Do yourself a favor, and get a text written by a PhD organic chemist. I think Klein, Smith and Carey did a great job. Nucleophilicity often parallels basicity, but this is only a very general rule that causes many students to make errors. I- is a very weak base, but a very good nucleophile. Lets talk Sulfur. SH- is less basic than OH-, but is a FAR GREATER nucleophile. Sulfur is large, and the electronegativity is less than Oxygen, hence the electrons are more loosely held. This renders it a greater nucleophile. Also.....another reason that I have NOT seen authors address is the fact that Sulfur is a relatively poor base because the orbital overlap between Sulfur and Hydrogen is not as favorable as between Hydrogen and Oxygen. I have recently seen studies involving CN-. This nucleophile is almost as good as Sulfur !!!! It reacts in BOTH polar aprotic and polar protic solvents, although at FASTER rates in polar aprotic solvents. 20 hours vs. 4 hours. These nucleophiles...SH- and CN- are favored by the SN2 process especially in an aprotic solvent. Notice that these two species are also not very basic. For the DAT exam......be alert for species such as CN-, CH3S-, I-, SH-.....these are strong nucleophiles that favor SN2. ( For homework, you will tell me WHY SN2 is favored by strong nucleophile !!!!) If we employ Oxygen nucleophiles, we also now have very good nucleophiles but also strong bases. The Oxygen atom especially when in the anionic form, has a very favorable interaction with the small 1s orbital of hydrogen, and the E2 operates as a competitor with SN2. Usually on a secondary or tertiary halide, elimination prevails. Sorry for a long explanation, but your question was quite general. To sum it all up......Yes....for your purposes.....SH- trumps HO- in all solvents when functioning as a nucleophile. Data on this is quite easy to find and it is studied extensively in the chemical literature. Thanks for such a delightful question.
Hope this helps
Dr. Romano
