Basically, when do you think there was a good opportunity for the earlier development of chemistry as a science? In Europe, at least, chemistry was pretty hindered by the adherence to the Aristotelian physical worldview, whose greatest hits included geocentrism, bad gravity, and four elements. At the same time, however, the advancement of chemistry was pushed along by relatively few individuals; Boyle, Dalton, and especially Lavoisier were some of the few people who truly developed chemical theory, so its not hard to see earlier versions of them following/leading their OTL counterparts' footsteps.
Personally, I would choose a POD in the Black Death, because it gives us plenty of time before the present day, and it's really easy to picture a young boy surviving, taking up an interest in medicine, and ending up making a form of proto-chemistry that is built upon by his students.
Personally, I would choose a POD in the Black Death, because it gives us plenty of time before the present day, and it's really easy to picture a young boy surviving, taking up an interest in medicine, and ending up making a form of proto-chemistry that is built upon by his students.