As it turns out, the crystals of “Martius yellow” that I recovered from the acetone washings are truthfully not Martius yellow. Unfortunately, I failed to realize this until after I recrystallized my entire supply. The compound, while also an intensely colored yellow-orange dye, is much darker in the solid form than Martius yellow acid. It does not dissolve in basic aqueous solution, nor does it react with conc. hydrochloric acid. Martius yellow acid should of course readily dissolve in aqueous ammonia. My best guess as to what happened is some sort of aldol condensation, though I’m not familiar enough with reaction mechanisms to work it out. I’ll have a sample of the mystery compound analyzed by GC/MS to see what it really is. Once that has been done, I’ll post an update.Not Martius yellow. Possibly a product of an aldol condensation.
Luckily, since Martius yellow is so easy to make and I have large amounts of the needed reagents, I prepared more. This time I used the workup prescribed by Fieser: dissolving the crude Martius yellow in hot aqueous ammonia to convert it to the ammonium salt, filtering while hot, and then chilling the solution and salting out the ammonium salt using ammonium chloride. The ammonium salt (which I suppose is truly Martius yellow? Fieser sometimes refers to the salt as Martius yellow, while other times referring to the acid, so it is rather confusing) is a deeper color than the free acid- orange, as opposed to golden yellow. It is, of course, much more readily soluble in water.
Martius yellow (ammonium salt)
The difference in properties is easily demonstrated by dissolving ~10 mg of the salt in a few mL of water, and then adding a couple drops of conc. HCl. The free acid immediately precipitates.
This new batch of Martius yellow will be used for making the other dyes in Fieser’s series. The next step is reducing it to 2,4-diaminonaphthol, which is not a dye itself, but is used as the starting point for all of the others.