Okay. PhD Chem admission process is very different than dental schools. For dental school admission, in order to be eligible, you only need to fulfill the first two years of prerequisite classes. No bachelor degree is required. As for PhD Chem admission, you need a minimum of a bachelor degree in Chemistry/Biochemistry/Chemical Engineering or strongly-related field. Most of PhD Chem admission committee tends to focus on, 1. your letters of recommendation 2. number of peer-viewed publications 3. research experiences (length, field) 4. industrial, national, international internships or fellowships 5. outreach activities (Chemistry-related only) 6. number of high-end, advanced, graduate-level Chemistry classes you have completed. So long story short, they aren't gonna care if you drop your organic chemistry 2 this semester or not, as long as you take it before you graduate.