Finkelstein reaction

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Finkelstein reaction
Named after Hans Finkelstein
Reaction type Substitution reaction
Identifiers
Organic Chemistry Portal finkelstein-reaction
RSC ontology ID RXNO:0000155

The Finkelstein reaction, named after the German chemist Hans Finkelstein,[1] is an SN2 reaction (Substitution Nucleophilic Bimolecular reaction) that involves the exchange of one halogen atom for another. It is an equilibrium reaction, but the reaction can be driven to completion by exploiting the differential solubility of halide salts, or by using a large excess of the halide salt.[2]

R–X   +   X′−   ⇌   R–X′   +   X−

Method

The classic Finkelstein reaction entails the conversion of an

alkyl iodide by treatment with a solution of sodium iodide in acetone. Sodium iodide is soluble in acetone while sodium chloride and sodium bromide are not.[3] The reaction is driven toward products by mass action due to the precipitation of the poorly soluble NaCl or NaBr. An example involves the conversion of the ethyl ester of 5-bromovaleric acid to the iodide:[4]

EtO2C(CH2)4Br + NaI → EtO2C(CH2)4I + NaBr

dimethyl formamide, ethylene glycol, and dimethyl sulfoxide.[6]

Use for analysis

Alkyl halides differ greatly in the ease with which they undergo the Finkelstein reaction. The reaction works well for primary (except for

aryl and tertiary alkyl halides are unreactive; as a result, the reaction of NaI in acetone can be used as a qualitative test to determine which of the aforementioned classes an unknown alkyl halide belongs to, with the exception of alkyl iodides, as they yield the same product upon substitution. Below some relative rates of reaction (NaI in acetone at 60 °C):[7][8]

Me–Cl
Bu–Cl
i-Pr–Cl
 t-BuCH2–Cl CH2=CH–CH2–Cl PhCH2–Cl EtOC(O)CH2–Cl MeC(O)CH2–Cl
179 1 0.0146 0.00003 64 179 1600 33000

Aromatic Finkelstein reaction

The aromatic chlorides and bromides are not easily substituted by iodide, though they may occur when appropriately catalyzed. The so-called "aromatic Finkelstein reaction" is catalyzed by

tri-n-butylphosphine have been found to be suitable catalysts as well.[10]

See also

  • Halex process, also a salt metathesis, but for conversion of aryl chlorides to aryl fluorides

References

  1. doi:10.1002/cber.19100430257
    .
  2. ISBN 978-0-471-72091-1
  3. PMID 23907310
    .
  4. doi:10.15227/orgsyn.076.0252
    .
  5. doi:10.15227/orgsyn.036.0040
    .
  6. doi:10.1002/047084289X.rp214
  7. doi:10.1021/cr50010a001
    .
  8. doi:10.1021/ja01075a025
    .
  9. S2CID 11338218
    .
  10. PMID 22422214
    .
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