Hi Krishna!
I took chem 420, aka orgo 3, during the winter semester 2019, and I really enjoyed it. I took it with Corey Stephenson even though I know in the past there have been different professors, so my experience may be different from what you may experience if you were to take it with a different teacher. I would recommend looking into the previous syllabi to see how the different professors run the class. The general topics of the class are H-NMR and stereoselective reactions. Mainly, these are reactions that were covered in orgo 1 and orgo 2, but you dive into a lot more detail. When I took it, the class was super laid back, and the lectures were recorded. Corey even brought us Washtenaw Dairy donuts a couple times during class and for our final exam. For grading, there were two exams during the semester, a final exam that was cumulative, and problem sets, which had 5-8 problems that were due weekly. However, there were three exams given during the semester, one of which you could drop, and the exams were free response. Additionally, there were discussion sections, but you were not required to go. If you are to take this class, I would highly recommend going because they clear up the more difficult and abstract topics in the class, and the GSI explains them in more detail.  Another note is that Corey had office hours that gave extra practice problems, and the office hour’s main focus was going over these problems; it acted as almost another discussion section. He also had normal office hours where the main focus was to ask questions you had about the material. Overall, chem 420 was a challenging course. There is a lot of material, and they purposely make the exams difficult, but they do end up giving a generous curve at the end of the class. The professor and the GSIs are focused on you learning something new and emphasize throughout the class not to lose sleep over your grade. This class does not have a lot of outside work except the homework problems and studying, but it is a challenging class. I would recommend it if you really enjoyed orgo 1 and especially if you liked orgo 2.
As for chem 419, Corey Stephenson also teaches this class, so I can imagine the environment and workload is similar to chem 420. However, other professors do teach this course as well, so again, I would highly recommend looking at historical syllabi. Based off the course guide, your grade is based on 3 exams, problem sets, and class participation. However, something different from chem 420 is that it does not have a discussion section. Looking at the topics covered, a big difference between the material is that chem 419 covers kinetics whereas chem 420 does not. I am sorry I cannot provide more detail about this class!
All in all, in my opinion, I think both courses will have a similar workload, difficulty, and grading scale especially if Corey teaches both of them. It honestly depends on what material you find more interesting.  If you have any more questions about these classes or anything about the BMS major in general, feel free to reach out again or stop by our advising hours!
Best,
Brooke, Finance Manager