Question
How would you prepare fluoroethane from chloroethane? Write the reaction, name the reagent used, and explain why direct fluorination of alkanes is not a practical method.
(NCERT Class 12, Haloalkanes and Haloarenes)
Solution — Step by Step
This is the Swarts reaction: a halogen exchange where chlorine is replaced by fluorine using a metallic fluoride (AgF, Hg₂F₂, or SbF₃).
The most common reagents are:
- AgF (silver fluoride) — used in labs
- SbF₃ (antimony trifluoride) — used industrially, also called Swarts reagent
- Hg₂F₂ (mercurous fluoride) — alternative
The reaction occurs under mild conditions — often just heating the chloroalkane with the metallic fluoride.
Direct fluorination () is extremely exothermic and violent. The C-F bond energy (~485 kJ/mol) is so high that the reaction releases enormous energy, leading to:
- Uncontrolled chain reactions
- Breaking of C-C bonds
- Mixture of products including carbon and HF
The reaction essentially blows the molecule apart. That’s why we use the indirect Swarts route: make the chloro compound first (via controlled chlorination), then swap Cl for F.
Why This Works
The Swarts reaction is a nucleophilic substitution where F⁻ (from AgF) replaces Cl⁻. The driving force is the precipitation of AgCl — removing a product from the equilibrium shifts the reaction forward (Le Chatelier’s principle).
Fluoride ion is actually a poor nucleophile in protic solvents (because it’s small and heavily solvated). But in the Swarts reaction, AgF provides F⁻ in a controlled manner, and the formation of insoluble AgCl drives the reaction to completion.
The order of reactivity of haloalkanes towards Swarts reaction follows: R-I > R-Br > R-Cl, matching the weakness of the C-X bond being broken.
Alternative Method
Another route to fluoroalkanes is the Balz-Schiemann reaction for aromatic fluorides:
This is specific to aryl fluorides. For alkyl fluorides, Swarts is the standard method.
Don’t confuse Swarts reaction (Cl → F using AgF) with Finkelstein reaction (Cl/Br → I using NaI in acetone). Both are halogen exchange reactions, but they go in opposite directions. Finkelstein works because NaCl/NaBr precipitate in acetone, while NaI is soluble. Same principle — remove product to drive equilibrium.
Common Mistake
Students sometimes write NaF as the reagent for the Swarts reaction. While NaF is a fluoride source, it’s not effective because F⁻ from NaF is too strongly solvated in common solvents and is a poor nucleophile. The reaction requires metallic fluorides (AgF, SbF₃, Hg₂F₂) that provide F⁻ more effectively. Always write AgF or SbF₃ — not NaF or KF — for the Swarts reaction.