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June 7, 2006
Best When Used By. . .
Most of the things we use in an organic chemistry lab can sit around for reasonable periods of time. I've used reagents from bottles that are older than I am (OK, this was twenty years ago, so it's getting a bit harder to do). As long as the stuff isn't air- or moisture-sensitive, it can hang around a long time. That lets out the violently reactive things - don't expect to find the same piece of potassium metal you left if you're silly enough to leave it out while you answer the phone, for example. (In fact, you'd better pick up a fire extinguisher on the way back, just on general principles).
But there are some reagents that don't react with air, but rather react with themselves, which can make them particularly hard to handle. If the reaction is exothermic, things can get dangerous. The heat given off by the first bit that reacts tends to set off some others, which really gets a good amount going, and ba-doom. Even if you're not in the ba-doom category, there are some things that you need to look out for. Styrene, for example, is always sold with some free-radical inhibitors in it, because if it gets a chance for a radical chain reaction to start, the whole bottle will seize up into a warm gunky block of polystyrene. (That's for small bottles - larger ones won't be able to transfer their heat so well to the surface of the container and can make a much bigger mess).
Benzaldehyde isn't so violent, but it slowly forms
a six-membered-ring trimer on standing. Update: I've been carrying this idea around for twenty-five years now, but it's wrong. The nonaromatic aldehydes love to trimerize, but benzaldehyde and the other aromatic ones don't. The solid gunk is benzoic acid, from air oxidation, which is a separate category of How Reagents Go Bad. Old bottles can have some crusty crystals of the stuff around the neck of the bottle. The reagent is one of those things that you really have to distill before using it if you want to trust your results. Update: This point is definitely still true!
The extreme case in the self-condensation category is probably cyclopentadiene. It does a Diels-Alder reaction with itself first chance it gets, so it's always sold as the dimer. If you want the pure monomer, you have to distill for it. The dimer cracks thermally, so the vapor condensing at the top of the still is a different substance than the stuff down in the pot. Collect it, keep it on ice, and use it - the diene's a ware that will not keep.
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