Halogen Exchange

The Finkelstein Reaction

Alkyl iodides are most conveniently prepared not from an alcohol but by swapping the halogen on an existing alkyl chloride or alkyl bromide. Treating the chloride/bromide with sodium iodide in dry acetone carries out this exchange:

This halogen-exchange reaction is called the Finkelstein reaction. Sodium iodide is soluble in dry acetone, but the sodium chloride or sodium bromide produced is not — it precipitates straight out of solution as the reaction proceeds. By Le Chatelier's principle, continuously removing a product this way pulls the equilibrium forward, driving the exchange to completion.

The Swarts Reaction

Alkyl fluorides are best obtained the same way — by halogen exchange rather than direct fluorination — because fluorine itself is too reactive to handle safely in a direct substitution. An alkyl chloride or bromide is heated with a metallic fluoride such as $\text{AgF}$, $\text{Hg}_2\text{F}_2$, $\text{CoF}_2$, or $\text{SbF}_3$:

This reaction is called the Swarts reaction. …