Everyone's heard of cyanide, whether they've spent any time in a chemistry classroom or not. And if you form a covalent bond to the carbon of that CN group, you've got a nitrile, and those are familiar compounds to any organic chemist. But what if you flip the group around and bond it via the nitrogen? That gives you a weird situation, where the nitrogen has a formal positive charge and the carbon is left with a formal negative one, which looks somehow unnatural. But that's an isonitrile (isocyanide) for you.
They're actually quite useful, although I'd guess that the majority of chemists have never encountered one. But if they have, they've remembered it, because isonitriles are not shy about announcing their alien character. Our noses can immediately tell the difference between garden variety nitriles and their evil twins. The former often have no smell at all, or run to a faint spicyness. The latter smell like. . . like. . .well, I've never actually been downwind of the Abominable Snowman's armpit or been had my eyeglasses fogged up by a Komodo dragon with stomach trouble, but those are the examples that come to mind.
Fragrance expert Luca Turin has described isonitriles as "the Godzilla of scent", and that's accurate, if you also try to imagine Godzilla's gym socks. "Penetrating" and "repulsive" are good words to describe your typical isocyanide. It feels like the odor is aggressively storming your nasal passages, and it really makes you want to be somewhere else very quickly. This abstract gets the point across well. Problem is, it can be one of those smells that stays with you ("I like it in here. Stop bothering me."), so going somewhere else - although a recommended first step - is not always enough to do the trick. As a paper on the synthesis of these fine compounds puts it:
It should also be noticed that due to the extremely distressing odour of isocyanides the application of the usual techniques of purification is especially difficult since the exposure to isocyanide vapours, even at very low levels, must be rigorously avoided.
Good advice! But hard to put into practice if you use the things at all, since it doesn't take much. Even the not-so-volatile isonitriles get up on the table and shout at you, but the low-molecular-weight ones are truly hard to take. And the pride of that bunch seems to be the n-butyl, which should come as no surprise. Straight-chain butyl compounds are well known to be just a poor match for human sensibilities. Butyl alcohol is stinky, butylamine foul, butyraldehyde reeks, butyric acid is famously disgusting, and butyl mercaptan is a standout even in the vile crowd of thiols.
So butyl isocyanide is, well, something to experience. I've never had the pleasure, and will take pains not to. I can do no better than to quote the 1937 observations of one of the first groups to figure out how to prepare this noble reagent in quantity:
Butyl isocyanide proved to be so disagreeable to manipulate that none of its physical constants except boiling point were determined. Even when a hood with an extra forced draft was used, the odor pervaded the laboratory and adjoining rooms, deadening the sense of smell and producing in the operator, and in others, severe headaches and nausea which usually persisted for several days.
Sounds great. Many of you may have had similar experiences at some point, but it usually takes more than just spilling a drop of stuff on the floor to bring them on. Here's the whole thing, in a bottle! Available wherever fine chemicals are sold, actually. Don't just live an average, boring life: go wild - go isocyanide!