04-07-03 10:18
No 424493
I have made several 2,5-dimethoxybenzaldehyde batches during the last couple of months, and I have noticed that my crystallized product may discolorize during the air-drying process. I recrystallize the benzaldehyde from water/EtOH, rinse the filtered crystals with a small amount of dH2O and dry them in the air to constant weight. However, during the drying process, the outer crystals may change color, from an attractive white/faintly yellow to a nasty and unattractive black. Now, my question: what is the reason for this?
A) 2,5-dimethoxybenzaldehyde auto-oxidizes freaking fast, and I should choose another recrystallizing solvent (one which evaporates faster) or dry in vacuo.
B) there are still some impurities left that change the color to black, or the impurities are oxidized during the drying process and give it this unattractive color.
C) something else.
My guess is A, but I'd like to verify with someone who knows for sure. If (s)he knows a good alternative, you are welcome to share
Also, I found that recrystallization from petroleum ether (of the already pure product) results in spectacular crystals, easily 2 cm long needles.
The faster you run, the quicker you die.
(Hive Bee)
04-07-03 22:45
No 424581
Just an idea, I think you could distill it using a short path. I assume it is to avoid the 2,5-DMB clogging the condenser. I have seen this recomended as a purification technique in Rhodium's page.
(Hive Addict)
04-07-03 22:52
No 424582
The purity (by GC) of the batches I add to my 2,5-DMB stock is never lower than 98%. The mp is always within range. I have reasons to believe my product is rather pure. I think it is air-oxidation of the benzaldehyde, but I'm not sure. I have to admit that I am unpleasantly surprised by the speed of this phenomenon. If I allow the crystals to dry during the complete afternoon, they all turn black. If I store them in a closed container (brown glass jar), the white/faintly yellow colour remains.
The faster you run, the quicker you die.
(Hive Addict)
04-07-03 23:51
No 424600
Vacumn dessicator.
(Hive Bee)
04-08-03 02:28
No 424630
BTW, if distilling it, do it with vacuum, preferably with a good vacuum pump. I believe the MP is around 50 C or so, maybe has to use warm water in the short path condenser.
If you believe it is 98 % or so pure, and only want to avoid the oxidation problem you think is happening, I suggest use a good vacuum packing machine, pack it in plastic bag and store in the fridge.
EDITED POST :
Sorry, I mean MP, not BP, above post corrected already.
As I have said before, after distilling ( or re-crystalizing and drying with vacuum ) can use a vacuum packing machine to store in plastic bag or mason jar.
Sorry for the mistake.
(Hive Addict)
04-08-03 10:47
No 424720
is that the hypothetical nitrostyrene derived from nitromethane and benzaldehyde?
(Hive Addict)
04-08-03 11:00
No 424723
I have a number of mono-, di-, and tri-substituted benzaldehydes. The purity is between 98-99%. None of them are particulary sensitive to air oxidation. Not even vanillin, which is supposedly one of the more sensitive benzaldehydes, changes that much from 24 hours exposure to air.
I even put two grams of 2,5-dimethoxybenzaldehyde on two pieces of filter paper over night, after reading your post yesterday. One gram was placed on one filter paper and left as it was, the other gram was wetted with some tap water and left to dry over night. The dry sample hadn't changed at all, which was verified by HPLC. The wetted sample had indeed changed color a bit to a more brown tone. The purity was down to 94%. Water/air seems to be a bad combination.
I actually assume you have a rotovap, get a flanged drying flask from buchi. Recrystallise from some suitable solvent (my choiche would be MeOH or EtOH) and dry the benzaldehyde at 35-40° and 10-20 mbar for an hour or so. If you have a buchi rotovap (don't know about the other brands) you can even change the atmosphere inside to a inert one if you like.
Freaky
(Hive Addict)
04-08-03 11:44
No 424731
Mmmm, Madmax? That orange looking thing that might be the nitrostyrene which might have been formed via the 2,5-DMB looks very much like the orange crystal-shaped structures which might be stocked in the freezer behind me and which might be formed from the same benzaldehyde
.Thanks for the tips. I recrystallized from EtOH now and discoloration occurred to a much less extent. I'm certainly going to try Ba's tip, as it happens I do have a Buchi
The faster you run, the quicker you die.
(Chief Bee)
04-08-03 23:25
No 424849
2,5-Dimethoxybenzaldehyde from Aldrich (98%, $50/100g) is coffee-stain-colored, while the same substance from an asian exporter (98%, $65/kg) is perfectly colorless, and stays that way. It all depends on which impurities are present.
However, any phenolic impurity is VERY likely to oxidize to a quinone compounds of some sort, and as we all know, they are very highly colored. Perhaps washing a solution of the benzaldehyde with saturated NaHCO3 followed by water & brine will remove such phenols and prevent discoloration of your product.