The Finkelstein reaction named after the German chemist
Hans Finkelstein, is an
SN2 reaction (Substitution Nucleophilic Bimolecular reaction) that involves the exchange of one
halogen
The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...
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.
:R–X + X′
− R–X′ + X
−
Method
The classic Finkelstein reaction entails the conversion of an
alkyl chloride
An organochloride, organochlorine compound, chlorocarbon, or chlorinated hydrocarbon is an organic compound containing at least one covalently bonded atom of chlorine. The chloroalkane class (alkanes with one or more hydrogens substituted by chlor ...
or an
alkyl bromide to an
alkyl iodide by treatment with a solution of
sodium iodide in
acetone. Sodium iodide is soluble in acetone while
sodium chloride
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...
and
sodium bromide are not. 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-bromo
valeric acid to the iodide:
:EtO
2C(CH
2)
4Br + NaI → EtO
2C(CH
2)
4I + NaBr
Potassium fluoride is used for the conversion of
chlorocarbons into
fluorocarbon
Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often has distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Fluorocarbons and their derivatives are commerci ...
s. Such reactions usually employ polar solvents such as
dimethyl formamide,
ethylene glycol, and
dimethyl sulfoxide
Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds a ...
.
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
neopentyl) halides, and exceptionally well for
allyl
In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, ...
,
benzyl, and α-carbonyl halides. Secondary halides are far less reactive.
Vinyl,
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):
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
copper(I) iodide in combination with diamine ligands.
Nickel bromide and
tri-''n''-butylphosphine have been found to be suitable catalysts as well.
See also
*
Halex process, also a salt metathesis, but for conversion of aryl chlorides to aryl fluorides
References
{{Reflist
Halogenation reactions
Substitution reactions
Name reactions