SolutionsNucleophilicity and Solvation
1. Protic and aprotic
Classify these solvents: water, methanol, ethanol, DMF, DMSO, acetone and acetonitrile.
Protic (with H bonded to O or N): water, methanol and ethanol. Aprotic: DMF, DMSO, acetone and acetonitrile.
2. Halides as nucleophiles
Order the nucleophilicity of F−, Cl−, Br− and I− in methanol. Does the order change in DMF?
In methanol: I− > Br− > Cl− > F−, because small anions are strongly solvated by hydrogen bonds. In DMF, without this solvation, the order is reversed and approaches that of basicity: F− > Cl− > Br− > I−.
3. Charge
Why is the methoxide ion, CH3O−, a better nucleophile than methanol?
Between two species with the same reactive atom, the one with a formal negative charge is the more nucleophilic: the oxygen of methoxide has much more available electron density.
4. HS− versus HO−
In protic solvents HS− is a better nucleophile than HO−, although it is less basic. Why?
Sulfur is larger and more polarizable and its charge is less concentrated: it is less solvated and its electron cloud easily distorts towards the carbon being attacked.