I thought I would share a series of photos from last week, where I had occasion to convert a pyridyl ketone into an oxime.
When I first added hydroxylamine hydrochloride to the flask, I noted that the clear solution immediately took on a pinkish hue. We are stirring on ice just to be safe.
After about 15 minutes, I checked back on the flask and noticed that the color had changed from pink to a dark orange, maybe the color of pulpy orange juice. You can see that at the right.
These first two color changes both took place at temperature around 0-5C. Of course, it is not uncommon for colors to change as a reaction proceeds to product. I wondered whether this would be the color of our pyridyl oxime product; I took the reaction out of the bath, and let it stir at room temperature to complete the conversion.
At left is the final color change as product formed. What once was pink, then orange, now took on a salmon color; I suspect this is the result of the reaction “splitting the difference,” and having the dark orange offset by the light pink. This is all pre-workup, too, so there is aqueous HCl and triethylamine floating around in there (we went with NEt3 over pyridine to aid the workup). Pyridine is not your typical tertiary base in many respects. So, the “fireworks” that we see here is not necessarily unexpected, but it cool to note anyway.