Consider the following specific laboratory solvents commonly used in organic synthesis: 1. Pure Wate — Haloalkanes and Haloarenes Chemistry Question
Question
Consider the following specific laboratory solvents commonly used in organic synthesis: 1. Pure Water 2. Ethanol 3. Dimethyl sulfoxide (DMSO) 4. N,N-Dimethylformamide (DMF) 5. Glacial acetic acid 6. Acetone Exactly how many of these solvents strongly and selectively favour the $S_N2$ mechanism over the $S_N1$ mechanism by actively leaving the anionic nucleophile relatively unsolvated and highly reactive?
💡 Solution & Explanation
The $S_N2$ mechanism is highly favoured by polar aprotic solvents. These solvents effectively dissolve ionic reagents by solvating cations (using their strong negative dipoles), but they lack acidic protons, so they cannot form hydrogen bonds to solvate anions. This leaves the anionic nucleophile "naked" and extremely reactive. Among the options, water, ethanol, and acetic acid are polar protic (they form hydrogen bonds). DMSO, DMF, and acetone are polar aprotic. Thus, exactly 3 solvents strongly favour $S_N2$.