote: Originally posted by Metacelsus  
Some years ago I tried that same procedure exactly as written. It didn't work (although the starting material was certainly consumed, and the zinc dissolved). I expect something went wrong with the workup.

So you're not the only one to have trouble with this.

[Edited on 2019-11-13 by Metacelsus]


It really is perplexing that some report good yields of 65-85% by running the reaction as written, (with proof of pictures, melting point, even NMR data) while others report yields of 0-30%.
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[*] posted on 13-11-2019 at 15:54


30- to almost 50% for me, on 3,4,5-TMNS.

In my opinion, forget that reaction, use NaBH4/CuCl2... with this reaction I achieved 70% on 2,5-DMNS with the first trial.
I posted about this on here.
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[*] posted on 13-11-2019 at 20:16


In my opinion, which is based on some experience, the reduction of nitrostyrenes with zinc and acid is unreliable and largely non-reproduceable. I have looked carefully at the report by "persona" and find it rather suspect and unbelievable. The yields reported have no meaning since the isolated products were not characterized nor tested for purity. Who knows what was there? Another related problem with zinc dust reactions is making sure that there is very efficient stirring. Even with small scale reactions magnetic stirring may not be sufficient. I am aware that there are bona fide reports of zinc/acid reductions of aliphatic nitro compounds but these are usually special cases. I have to agree with karlos on this: use a hydride based system for these sorts of reductions.

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[*] posted on 14-11-2019 at 04:28


Quote: Originally posted by AvBaeyer  
In my opinion, which is based on some experience, the reduction of nitrostyrenes with zinc and acid is unreliable and largely non-reproduceable. I have looked carefully at the report by "persona" and find it rather suspect and unbelievable. The yields reported have no meaning since the isolated products were not characterized nor tested for purity. Who knows what was there? Another related problem with zinc dust reactions is making sure that there is very efficient stirring. Even with small scale reactions magnetic stirring may not be sufficient. I am aware that there are bona fide reports of zinc/acid reductions of aliphatic nitro compounds but these are usually special cases. I have to agree with karlos on this: use a hydride based system for these sorts of reductions.

AvB


Would you recommend NaBH4/CuCl2 or some other variant? I have come across NaBH4 of reduction of alkene followed by Zn/HCl for reduction of nitro in a one-pot reaction, but I am suspicious of the Nef reaction taking place.
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[*] posted on 14-11-2019 at 04:41


I did this reaction on 2,5-DMNS in methanol and with concentrated HCl. I dissolved the styrene in methanol and HCl and slowly added the zinc, worked pretty well.
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[*] posted on 14-11-2019 at 04:52


I confess I never tried the nitrostyrene reduction with zinc and HCl as written at those papers.

@Karlos have send to me, posting here at the Forum, a work up procedure with NABH4 and CuCl2 that appears very sexy, but I didn't have tried it yet to give a personal verdict.

In any case, here's the reproduction of @Clearly not Atara work up procedure that him and @Karlos claims that works with nitrostyrenes and phenyl nitro propenes too:

"With vigorous stirring, to 5ml of IPA / 1g of P2NP and 0.4 times volume of H2O in RBF was dumped 6 mol eq. of NaBH4.
Recrystallized P2NP was washed with sodium bissulfite solution, then with and water and it was then added in small portions to the mixture to minimize frothing. No external cooling applied. After all substrate was added and near-colourless solution is obtained, the solution left stirring for 30 minutes. Claisen adapter was fitted to the rbf with reflux setup and addition funnel. With one portion, 10%-mol equivalent (for the nitroalkene) of CuCl2.2H2O dissolved in IPA was added to the solution via the funnel. It was refluxed for 30 minutes. After which the remaining NaBH4 was destroyed by adding about 50% acetic acid untill the fizzing stopped and the solution was fairly acidic. With the claisen adapter still attached, short path distillation setup was rigged and everything below 100'C was stripped.
Then the solution was made basic dripping strong NaOH solution to the reaction flask and steam distilled. Additional very dilute NaOH solution was added through the addition funnel every now and then. The distillate was then made slightly acidic with H2SO4 and evaporated.
Total yield after recrystallization: 80% mol wise from the P2NP due to very sloppy work"

I hope it helps!

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[*] posted on 14-11-2019 at 06:08


Are you trying to remove residual benzaldehyde with the bisulfite wash? I don't see how that'd be effective considering the adduct is a solid and there's no mention of filtering.

Or am I missing something obvious?
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[*] posted on 14-11-2019 at 12:12


Quote: Originally posted by Mesa  
Are you trying to remove residual benzaldehyde with the bisulfite wash? I don't see how that'd be effective considering the adduct is a solid and there's no mention of filtering.

Or am I missing something obvious?


What I have read here at the Forum about residual benzaldehyde is that it fucks up with the yield of borohydride reductions of P2NP. That means the Persona who wrote this work up may be are thinking about a way to neutralize the benzaldehyde effect by turn it into bissulfite aduct, which would not interfere in the rest of the work up, I guess.

I found another work up with NaBH4 and CuCl2 @Karlos have sent that I think it's easier, without needing of steam distillation. Here it is:

"2,51g 2,5-Dimethoxynitrostyrene(12mmol) was added to
3,5g NaBH4(93mmol) in 32/16ml of IPA/H2O, 20min after the reaction(no cooling applied), a solution of
0,2g CuCl2*2H2O(1,2mmol) in 6ml 1:1 IPA/H2O was added at once, then refluxed for 40min
After reaching r.t, a 25% solution of NaOH (20 ml) was added, the phases separated and
aqueous phase extracted again with IPA(2x30 ml).
The extractions combined, dried with Na2SO4, filtered, acidified.
The IPA was distilled off then, but the residue was still dirty.
So it was extracted with 3x15ml DCM, then basified and the freebase extracted with 3x30ml DCM,
the extracts acidified using diluted HCl, the aq. portion separated and
evaporated, boiled in dry acetone then filtered, washed with acetone to give
-1,84g of white HCl salt(8,4mmol or 70%)"
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[*] posted on 14-11-2019 at 13:53


I'd guess that your product will be significantly less pure with a solvent extraction compared to distillation. Admittedly I don't know this reaction particularly well so I could be wrong.
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[*] posted on 17-11-2019 at 06:01


Quote: Originally posted by Tsjerk  
I did this reaction on 2,5-DMNS in methanol and with concentrated HCl. I dissolved the styrene in methanol and HCl and slowly added the zinc, worked pretty well.


Can you remember the specific details of the rate of addition of zinc?

I have repeated the experiment another time, keeping the ratios of solvent to reactants the same, but instead grinding the zinc and nitrostyrene together and adding this over the course of 45 minutes to a beaker of IPA/HCl. This was stirred for another 30 minutes. An additional 10 ml HCl added, then this was stirred for another 1 hour. The yellow color disappeared at this point, yielding a clear-gray solution with undissolved zinc. This was stirred for another 5 hours. Reaction temperature was < 10 C at all times. A small amount of zinc was filtered away and washed with IPA/water. The clear-gray solution was made basic with 80 g NaOH in 80 g water with reaction temperature < 25 C, extracted with 150 ml IPA total, dried, and made acidic with H2SO4 in dry IPA. Even after freezing this formed only a suspension and not glittery solids as before. All liquids were evaporated overnight to yield a yellow solid. The solid was practically insoluble in water but readily soluble in acetone, indicating it was most likely unreacted nitrostyrene with a very small proportion of amine.

I believe this shows that the reaction is extremely sensitive to the rate of addition of zinc. Persona's posts on HyperLab provide some detail over this, saying that if zinc is added too slowly it will be consumed entirely by the HCl and not have time to react with the nitrostyrene. Likewise, if HCl concentration is too high (either from too much HCl or not enough solvent) then zinc will be consumed rapidly. But Persona's PDF also states that too little HCl promotes oxime formation...

What a difficult reaction.

Next trial will be in methanol (better solubility than IPA) with smaller amounts of solvent and HCl for the same amount of zinc. Something like 15-20 ml methanol, 15 ml 31% HCl, 5 g zinc, 1 g nitrostyrene. Add methanol/HCl/nitrostyrene at once and stir until completely dissolved. Add zinc over 20-30 minutes RT < 10 C to prevent consumption of zinc by acid. Stir for hours until all zinc is dissolved, possibly with addition of more HCl. Boil off all methanol to prevent co-solvent effects or the complications of toluene solubility in methanol. Make basic with 25% NaOH. Methanol cannot be salted out so amine should float atop the basic post-reaction mixture as an indicator of success or failure before workup. Workup will simply be extraction with toluene, drying, addition of dilute solution of H2SO4 in toluene, freezing, filtration, and washing with acetone. Or maybe just wash the toluene with acidic water, separate, and evaporate to dryness overnight to judge the contents of the acidic washings as unreacted nitrostyrene (yellow) or white crystals (amine salt.)

[Edited on 17-11-2019 by horribilis]
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[*] posted on 1-12-2019 at 02:35


Quote: Originally posted by horribilis  

Next trial will be in methanol (better solubility than IPA) with smaller amounts of solvent and HCl for the same amount of zinc. [...] Add zinc over 20-30 minutes RT < 10 C to prevent consumption of zinc by acid.


Actually, NO. You need a lot of HCl and an ice bath or something similar that will absorb the heat from Zn reacting with HCl, also VERY efficient stirring is crucial.

This reaction works as advertised, when conducted as advertised. The failures reported here can summarily be attributed to operator error.

Quote: Originally posted by Chemi Pharma  
I confess I never tried the nitrostyrene reduction with zinc and HCl as written at those papers.


Oh really, but you keep insisting on your "absolutely superior methods which end any and all discussions about alternatives".

Looks like someone finally beat some modesty into you, huh?

[Edited on 1-12-2019 by stoichiometric_steve]
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[*] posted on 8-12-2019 at 10:14


This is representative of another failure but perhaps it will be useful to some. The reaction was conducted on the 1.0 g scale using an alternative source of zinc that was purer in appearance and a freshly prepared batch of nitrostyrene that was recrystallized as very fine needles, bright yellow.

1.03 g finely powdered nitrostyrene was added to a solution of 16 ml IPA, 5 ml water, and 30 ml HCl to form a bright yellow suspension. With stirring, 5.04 g zinc was added in 5 batches over 40 minutes, another 5ml HCl was added, and then the reaction was stirred for another 2 hours. A clear yellow solution was then observed with no zinc solids present. Another 2.09 g zinc was added in two batches and the reaction was stirred for another 1.5 hours to yield a clear solution with a slight gray tinge and some undissolved zinc at the bottom. Reaction temperature was between -10 C and 0 C at all times. The mixture was made basic with 38 g NaOH in 40 g water with a reaction temperature < 15 C over the course of 1.5 hours. As base was added the zinc salts formed as white flakes, then as a thick white paste, then as a free-flowing white liquid. A small sliver of clear IPA was observed floating above the zinc salts, with some solid zinc in suspension. All solids were filtered off and washed with a small portion of IPA and water. The mixture was extracted with 3 x 30 ml IPA. The slightly cloudy IPA extracts were dried over MgSO4, filtered to absolute transparency, stored overnight in a sealed container in a freezer, and then made acidic to pH = 5 with dilute H2SO4 in IPA. Yield was 0.35 g of a flat white solid with some very small shiny crystals embedded in it. As the appearance was inconsistent with the desired product, no MP was taken.

Maybe the reaction is "on the right track" because the IPA extracts were almost transparent and not yellow-orange or amber. Perhaps the reaction went wrong because the zinc was consumed too rapidly. This explains why the reaction mixture was initially a yellow suspension, and then a clear yellow solution with no undissolved zinc. It's possible that the additional 2.0 g zinc was added too late, or that it was not given enough time to stir and react. Perhaps the product was largely an imine intermediate (flat white) with a small amount of amine sulfate salt (shiny crystals). Or maybe not, who knows.

Corrective action will be to increase the amount of zinc, adding enough to keep the reaction mixture gray in appearance for the first hour. Persona's write-up mentions keeping the mixture gray for the first hour or so of the reaction, but he also recommends 5.0 g zinc as enough to accomplish this. Perhaps 8.0 g zinc within the first 40 minutes will be sufficient to maintain this gray appearance, or maybe more.
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[*] posted on 8-12-2019 at 14:08


So I know it's a bit haphazard, but I'm really wondering if anyone has tried adding a bit of CuCl2 to the Zn/HCl reduction. Or, if you have a little NaBH4, you can try forming copper boride in situ (by CuCl2 + NaBH4) and then adding Zn/HCl to provide most of the actual H atoms.

Thus we have the "Zn/HCl/CuCl2" and the "Zn/HCl/CuCl2/NaBH4" methods.

It seems like everyone knows Zn/HCl is not ideal, but it might be better than nothing. NaBH4 is a little expensive as a primary reductant, but maybe it could be a little more practical as a catalyst?





[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 8-12-2019 at 14:55


Quote: Originally posted by clearly_not_atara  
So I know it's a bit haphazard, but I'm really wondering if anyone has tried adding a bit of CuCl2 to the Zn/HCl reduction. Or, if you have a little NaBH4, you can try forming copper boride in situ (by CuCl2 + NaBH4) and then adding Zn/HCl to provide most of the actual H atoms.

Thus we have the "Zn/HCl/CuCl2" and the "Zn/HCl/CuCl2/NaBH4" methods.

It seems like everyone knows Zn/HCl is not ideal, but it might be better than nothing. NaBH4 is a little expensive as a primary reductant, but maybe it could be a little more practical as a catalyst?



Is there any literature to support CuCl2 catalysis of dissolving metal reductions? I've never heard of it being used. It's an interesting idea, if it works.
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[*] posted on 15-12-2019 at 17:33


Sounds like somewhat of a crossover between a ZnCu-couple and a dissolving metal reduction. Probably works, not sure if necessary.
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[*] posted on 12-1-2020 at 11:34


Another attempt was made with some slight changes.

17 ml IPA, 5 ml water, 1.01 g powdered nitrostyrene were combined in stirring and placed on a salt water bath. 5.0 g zinc and 30 ml HCl were added in 5 batches over the course of 30 minutes with stirring. Reaction temperature was between -5 C and 10 C at all times.

After the final addition the beaker was stirred for another 30 minutes. The solution was still slightly yellow, but becoming transparent. Another 1.0 g zinc and 6 ml HCl was added. After another 45 minutes of stirring the solution was clear with some undissolved zinc. The beaker was stirred for another hour and almost all zinc had dissolved while the transparent color persisted. Reaction temperature was between -5 C and 10 C at all times. The beaker was stirred for another hour and then placed into a -10 C freezer overnight. The few remaining pieces of zinc were removed via vacuum filtration. The solution was made basic with 40 g KOH in 40 g water over the course of an hour. The reaction temperature between 0 C and 10 C at all times.

The milky white solution was immediately extracted with 4 x 25 ml IPA and dried over a small amount of MgSO4. The clear extracts were slightly cloudy after drying due to some residual water. The solution was made acidic to pH = 3 with H2SO4 in dry IPA and placed in a freezer. No precipitate was observed and so 25 ml acetone was added, causing precipitation of some solids. The solids were vacuum filtered, washed with acetone, and dried over vacuum to yield ~0.34 g of a flat white solid. Recrystallization produced a solid with a similar appearance.

Assuming this is the intended product, which I doubt considering its appearance, this is a ~25% yield. Nitrostyrene was fresh and recrystallized, bright yellow needles. Zinc was new although technical grade. The reaction is doing something as the yellow nitrostyrene is being consumed...but why are the yields so terrible. God, what a pain in the ass.
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