Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: willijung1 on February 19, 2020, 05:12:19 AM
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I am a high school student who is interested in organic chemistry.
I was searching for syntheses of indigo when I came across two interesting papers on ortho-nitration of benzaldehyde.
This is the first one:
Synthesis of 2-nitrobenzaldehyde
Concentrated HNO3 (5 mL) was cautiously added to the benzaldehyde at 0 degrees Celsius. Then the mixture was stirred for 40 min at 15 degrees Celsius. On pouring the reaction mixture into ice water, insoluble material precipitated. The precipitate was filtered out to afford the crude product.
Yield: 90%; yellow colour compound; mp: 42 degrees Ceisius; IR (KBr, cm1): CHO (1685), C–N (1227), C–H aromatic (3122), C=C aromatic (1574), NO2 (1337 symmetric), NO2 (1500 asymmetric).
Now when I saw this for the first time, I was rather skeptical because they did not provide the amount of benzaldehyde or the NMR spectra values. Also, the reaction looked unlikely since sulfuric acid was not added.
That was until I managed to search up another paper they mentioned in the references section:
5-(Benzyloxy)-4-methoxy-2-nitrobenzaldehyde (3a)
3-(Benzyloxy)-4-methoxybenzaldehyde (10 g, 41 mmol) was added cautiously to 40 mL of concentrated nitric acid at 0 ºC. The mixture was then stirred at 15 °C for 40 min. On pouring the reaction mixture into ice water, the precipitate was filtrated to afford 5-(benzyloxy)-4-methoxy-2-nitrobenzaldehyde (10.4 g, 93%) as a yellow solid. Mp 131 ºC (lit[28], mp 133 ºC). 1H NMR (CDCl3): δ 10.4 (s, 1H, CHO), 7.6 (s, 1H, Ph-H), 7.3–7.5 (m, 5H, Ph-H),
7.20 (s, 1H, Ph-H), 5.25 (s, 2H, PhCH2O), 4.0 (s, 3H, CH3O).
4-(Benzyloxy)-5-methoxy-2-nitrobenzaldehyde (3b) The compound was prepared in 91% yield according to the procedure for 3a using 4-(benzyloxy)-3-methoxybenzaldehyde. Mp 131 ºC (lit[28], mp 133 ºC). 1H NMR (CDCl3): δ 10.4 (s, 1H, CHO), 7.60 (s, 1H, Ph-H), 7.3–7.5 (m, 5H, Ph-H), 7.2 (s, 1H, Ph-H), 5.25 (s, 2H, PhCH2O), 4.0 (s, 3H, CH3O).
Now when I saw this paper I was like "what..?"
I knew that benzaldehyde is a meta-directing deactivator, so I was quite surprised that the ortho-nitration was possible in such high yields. (I don't think the the benzyloxy or methoxy groups gave much effect in regio-determination since the 3a and 3b compounds both reacted well, at least seemingly.) I looked it up on the Internet and found a few more papers on this phenomenon. It was interesting enough that I decided to try out the reaction in the school lab.
I did a dropwise addition of 7.5mL of 60% nitric acid to 2mL of benzaldehyde while stirring vigorously. The whole addition step took about 30~40 minutes, and I then left it stir for 40 minutes, making sure that the temperature stayed around 14~16 degrees Celsius. The reaction mixture when I stopped stirring it had two layers: the top one was yellow and relatively clear, but the bottom one was whitish and opaque. Then I poured the reaction mixture into ice water as they said, only to find out that there were only a few pink oily drops resting at the bottom and a big drop of transparent oil floating on the top. No precipitate resulted.
I think the transparent oil floating on the top was the benzaldehyde that did not react for some reason, but I can't seem to get a clue of what the others are and why the reaction did not take place. :(
At first I thought the product did not separate because it somehow was produced in an oily state. However, coming across another paper which reacted conc. nitric acid and benzaldehyde in dichloromethane, I tried that method too only to fail again. (It said the benzaldehyde and only the HNO3 molecules react in the dichloromethane layer, so this brings an increase in the nitric acid concentration in the dichloromethane layer...)
My school isn't really into chemistry, so there are no IR or NMR equipment. We ran out of TLC plates recently, and it will take a while for them to arrive. So I'm just left with my head to assume what might have happened....
Can anybody guess or explain or at least hint about what might have happened during the experiment? I believe that my starting materials are in a fairly pure state because I just opened the benzaldehyde container lid yesterday (though it probably sat around for two years or more, of course in a dark room) and the nitric acid container lid was very tightly shut.
I didn't think that it would be such a fail since those papers "had been published," but.... :(
P.S. It would be great if we had 2-nitrobenzaldehyde in the first place, but we don't, and the purchase would take at least a few weeks... :(
Thanks in advance!
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I am surpriced that you can make o-nitration of benzaldehyde, main product should be m-nitro? It sounds very strange, at my first workplace they made a lot of nitrations and one compound we wanted was the o-nitrobenzaldehyde, we made it through ozonolysis of o-nitrostilbene, it was not possible to nitrate benzaldehyde directly.
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https://en.m.wikipedia.org/wiki/2-Nitrobenzaldehyde
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I am surpriced that you can make o-nitration of benzaldehyde, main product should be m-nitro? It sounds very strange, at my first workplace they made a lot of nitrations and one compound we wanted was the o-nitrobenzaldehyde, we made it through ozonolysis of o-nitrostilbene, it was not possible to nitrate benzaldehyde directly.
Yes, and that was why I was astounded to see those procedures in a published thesis. It has something to do with the carbonyl oxygen which perhaps might form a six membered transition state.
But what I am curious about is that even if I did not get o-nitrobenzaldehyde, I should have gotten some product as a nitration product. (or at least some "solid" from a side reaction, perhaps) Even if those who wrote those theses were mistaken for the o-nitration, they should have seen something that made them write "precipitated solid" and 90% or 91% as their yield...
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The two schemes with the alkoxy groups I can understand but the first scheme is strange, it should not work.
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The two schemes with the alkoxy groups I can understand but the first scheme is strange, it should not work.
Perhaps it would help to ask the publisher of the procedure..?
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Having recently purchased ortho-nitrobenzaldehyde, I can say that the commercial material was of good quality for our work.
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Remember that benzaldehyde in aqueous solution (like nitric acid) is in equilibrium with the hydrate. The hydrate is similar to the acetal - and the acetal is an o-p director (not m), and activating. So the fastest reaction is with the hydrate at the o and p positions!
There is an preference for ortho with benzyl alcohols - a nitrate ester is possible involved with intramolecular nitration at the o position.
Just be careful - when I did some of this chemistry decades ago, the nitrate esters can lower your blood pressure - even being very careful it can be noticeable.
This is the reverse of amines - the amine group is activating o,p; but if protonated to ammonium is deactivating and m.
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It seemes strange even so, I think there is something wrong here. Read the wikipedialink I provided its just not possible with just nitric acid and benzaldehyde.
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Remember that benzaldehyde in aqueous solution (like nitric acid) is in equilibrium with the hydrate. The hydrate is similar to the acetal - and the acetal is an o-p director (not m), and activating. So the fastest reaction is with the hydrate at the o and p positions!
Does benzaldehyde mix well with water? During my experiment, I observed two layers form, so I thought the reason for the failure was because benzaldehyde was not very soluble in water. Thus it couldn't get into the reaction mixture, perhaps...? What should I do to facilitate the process?
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Having recently purchased ortho-nitrobenzaldehyde, I can say that the commercial material was of good quality for our work.
I'm sorry, but I don't seem to understand what you mean. Do you mean that you have succeeded in the aforementioned ortho-nitration process, or is it just that the purchased ortho-nitrobenzaldehyde was of good quality?
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I wright this again, have you read the wikipedia article on nitration of benzaldehyde? You get mainly the m-isomere just as would be expected. If they use some kind of catalyst or special solvent then maybe something else is possible but it seems not to be what you are using?
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I wright this again, have you read the wikipedia article on nitration of benzaldehyde? You get mainly the m-isomere just as would be expected. If they use some kind of catalyst or special solvent then maybe something else is possible but it seems not to be what you are using?
Yes I read the wiki article and several other sources, and I know well that the m-isomer should be the major product. But it says in the sources I mentioned above that with only nitric acid ortho-nitration is possible. So that's why I brought this matter to the forum: how did they do that, and why couldn't I get it?
So you've said that the method in the first source (2-nitrobenzaldehyde synthesis) is wrong. I get the message. I'll try again with different conditions or perhaps an additional catalyst or reagent. Thank you anyway :)
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Have you done a thorough litterature-search on this matter? The two references with the alkoxygroups are different and this chemistry can not be used for the non-substituted benzaldehyde. If you do find a catalyst it would be of great value for the industry.
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https://www.researchgate.net/figure/Scheme-of-aldehyde-group-assistance-in-the-nitration-of-benzaldehyde_fig1_226387985
Paper can be downloaded.
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interestingely the mention the ozonolysis of o-dinitrostilbene, I made that in -84. They get a little more if the o-isomer but even with there method they get mainly the m-isomer when doing direct nitration. To go via the dioxolane is not very impressive, more steps.
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Maybe it has to do with the concentration of HNO3.
After reading more research papers on the matter, I learned that fuming nitric acid could catalyze nitration for various aromatic compounds in dcm (but also have a danger of detonation in dcm). Apparently they made fuming nitric acid in situ by protonating the NO3- ion, and the protonated form then separated from the aqueous layer into the dcm layer.
When I first read the procedure, I thought of 'concentrated nitric acid' as the 68% one present in my school lab. But now as I think again about it, they might have meant fuming nitric acid. Since 68% nitric acid is an azeotrope with water, the species mostly exists as NO3- ions which definitely wouldn't do the job.
Now somehow I'll try and find a less dangerous step which involves the in situ generation of the protonated form. (I well know that fuming nitric acid is extremely dangerous...)
Many thanks to those who gave me advice!
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Have you done a thorough litterature-search on this matter?
Maybe highly concentrated nitric acid might do the job...!
Have you ever tried making HNO3 in situ by mixing KNO3 or any other nitrate with sulfuric acid and then adding some dichloromethane? Several papers show this procedure for producing the designated nitrating mixture which results in ortho-nitration due to something called the 'chaperone effect.'
I'm slightly worried about this procedure because I have learned that nitric acid can detonate when mixed with organic solvents. Is there a difference between directly mixing nitric acid and an organic solvent and letting the protonated form separate into the organic layer..? Would it be safe enough to try out..?
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You should be aware that people has worked with this problem for a very long time. That does not mean that its hopeless but very difficult to get the o-nitration. I think you need something new, to generate HNO3 this way will not fix the problem I think.
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You should be aware that people has worked with this problem for a very long time. That does not mean that its hopeless but very difficult to get the o-nitration. I think you need something new, to generate HNO3 this way will not fix the problem I think.
I'll definitely keep that in mind. Thanks!!