Answer: Isocyanides

Isocyanides are a class of organic compounds related to cyanide that serve as the building blocks for synthesizing more complex compounds. Anything that serves as a building block for bigger reactions seems like the perfect thing to study and explore in the lab, no?

In the case of isocyanides, however, research into them (and the practical applications thereof) is greatly hindered by one factor: smell. Not just any smell, not just a sulfurous whiff, momentarily unpleasant and then sent up the fume hood, but a horrible building-permeating stench. The smell of most isocyanides is so potent as to be legendary among chemists, and it takes days for the smell of even a minute amount of the substance to dissipate (there are a few isocyanides like malt, natural rubber, and creosote that convey less offensive odors).

In fact, the smell is so bad that the most practical application for the compounds, at the moment, seems to be deploying them in non-lethal weapons—who needs water cannons or microwave beams to disperse a crowd if they’ll run screaming from the smell, after all?