You already know that alkyl halides can be made by replacing the –OH group in alcohols. But when you specifically want an alkyl fluoride (R–F), the usual methods like using HF or KF often fail — HF is dangerously corrosive, and KF doesn't dissolve well in organic solvents. So how do chemists put a single fluorine atom onto a carbon chain?
The answer is the Swarts reaction.
The Intuition
Think of a halogen exchange. You start with an alkyl chloride or alkyl bromide (R–Cl or R–Br). You treat it with a metallic fluoride — typically silver fluoride (AgF) or antimony trifluoride (SbF₃). The metal atom (Ag or Sb) has a strong affinity for the heavier halogen (Cl or Br), pulling it away. In exchange, the fluoride ion (F⁻) from the metal fluoride bonds to the carbon, giving you R–F.
The driving force is the formation of a stable metal–halogen bond (Ag–Cl or Sb–Cl/Br), which is energetically favourable. The reaction is essentially a halide exchange — swapping a heavier halogen for fluorine.
The Precise Statement
Swarts reaction: Alkyl fluorides are prepared by heating an alkyl chloride or alkyl bromide with a metallic fluoride such as AgF, Hg₂F₂, or SbF₃.
The general equation:
R–X+MFΔR–F+MX
where X = Cl or Br, and MF = AgF, SbF₃, etc.
A Concrete Example
CH3CH2Br+AgFheatCH3CH2F+AgBr
Ethyl bromide reacts with silver fluoride to give ethyl fluoride and a precipitate of silver bromide. The AgBr is insoluble and falls out of the reaction mixture, pulling the equilibrium forward.
Replacing the halogen of an alkyl halide by fluorine using a metal fluoride is the Swarts reaction.
Concept. Alkyl fluorides are best made by heating an alkyl chloride or bromide with a heavy-metal fluoride such as AgF, Hg2F2, CoF2 or SbF3. This halogen-exchange reaction is named after its disco …
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2026Set ANNUAL1 markMCQ
Q.The best method for the synthesis of alkyl fluoride is
(a) Finkelstein reaction
(b) Swarts reaction
(c) Kolbe reaction
(d) Wurtz reaction
›Reveal solutionSolution
Alkyl fluorides are best prepared by the Swarts reaction, in which an alkyl chloride or bromide is heated with an inorganic metallic fluoride.
In the Swarts reaction, an alkyl chloride/bromide reacts with a metal fluoride salt such as AgF, Hg2F2, CoF2, or SbF3, and the halogen is replaced by fluorine:
R-Cl + AgF -> R-F + AgCl
This works well because fluorides are generally not obtained by direct halogenation with F2 (too violent/uncontrollable) or by simple nucleophilic substitution with common fluoride salts, making this metal-fluoride exchange the standard, reliable method.
Q.Methyl bromide reacts with AgF to give methyl fluoride and AgBr. This reaction is called
(a) Finkelstein reaction
(b) Fittig reaction
(c) Swarts reaction
(d) Wurtz reaction
›Reveal solutionSolution
Treating an alkyl bromide/chloride with an inorganic fluoride such as AgF replaces the heavier halogen with fluorine — a halogen-exchange reaction named the Swarts reaction, the standard method for preparing alkyl fluorides.
CH3Br+AgF→CH3F+AgBr
Alkyl fluorides are difficult to prepare by direct fluorination (too violent/uncontrollable), so instead an alkyl chloride or bromide is heated with a metallic fluoride such as AgF, Hg2F2, CoF2, or SbF3; the metal's strong affinity for the heavier halide ion drives the halogen exchange, depositing the corresponding insoluble metal halide (here AgBr) and releasing the alkyl fluoride. This substitution reaction is named the Swarts reaction.
Q.Name the following reaction. H₃C – Br + AgF → H₃C – F + AgBr.
›Reveal solutionSolution
Replacing the halogen of an alkyl halide by fluorine using a metal fluoride is the Swarts reaction.
Concept. Alkyl fluorides are best made by heating an alkyl chloride or bromide with a heavy-metal fluoride such as AgF, Hg2F2, CoF2 or SbF3. This halogen-exchange reaction is named after its disco …