4.8 More Thio Compounds

If you are wondering whether there are thio equivalents to other compounds like there are for alcohols and ethers, the answer is absolutely yes!

Thioaldehydes and thioketones are known, though not common. Some of the stinkiest compounds known belong to these classes. An oxide of a thioaldehyde is responsible for the phenomenon of crying while chopping onions: (Z)-propanethial-S-oxide CH₃CH₂CH=SO. The molecule has a positive charge on the sulfur atom and a negative charge on the oxygen atom. It is the (Z) isomer because the ethyl group and the oxygen atom are on the same side of the C=S bond:

Thio versions exist for carboxylic acids. Their structures can incorporate a -C(=S)OH group, forming a carbothioic O-acid, a -C(=O)SH group, forming a carbothioic S-acid, or a -C(=S)SH group, forming a dithiocarboxylic acid. Thio S-acids are much more acidic than normal carboxylic acids.

Thiocyanates and isothiocyanates occur in some plants. Allyl isothiocyanate CH₂=CHCH₂N=C=S occurs in cruciferous vegetables and causes the pungency of mustard, horseradish, wasabi, and radishes.

Besides thio compounds, there are also seleno compounds, where a selenium atom replaces an oxygen or sulfur atom. There are anecdotes about entire buildings having to be evacuated because everyone in a thousands-of-feet radius could immediately smell when someone accidentally spilled a few drops of benzeneselenol C₆H₅SeH. I don't know if I believe that a stink could propagate that quickly, but I don't doubt the molecule's potency.

I've had occasion to smell selenium dioxide SeO₂, they say it resembles rotten horseradish. To me it's more like if someone slathered a chicken sandwich with a ridiculous amount of horseradish sauce, gobbled it up, then immediately crapped it out.

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