Finkelstein Reaction

Introduction

Named reactions are organic reactions that are named after the discovery made by the corresponding scientist. Many named reactions are economically and commercially very useful. Finkelstein reaction is one such named reaction present in organic chemistry. Finkelstein's reaction is introduced by the German chemist Hans Finkelstein. It is an important organic reaction and is named after the scientist's name. It is also an equilibrium reaction that follows substitution nucleophilic bi molecular reaction. It is an organic exchange reaction in which one halogen atom present in a particular compound has been exchanged with another halogen atom from a different compound.

What is the Finkelstein Reaction?

The name halogen exchange or Halex reaction is provided two to Finkelstein reaction since it involves the halogen exchange process. 𝑆𝑁2 mechanism is followed for the replacement of halogen atoms. The reaction involves the formation of alkyl iodide by the reaction of alkyl halides with metal iodide in the presence of a polar aprotic solvent. The reaction only proceeds when primary halides are present. The reaction that is taking place is,

The halogens used for this reaction may be Chlorine or Bromine. The solvent used for the reaction is acetic acid which is a polar aprotic solvent. It is a covalent compound so it will dissolve covalent molecules. The driving force for the reaction to proceed is the formation of a slightly soluble precipitate or salt of corresponding a metal. Among the halides, primary halides are more reactive than secondary halides.

Examples of Finkelstein Reaction

Finkelstein's reaction helps in the production or synthesis of many alkyl halides. So, there are many examples of Finklestein reaction. Some of the examples of this reaction are,

• The reaction of sodium iodide with methyl bromide results in the formation of methyl iodide is an example of Finkelstein's reaction. And sodium bromide is obtained as a by-product. The reaction is,

$$\mathrm{Nal\:+\:CH_{3}Br\:\rightarrow\:NaBr}$$

• The reaction of sodium iodide with ethyl chloride results in the formation of ethyl iodide is also an example of Finkelstein's reaction. And sodium chloride is obtained as an acid by-product. The reaction is,

$$\mathrm{CH_{3}CH_{2}Cl\:+\:Nal\:\rightarrow\:CH_{3}CH_{2}l\:+\:NaCl}$$

• The reaction of Sodium iodide with ethyl bromide results in the formation of ethyl iodide is also an example of Finkelstein's reaction. And sodium bromide is obtained acid by-product. The reaction is,

$$\mathrm{CH_{3}CH_{2}Br\:+\:Nal\:\rightarrow\:CH_{3}CH_{2}l\:+\:NaBr}$$

• 5-bromovaleric acid is converted to iodide using this reaction. The reaction that happens here is,

$$\mathrm{Nal\:+\:EtO_{2}C(CH_{2})_{4}Br\:\rightarrow\:EtO_{2}C(CH_{2})_{4}4l\:+\:NaBr}$$

Finkelstein Reaction Mechanism

The reactants used for the proceeding of this reaction are, Sodium iodide, 𝑁𝑎𝐼, Alkyl bromides or Alkyl chlorides, and Acetone, $\mathrm{C_{3}H_{60}}$. And it follows an $\mathrm{SN_{2}}$ mechanism, so the reaction mechanism of this reaction is easy and simple to understand. So, there is an inversion of stereochemistry is present. The poor solubility of metal halides is responsible for the forward reaction. It is a single-step reaction in which the attack of the nucleophile present in the sodium iodide takes place on the alkyl halides. And which further results in the formation of sodium halide and alkyl iodide. During the reaction, the sodium bromide or sodium chloride is formed as a precipitate and is not soluble in acetone. The mechanism which takes place in this reaction is,

$береста$, Finkelstein reaction mechanism V1, CC0 1.0

Aromatic Finkelstein Reaction

Normally the substitution of iodide in aromatic compounds does not happen so easily. But the process can be made efficient using a catalyst in addition to the Finkelstein reactants. The catalyst used for the proper functioning of aromatic Finkelstein reaction is 𝑐𝑜𝑝𝑝𝑒𝑟(𝐼) 𝑖𝑜𝑑𝑖𝑑𝑒 combined with diamine ligands. And some other catalysts such as 𝑡𝑟𝑖 − 𝑛 − 𝑏𝑢𝑡𝑦𝑙 𝑝ℎ𝑜𝑠𝑝ℎ𝑖𝑛𝑒 and Nickel bromide can also be employed. The below reaction shows the formation of Benzyl iodide from the benzyl chloride by the reaction of sodium iodide in presence of acetone.

MegaByte07, Synthesis Benzyl iodide, CC BY-SA 3.0

The first step of aromatic Finkelstein reaction with the help of copper catalyst involves the oxidative addition reaction. Then halide exchanges take place and in the last step of this reaction reductive elimination is taking place and which will further result in the regeneration of a corresponding catalyst. The use of 𝐶𝑜𝑝𝑝𝑒𝑟 (𝐼) 𝐼𝑜𝑑𝑖𝑑𝑒 with diamine ligand is an ideal catalyst in aromatic Finkelstein reaction. Copper-catalysed reaction happening in aromatic halides is shown below.

Uses of Finkelstein Reaction

The named reaction is of great use. Some uses of Finklestein reactions are,

• As it deals with alkyl halides it can be used for the analysis of alkyl halides. Since by proceeding with the reaction the amount of alkyl halide will decrease.

• For the industrial use of alkyl iodides, this reaction is used extensively to produce alkyl iodides.

• It is also used for the detection of a class of alkyl iodides.

• Chrysochlamic acid is synthesized by using this reaction.

• It is used for the preparation of 𝛼 − 𝑖𝑜𝑑𝑜𝑎𝑙𝑘𝑦𝑙 esters.

Conclusion

Finkelstein reaction is an organic named reaction that involves the production of alkyl iodide in a better way. The production of alkyl iodides is a complex mechanism, but it is made easy with the help of this reaction. It involves the use of alkyl halides and metal iodide such as sodium iodide. An organic polar aprotic solvent, acetone is used in the reaction. It is a single step by the molecular reaction which follows the 𝑆𝑁2 mechanism. The first step in the reaction is the nucleophilic attack. And then the corresponding replacement of halogens takes place and alkyl iodides will be formed. The sodium salt is also precipitated in the reaction. The reaction also has some other applications, especially in the analysis of alkyl halides.

FAQs

1. Why only sodium iodide is used, not sodium chloride sodium bromide is used in Finkelstein's reaction?

This is because of the Fajans rule in which the larger the anion and smaller the cation greater will be the covalent character of a corresponding compound. So, sodium iodide is a more covalent character than other sodium halides and it made the use of this instead of other halides.

2. Why is acetone used in Finkelstein's reaction?

Acetone is an important organic solvent that is widely used in several organic syntheses. In Finkelstein's reaction acetone is used as a solvent because of the reason that will help in the forward reaction to proceed in this reaction. And is based on Le chatelier's principle.

3. What are the conditions under which Finkelstein Reaction takes place?

Finkelstein's reaction depends on several factors for the proper functioning of the process. Some of the necessary conditions that must be followed in the Finklestein reactions are,

• The nucleophilicity of the Sodium Iodide since the first attack is taking place from this reactant.

• The bond between carbon and halogen must be weak enough to undergo the reaction.

• The nature of the group is attached to the alkyl halides.

• If the reactivity of alkyl halide increases the rate will also increase.

4. What are the advantages of Finkelstein's reaction?

There are some advantages of this organic reaction are the process of bromination has made easy with the help of this reaction, the reaction involves the formation of some precipitates and so it is used as a driving force for the reaction to complete, and the catalyst used in the aromatic reaction is easily regenerated after the completion of the reaction.

5. What is Frankland's reaction?

It is also a named reaction that involves the formation of dialkyl zinc with the use of the same reactant, alkyl iodide formed in the Finkelstein reaction.