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.