Undergrad,
Many students consider organic chemistry to be massively difficult, and rightfully so. It doesn't have to be a tough class, but it certainly can be made so. Furthermore, many chem departments consider it a point of pride that their organic chemistry courses are "real" orgo courses, i.e. legitimate chemistry-major-caliber courses and not just enough for premeds to get by on.
All this, along with the fact that students are reaching the point of no return in their majors, combines to a lot of people dropping premed as too difficult midway through organic chemistry. Once the C's and D's start rolling in, those who breezed through freshman year start to reevaluate their choice of profession.
Regarding the junior college thing. There are two reasons people take organic chem at a junior college, one good, one not.
The better of the two reasons to take organic chem at a junior college is simply to have an easier time at it. Junior colleges tend not to be able to front the level of professors that are tenured at four-year colleges, and since the professors aren't as specialized, the courses are easier. This is a mixed blessing, though, as nearly all med schools, allopathic and osteopathic, prefer that prereqs be taken at four-year colleges. Not required generally, but strongly preferred.
The worse of the two reasons is also common. Many students will take a prereq course like organic chem at a junior college and then take it again at their regular university. This way they know the stuff going into the class and set themselves up for a better grade. Those who do this tend not to report the first course on their application; the med schools only see the good second grade on the application and assume that the student took the course once and did well at it.
This is unethical for all the obvious reasons, and specifically violates the agreements made when applications are sent in. Applicants have to report ALL their college coursework, bar none. Omitting courses like this is not only unethical, but very, very dangerous. If a school finds out that the student did this (which is easier and easier to do now that so many JCs have linked course records and the med schools are savvy to the cheat), the school will almost certainly reject the applicant, and it will be awfully tough to get into any US schools thereafter without a LOT of explaining.
Hope this clears it up! I would recommend taking organic chem and any other prereqs you can at a four-year university if at all possible. It can't hurt to study ahead. Kaplan offers an orgo edge program which gives a good basic preview into organic chem. Since no grade is offered, it's not a reportable class and is totally legitimate. You can also buy an orgo review book the summer before you take the class, get a basic feel for what's going on, and then have a head start when you take the class.
I dropped organic chem my first time around and took it again during the summer (I did the same thing for physics.) Weed out courses only weed out the truly unable and those who aren't fully convinced of their calling. If you have the ability and are convinced of your calling, the worst a course can do is delay you. Persistence is, as in most things, key.
