Thiophene (Sulfur, not quite stinky)

Thiophene is a simple five-membered sulfur-containing heterocycle - it's the sulfur analogue of furan.

It's an interesting heterocycle in its own right but it's probably best known as a contaminant in benzene - almost all the bottles you get a hold of today proclaim they're "thiophene free."

It wasn't always the case, as chemists who were trained more than a decade or two ago will tell you. My advisor in undergrad told me stories about having to remove the thiophene from benzene, which entailed shaking the benzene with concentrated sulfuric acid in a giant separatory funnel. Pressure builds up as you use them, so you have to open the valve to vent them. Inevitably this releases some liquid; in this case a caustic/carcinogenic sulfuric acid/benzene mixture.

My boss was, generally, far from cavalier about chemical hazards, but his quips were the best. "You'll get hit by a bus before this stuff gets you." "We all have to go sometime!" Or, when wiping up (gloveless) a concentrated (deep red) ethidium bromide spill, "I'm too old to worry about this intercalator shit!"

For the record, he was a careful guy in general (and all in all, pretty safe). Hi, if you're reading :) The ethidium one even made my jaw drop a bit, though (and I'm skeptical about the level of risk 1 micromolar ethidium stain really poses, as I mentioned). I still wear gloves, though!