Chemistry Forums for Students > Organic Chemistry Forum

Ortho-nitration of benzaldehyde

(1/4) > >>

willijung1:
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!

rolnor:
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.

rolnor:
https://en.m.wikipedia.org/wiki/2-Nitrobenzaldehyde

willijung1:

--- Quote from: rolnor on February 19, 2020, 10:03:39 AM ---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.

--- End quote ---

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...

rolnor:
The two schemes with the alkoxy groups I can understand but the first scheme is strange, it should not work.

Navigation

[0] Message Index

[#] Next page