Finkelstein Reaction — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Finkelstein Reaction The Finkelstein reaction is the exchange of one halogen atom for another in an alkyl halide by treatment with a sodium halide (typically NaI) in a solvent where the sodium halide byproduct (NaCl or NaBr) is insoluble. Dry acetone is the preferred solvent because NaCl and NaBr are insoluble in it, driving the equilibrium forward. A white precipitate of NaCl (or NaBr) forms in the acetone solution as the reaction proceeds. The reaction is faster with primary halides (SN2) and slower with tertiary halides (steric hindrance). The reaction is an equilibrium process that is driven to completion by the precipitation of NaCl or NaBr from acetone. Without acetone as solvent, the reaction would reach an unfavorable equilibrium. The choice of solvent is the thermodynamic key. Iodide ion (I⁻) from NaI acts as a good nucleophile (large, polarizable) I⁻ attacks the electrophilic carbon from the back side (opposite to the leaving group) In a concerted single step, the C-I bond forms while the C-Cl/C-Br bond breaks NaCl or NaBr precipitates out of acetone solution (insoluble), driving the equilibrium forward (Le Chatelier's principle) Confusing with Swarts reaction — Finkelstein converts R-Cl/Br to R-I using NaI/acetone. Swarts converts R-Cl/Br to R-F using AgF. Forgetting the role of acetone — it's not just a solvent, it drives the equilibrium by precipitating NaCl/NaBr. Thinking the reaction works equally well for all halides — primary halides react fastest (SN2). Tertiary halides prefer elimination. Not realizing that alkyl fluorides cannot be made by Finkelstein — NaF is insoluble in acetone.