The only time I would say withdraw is when you think you are going to fail. I know a lot of people who have done what you are doing, and ended up with a C+ the next time around (not exactly an improvement). If I were you I would just take the C and move on. Get an A in Ochem 2 or something. It's up to you though. If you know for sure you can bust your ass and get an A the next time around than consider it... Otherwise move on.