This word might seem to have a nautical air to it, with its resonances of mermaid, capstan, and captain, but the nose of its object tends more towards the mephitic – or Stygian. Indeed, it names a whole family of chemicals that have in common an SH (sulphur-hydrogen) group. The ugliest sister of the family is surely ethyl mercaptan, which sounds like a name for a singer but is actually C2H5SH, which is coming up not roses but rotten eggs. It has been listed in the Guinness Book of World Records as the foulest-smelling substance in the world. Other mercaptans are generally also noted for their pungency, but they are not necessarily all as noisome.
This is not a word one hears often, but I did hear it once on CBC Radio 2: Jurgen Gothe (a man with a fine palate for words – and wines) spoke of the “faint whiff of mercaptans” one gets when opening a new CD. He could have said “faint whiff of thiols” and meant the same thing, but the two words do taste quite different, don’t they? Thiol comes from the Greek theion, “sulphur,” while mercaptan is a portmanteau word distilled from corpus mercurium captans, “body that seizes mercury,” so named because the SH group binds tightly to the element mercury.
But where mercaptan has the sea sound (“Oh captain, mercaptan!”) and the ripply shape and those crisp stops bookended by the nasals and padded with short vowels, thiol has letters that stand up and the sound gives just a lisp and a liquid and swivels on a long central tripartite vowel movement: a floppy, arch word that sounds of sigh and thigh and vile, and less of sea and more of, say, seat of pants. Which may be more fitting given its referent. But would you really want to say ethane thiol instead of ethyl mercaptan?