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May 29, 2009
Wait For It. . .Wait For It. . .
Here's something that I'll bet every bench chemist has experienced: thinking that you've quenched some nasty reagent (it has to be gone by now!) only to find that it's very much still with you. These guests that won't leave can be smelly, corrosive, or downright dangerous when they finally yawn, stretch, and decide that it's time to move off the couch.
Alkylaluminum species, in my experience, take their time for longer than you'd think possible, and then depart in a tearing hurry. I used to use several diethylaluminum-X things (cyanide, alkynes, and so on), and was taken by surprise early on by their lackadaisical response to methanol or water at the end of the reaction. "Surely there's some excess aluminum junk in there", I remember thinking the first time this happened, "but there's nothing happening. Maybe I should just squirt in some more." That last phrase has been the prelude to many exciting chemistry moments, and so it was here. Not long after I acted on that impulse, the reagent caught on the fact that it had lots of methanol surrounding it ("Hey, I react with this stuff, don't I?"), and another geyser was born.
Perhaps the king of the "I thought it was hydrolyzed" bunch is phosphorus oxychloride. That stuff takes forever to get around to reacting with water, although on the face of it, you'd imagine it fizzing and sputtering as soon as it got within range. But no, many chemists who've used this reagent have returned to their fume hoods to find the contents of their sep funnels or waste jars gradually coming back from the dead. Milkshake can tell you all about it at Org Prep Daily, and so can many others: never take this one for granted.
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