Finkelstein Reaction: Definition, Advantages and Equation

Finkelstein reaction is used in the production of alkyl halides or haloalkanes. The reaction is considered an essential topic of organic chemistry Class 12. It can be used in the place of the nucleophilic bimolecular halogen exchange reaction. Finkelstein reaction is also known as halogen exchange and halex reaction. In this article, we will discuss the Finkelstein reaction, mechanisms, and conditions relevant to it. 

Key Terms: Organic chemistry, alkali metal, Finkelstein reaction, solvent, halogen, halex reaction, alkyl halides, haloalkanes

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What is the Finkelstein Reaction?

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Alkyl halide (alkyl bromide or alkyl chloride) when combined with metal iodide reacts in the presence of a polar aprotic solvent, i.e., acetone which results in precipitation in the form of metal halide salt and alkyl iodide. This reaction is known as the Finkelstein reaction. Replacement of halogen through sn2 Reaction happens when primary alkyl halide is treated with an alkali metal halide. 

The chemical equation of the Finkelstein reaction is :

Finkelstein Reaction Equation

where, X= Cl, Br

The reaction between bromide, Nal, alkyl chloride, and dry acetone results in alkyl iodides. The chemical equation for this reaction is:

CH₃CH₂-Br + NaI → CH₃CH₂-I + NaBr

Ethyl Bromide + Sodium Iodide (in presence of acetone) → Ethyl Iodide + Sodium Bromide

Acetone used as a catalyst in Finkelstein Reaction

Alkyl iodide can be formed from acids if red mercuric and iodine are used. Esters and alkyl halides can be formed from the aliphatic carboxylic acids. The chemical equation of such a reaction is :

Through halogen exchange, two types of reactions take place: Finkelstein Reaction and Swarts Reaction. Swarts Reaction is the best way to produce alkyl fluorides by heating bromide or alkyl chloride in the presence of AgF, Hg₂F₂, CoF₂, or SbF₃ which are also known as metallic fluoride.

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Conditions for Finkelstein Reaction

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The Finkelstein reaction will not take place if we use metal bromide or metal chloride as a substitute for metal iodide and alkyl iodide is used in the place of alkyl bromide or alkyl chloride because of certain reasons:

• Acetone can dissolve covalent compounds in it as a polar aprotic solvent which can be considered as a covalent compound. 
• NaCl, Nal, NaBr along with all metal halides are ionic compounds. A higher covalent compound can be formed when there is an involvement of a large anion and a small cation.
• The cation Na⁺ is unchanged in NaCl and Nal but anion Cl^-, Br^- and l^- in NaCl, Nal, and NaBr changes its size. 
• Nal displays the highest covalent character as anion I^- (iodide) is larger. Nal gets easily dissolved in acetone whereas the other two metal halides which are NaBr and NaCl remain insoluble in acetone. 

Thus, the Finkelstien reaction depends on several conditions such as :

• Carbon halogen bond
• Nature of group
• Alkyl halide reactivity
• Nucleophilicity

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How does a Finkelstein reaction work?

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Finkelstein Reaction is a simple and easy process that maintains equilibrium. The insolubility of the newly formed metal halide salt in acetone supports the forward reaction. The reaction is known as a single step bimolecular nucleophilic substitution reaction which is also known by the name of an S_N2 reaction. The S_N2 reaction happens with the alteration of stereochemistry. 

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Advantages of Finkelstein Reaction

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The advantages of the finkelstein reaction are:

• Suitable production of alkyl iodide.
• The Nal in acetone can be used as a qualitative test to detect the class of an anonymous alkyl halide. 
• Alkyl halides excessively vary in the capacity with which they undergo the Finkelstein reaction which is why it is used for analysis. 

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Examples of Finkelstein Reaction

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• The reaction between sodium iodide and methyl bromide

CH₃Br + NaI → CH₃I + NaBr

• The reaction between sodium iodide and ethyl bromide 

CH₃CH₂Br + NaI → CH₃CH₂I + NaBr

• The reaction between sodium iodide and ethyl chloride 

CH₃CH₂Cl + NaI → CH₃CH₂I + NaCl​

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Things to Remember

• The Finkelstein reaction shows the exchange of one halogen for another.
• The reversible reaction of halide exchange occurs because of the varied Solubility of metal halide salts in acetone solvent
• There is an increase in the rate of reaction of the electron donors on the alkyl halide whereas the rate decreases for the electron-withdrawing groups.
• To achieve the desired alkyl halide in the modified version of the Finkelstein reaction, alcohol is initially converted into a mesylate or tosylate and then treated with metal halide. Mesylate and tosylate are appropriate withdrawing groups which is why the reaction works perfectly.
• Catalysts such as FeCl₃, ZnCl₂, and many more in CS₂ solvent are used to increase the reactivity of secondary, tertiary, vinyl, and aryl halides which are less reactive in Finkelstein reaction.
• Cul copper (I) iodide when mixed with diamine ligands is the only ideal catalyzed process for aromatic Finkelstein reaction.

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