Sciencemadness Discussion Board


Geomancer - 24-10-2005 at 19:59

Or: 5-hydroxy naphthoquinone
Or: A novel method for the purification of commercial xylene

Some days ago, I was extracting black walnuts (Juglans nigra) from thier husks. For those of you not familiar, the nuts are contained within large, soft green husks with a staining juice (I believe these properties to be shared with other walnut species). During this process, several things caught my attention. First, the juice from green husks is at first clear, slightly yellowish to greenish. Immediately upon exposure to the atmosphere, it becomes turbid and brownish. Second, although I was wearing thin latex gloves, they failed (physically, perhaps chemically as well). The exposed areas of my skin became stained, first yellowish to brownish, becoming black with time. The stain does not wash off. Moreover, several of the more exposed and sensitive regions (the back sides of my fingers) became irritated, red and swollen. The overall impression is uncannily like a severe bruise at first, before turning entirely black. The irritation subsided within a day or so, and the skin in those regions began to peel after about 5 days. The stains in the other regions lasted about 2 weeks, although my fingernails retain some bronze stains.

Naturally, these facts piqued my interest. It appears the primary responsible compound in all the above phenomena is juglone, 5-hydroxy naphthoquinone, present either as the hydroquinone or a glycoside thereof in the green husks and oxidising with age or exposure to air. It is an interesting compound; breifly:

According to the Merck, it gives yellow needles from benzene+pet. ether, is freely soluble in benzene or chloroform, soluble in many organic solvents and slightly soluble in hot water. It dissolves in alkali giving purple solutions. (All this is from memory, any errors are mine.)

The less said about my early extraction efforts the better. I worked mostly from dried husk, it being easier to collect and dry them and extract at my leisure. Even during rapid drying, though, a complex oxidation seems to happen and the entire material turns dark brown to black , with many of the brown pigments being highly water soluble. Some small deposits of yellow juglone may be observed on rapidly dried surfaces, though. It appears that any heating of the crude product causes polymerization. For this reason continuous extraction with alcohol was a failure. Direct recrystallization from xylene and/or naphtha also failed: although yellow solutions were obtained, they failed to cystalize or even precipitate much, even with partial evaporation.

In my latest experiment along these lines, I powdered a handfull of dried husks using a food processor, mixed with water and filtered off the very dark liquid. I continued washing (in the funnel) with water and bicarbonate solution until the liquid from the filter was only lightly colored. I then poured an NaOH solution through the filter, collecting the dark purple liquid produced. I washed this twice with xylene, then acidified and extracted with xylene again. Acidification caused a large amount of dark colored precipitate to form. Upon settling the aqueous layer was seen to be light tea colored. After letting it sit a day and a half to try and get the nasty emulsion to break (and adding a small amount of brine), I separated as best I could and filtered the red organic layer, along with the remaining emulsion, through a filter loaded with DE. I added a bunch of MgSO4 to suck the water from the wet DE and let the nonpolar layer through. The filtrate was light yellow, the red color apparently caught up on the drying agent. I shook this with activated carbon, filtered, and then shook with bicarbonate solution. After settling, there was a small amount of precipitate present in the aqueous layer. Sitting above that was an expanse of perfectly clear xylene.

I had been led along these routes by something on the internet (I'm not in the mood for cites right now; if anyone's interested--ask) that said the commercial method of extraction was by fermenting the husks for 2 days to oxidize the hydroquinone and extracting with water. Liars.

I tried direct sublimation (juglone sublimes ~150C, I believe). This showed some promise. In all, I got maybe 1-10mg of crude product. Unfortunately much of this dropped onto a lightbulb as I was trying to dry and examine it. The odor produced was slightly sweet, and did not make me feel terribly healthy. I was feeling somewhat under already, so this may not be a direct effect. Either way, do not "concentrate and inhale".

The most promising thing I've tried has been removing the green husks under an ascorbic acid solution. The liquid immediately begins to turn yellow (!), however the undesirable brown-stuff forming reactions are suppressed. When the whole mess is thrown into the blender it results in a mixture resembling pulpy orange juice, which quickly separates into a layer of foam and a layer of brownish orange liquid and pulp. When this (filtered or not) is mixed with xylene the xylene turns yellow, but it appears xylene is a less than ideal solvent and the solution in it is not too concentrated. I haven't done anything with the yellow xylene yet.

So. Insights? Ideas?