What chemical compound is shown in episode 17 of Monogatari Second Season? In episode 17 of Monogatari Second Season (the first episode of Onimonogatari), when Koyomi and Mayoi see the strange "darkness", an image of the structure of a chemical compound is displayed, repeatedly:



Shortly afterwards, a number of different chemical compounds are displayed (numbers not present in original):



What are these compounds?
The first image is the structural formula of epinephrine (or adrenaline, depending on where you live). 



Epinephrine is a neurotransmitter that plays a significant role in the fight-or-flight response, and so its appearance here makes sense - Koyomi is certainly about to take flight upon seeing the "darkness".



The second image contains the following compounds:


dopamine
serotonin
α-Neoendorphin
epinephrine (again)
(Leu-)enkephalin
norepinephrine (or noradrenaline)


All of these are neurotransmitters, as well, though none of them are as localized to the fight-or-flight response as epinephrine is.