Finkelstein reaction: practice and mechanism
Iodide replaces Cl or Br via SN2; because NaCl/NaBr precipitates in acetone, the reaction runs completely to the iodoalkane.
Example
Iodide replaces chlorine or bromine via SN2. The equilibrium lies on the product side because NaCl or NaBr is insoluble in acetone and precipitates.
Mechanism step by step
- 1
Iodide attacks the primary chloro- or bromoalkane from the back (SN2).
- 2
Chloride or bromide leaves and forms a salt with Na⁺.
- 3
NaCl/NaBr is insoluble in acetone and precipitates, driving the equilibrium to the iodoalkane (Le Chatelier).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
Why the reaction goes to completion
Sodium iodide dissolves well in acetone, sodium chloride and bromide hardly at all. The NaCl or NaBr formed precipitates and is removed from the equilibrium. Solubility (Le Chatelier) thus drives a reaction that would otherwise stop at equilibrium.
Which substrates work
It is an SN2 reaction: primary alkyl halides react well, allyl, benzyl and α-carbonyl halides especially so. Secondary ones are slow, tertiary, vinyl and aryl halides practically do not react. Why bother? Iodoalkanes are the best substrates for further substitutions.
Related halogen exchanges
Fluoroalkanes are made in a similar way with potassium fluoride in polar aprotic solvents (Swarts reaction with metal fluorides). For aryl halides there is a copper-catalysed “aromatic Finkelstein reaction”.
Typical mistakes
Often confused with: SN2 with hydroxide · SN2 with cyanide (chain extension)
Frequently asked questions
Why would you want an iodoalkane at all?
Iodide is the best leaving group of the halogens; iodoalkanes react much faster in follow-up reactions.
Is I⁻ a good nucleophile?
Yes; in aprotic solvents such as acetone it is large, polarisable and nucleophilic.
Why does the Finkelstein reaction go to completion?
NaI dissolves in acetone but NaCl and NaBr do not; they precipitate and are removed from the equilibrium (Le Chatelier).