The entry caffeine says that the German Kaffein was coined by Runge. However, he critically mentioned the word Coffein with an o instead of an ain his publication from 1820. He ultimately called the substance Kaffebase in this work.
The French caféine is already found in 1818. The Dictionnaire de médecine (1822) says: “[…] Chenevix en a également retiré une substance végétale particulière, que ce chimiste considère comme un principe immédiat nouveau, auquel il donne le nom de cafeine. L’existence de cette substance n’est point admise par tous les chimistes.”
Be that as it may, it seems like the form Kaffein (or Caffein) was not Runge’s invention, though it was apparently more common to say it with a than with o for a time; nowadays, the reverse is the case (compare with the genus name Coffea; though caffeum and cafea can also be found as New Latin terms for coffee).
The entry for caffeine suggests that the word originated in the German but may have entered English via the French. Are you proposing that it originated in the French instead? As a word from chemistry, this should be relatively knowable compared to many other word origins.
Thanks. So, I went looking into this and noticed your 1818 French is a translation of an English text, with the result of antedating caffeine into English to 1817. But this is two years before Runge is supposed to have discovered the stuff.
It might be a case like alcohol where the word originally referred to something other than what we now mean, but in any case the German is apparently out. I’ve revised the caffeine entry to match what we’ve found, and Doug or myself will (hopefully) get back into it to untangle the details at a future date.
From old texts, I get the understanding that caffeine was already discovered by Chenevix (not sure whether this is refuted by now) but obtained in a more pure form by Runge.