Finkelstein Reaction

The Finkelstein reaction is another one of the important name reactions of organic chemistry. It is used in the preparation of haloalkanes or alkyl halides. It is a substitution nucleophilic bimolecular (SN2 reaction) halogen exchange reaction. It is named after German Chemist Hans Finkelstein. It is an equilibrium reaction if carried out without solvent. Reaction completion depends on differential solubility of halide salts in the polar aprotic solvent such as acetone. 

What is Finkelstein Reaction? 

Alkyl halide (alkyl chloride or alkyl bromide) reacts with metal iodide in presence of polar aprotic solvent such as acetone to give alkyl iodide and metal halide salt as precipitate. This reaction is called Finkelstein Reaction. Finkelstein reaction can be represented as follows –

Where X = Cl, Br

M = Metal such as Na etc. 

R= Alkyl group

If we take alkyl iodide in place of alkyl chloride or alkyl bromide and metal chloride or metal bromide in place of metal iodide then reaction will not take place, due to following reason –

Acetone is a polar aprotic solvent which is a covalent compound so it can dissolve covalent compounds in it. Although NaCl, NaBr and NaI (or all metal halides) all are ionic compounds. Now according to Fajan’s rule, small cation and large anion leads to higher covalent character. So, in NaCl, NaBr and NaI cation Na+ is the same but anions Br-, Cl- and I- vary in size. Anion I- (iodide) is largest in size, due to this NaI shows greater covalent character and gets dissolved in acetone while remaining two metal halides NaCl and NaBr remain insoluble in acetone. This is the reason it is an exclusive method to form alkyl iodide. Thus, we can write –

Examples of Finkelstein Reaction 

We are listing here few examples of Finkelstein Reaction-

  • Reaction of ethyl chloride with sodium iodide-

  • Reaction of ethyl bromide with sodium iodide –

Reaction of methyl bromide with sodium iodide –

Finkelstein Reaction Mechanism 

As discussed, it is an equilibrium process and forward reaction is supported by taking advantage of the poor solubility in acetone of the newly formed metal halide salt. The mechanism for this reaction is easy and simple. It is a single step bimolecular nucleophilic substitution reaction or SN2 reaction. So, its mechanism is the same as SN2 reaction mechanism which occurs with inversion of stereochemistry. Reaction mechanism is given below –

Where X = Cl, Br and Y = I. 

Aromatic Finkelstein Reaction 

Aromatic chlorides and bromides are not easily substituted by iodide but can be substituted when appropriately catalyzed. Aromatic Finkelstein reaction is catalyzed by CuI [copper(I)iodide] in combination with diamine ligands. Apart from CuI, nickel bromide and tri-n-butylphosphine are also suitable catalysts for aromatic Finkelstein reaction. One example of aromatic Finkelstein reaction is given below –

Uses of Finkelstein Reaction 

Few uses of Finkelstein reaction are listed below –

  • It is used for analysis as alkyl halides greatly differ in the ease with which they undergo the Finkelstein reaction.

  • It is used for easy production of alkyl iodide. 

  • It can be used (NaI in acetone) as a qualitative test to determine from which class an unknown alkyl halide belongs to. 

  • Synthesis of chrysochlamic acid is also done by Finkelstein reaction. 

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