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Finkelstein reaction

The Finkelstein reaction was proposed by a German chemist named Hans Finkelstein. Finkelstein reaction is classified as a substitution nucleophilic reaction that involves the exchange of halogen atoms. Many significant organic compounds such as chrysochlamic acid are prepared via the Finkelstein reaction. In this article, we'll discuss in detail the Finkelstein reaction mechanism along with a suitable example.

Finkelstein reaction
The organic compounds capable of undergoing Finkelstein reaction are the alkyl halides. Alkyl halides undergo a Finkelstein reaction where the halide ion is replaced with another halide ion. Because the reaction involves the substitution of a nucleophile, this reaction is referred to as the substitution nucleophilic reaction.

The Finkelstein reaction takes place in the presence of a metal salt such as sodium iodide and acetone. The end product of the Finkelstein reaction, i.e., newly synthesized alkyl halide, is poorly soluble in acetone. The reaction thus takes place at an equilibrium phase, thereby carrying the reaction forward.
Finkelstein's reaction is applicable for the conversion of alkyl halides containing bromine or chlorine into alkyl iodide. Haloalkanes containing bromine or chlorine are treated with a solution containing sodium iodide and acetone.

On the other hand, sodium chloride or sodium bromide cannot be used in the Finkelstein reaction because of their insolubility nature in acetone. The reason why acetone is used in the reaction is that it facilitates the forward movement of the reaction. Finkelstein's reaction is very well applicable to primary haloalkanes. On the other hand, secondary haloalkanes are not as reactive as primary haloalkanes. However, organic compounds such as allyl benzyl and α-carbonyl halides react well towards the Finkelstein reaction.

The general equation used to represent the Finkelstein reaction is given below.
R–X
+ X′− ⇌
R–X′ + X−
Where R-X is the alkyl halide generally consisting of chloride or bromide ion.
X' is the halide ion that will be replacing X in the alkyl halide.
R-X' is the newly formed alkyl halide.
X- is the halide ion released from the reactant alkyl halide.
The symbol "⇌" represents that this reaction is taking place at an equilibrium.

Mechanism of Finkelstein reaction

The alkyl halide involved in the Finkelstein reaction undergoes a substitution nucleophilic reaction mechanism. This reaction involves a single step where one halide ion replaces the other in the presence of sodium iodide and acetone.

Let us now understand how the substitution of the halide ion is taking place in the alkyl halide.
As said earlier, the Finkelstein reaction takes place in sodium halide (generally sodium iodide) and acetone. In the presence of acetone, sodium halide dissociates and gives rise to sodium cations and halide anions. The halide anion so produced attacks the reactant alkyl halide (RX). The halide ion is nucleophilic and attacks the alkyl chain of the haloalkane. Partial bond formation takes place between the attacking halide nucleophile and the carbon atom of the reactant.

On the other hand, partial bond breakage occurs between the alkyl chain and the halide ion already attached to it. The sodium cations so released attract the halide ion (X-) already attached to the reactant. This is how bond formation and bond breakage takes place simultaneously. The entire process takes place in a single step and results in the formation of substituted alkyl halide. Sodium halide is obtained as a precipitate.

Example of Finkelstein reaction

The Finkelstein reaction is a bimolecular substitution nucleophilic reaction. To understand the mechanism of the Finkelstein reaction better, let us have a look at the example of chloroethane.
Chloroethane is a haloalkane consisting of an ethyl group attached to a chloride ion. Chloroethane readily reacts in the presence of sodium iodide and acetone. Before chloroethane starts reacting, sodium cations and iodide anions are dissociated from the sodium iodide molecule. The halide ion is a nucleophile and attacks the carbon atom to which chloride ion is attached. The iodide ion begins to form a partial bond with the carbon atom that has acquired a partial positive charge.

On the other hand, the chloride ion begins to dissociate itself from the reactant because it acquires a partial negative charge. As a result, iodoethane is formed as the end product. The halide ion dissociated from the haloalkane combines with sodium cations and precipitates out as sodium chloride salt.

Chloroethane undergoing the Finkelstein reaction

The success rate of the Finkelstein reaction is influenced by numerous parameters such as nucleophilicity of the reactant, carbon-halogen bond in the reactant, nature of alkyl group present, and the reactivity of alkyl halide. As said earlier, primary alkyl halides are more reactive than secondary haloalkanes.

Applications of Finkelstein reaction

As discussed earlier, the Finkelstein reaction is used for synthesizing many significant organic compounds. Apart from this, the Finkelstein reaction has got numerous applications which are as follows.

● The Finkelstein reaction is used to produce alkyl iodides effortlessly. This is one of the easiest methods for the synthesis of various         alkyl iodides.
● The same reaction is also used for analyzing various alkyl halides.
● The reaction mixture of sodium iodide and acetone is additionally utilized as a qualitative test to determine the type of halide ion           present in the given sample.
● The Finkelstein reaction is used for synthesising Chrysochlamic Acid.

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