I hardly believe you have carte blanche to choose any topic you like, without guidance. When I was an undergraduate (long, long ago) the department circulated a list of the lecturers and their areas of research, and you went and talked to those you were interested in, before making your choice. If your department doesn't do the same, go to their website (something we couldn't do) and check out the research interests of your lecturers, and talk to those whose interests most match yours. If possible, also talk to people who work/have worked with them, find out a bit about the working environment (does he use undergraduates as bottle-washers, etc....).
It's also worth thinking about where your interests lie within the area of organic/medicinal chemistry - e.g. are you more interested in organic synthesis per se, or in mechanistic studies (you'll probably have to do some of both). Good luck with your search.