Hunsdiecker Reaction

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What is the Hunsdiecker Reaction?

The Hunsdiecker Reaction is defined as a chemical reaction which involves in the carboxylic acid silver salts reacting to halogens to create an unstable intermediate that further undergoes decarboxylation thermally leading to the formation of the final product referred to as alkyl halides.


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This reaction is also known as either Borodin reaction or Hunsdiecker–Borodin reaction. It is also an example of both decarboxylation and halogenation reactions.


History

Firstly, Alexander Borodin was the one to demonstrate this reaction type in 1861 by the preparation of methyl bromide by silver acetate. This mechanism was also later applied by Angelo Simonini, a pupil of Austrian- Jewish chemist Adolf Lieben while performing experiments with the degradation of fatty acids, which included the reactions between silver carboxylates iodine. However, the name of the reaction was reserved particularly for the German chemist Heinz Hunsdiecker and his companion, Clare Hunsdiecker. Basically, they improved the reaction, and also their contributions led to it being the common method to form organic halide.


Hunsdiecker Reaction Mechanism of Hunsdiecker Mechanism

The hunsdiecker reaction mechanism or the mechanism of hunsdiecker reaction primarily involves the radical organic intermediates where;

  • Formation of the reactive intermediate.

  • The occurrence of decarboxylation to make a diradical pair.

  • Recombination of reactant to produce the desired product.

To break down this particular process further, the reaction starts by heating silver carboxylate in CCl4, including with bromine. At the time of this reaction, the silver carboxylate transforms into acyl hypobromite, which is primarily because of the bromine presence. Then the stable silver bromide precipitation occurs. Consequently, a radical chain reaction also occurs involving weaker oxygen-bromine bond homolysis. This results in the bromine atom formation and the carboxyl radical. This particular carboxyl radical decarboxylates, resulting in the formation of either a diradical pair of a hydrocarbon radical or an alkyl radical, which then recombines to produce the desired halide, which is an alkyl bromide in this case.


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Some Variations of the Reaction

The Hunsdiecker reaction also contains several variations. There exist cases where silver(I) carboxylate is exchanged with thallium(I) carboxylate, and then the carboxylic acid is either reacted or treated with halide ions of chloride and iodide bromine, and lead to tetraacetate majorly to effect decarboxylation and halogenation. In the same way, in 1-Bromo-3-chlorocyclobutane preparation from 3-chlorocyclobutane carboxylic acid mercury oxide and the bromide can be used.


Importance of Hunsdiecker Reaction

This reaction is one of the first decarboxylative radical-intermediate forming reactions to be proved useful in the arylation, acylation, and alkylation of specific classes of compounds. It is easy to implement, moderate conditions are required, starting materials are cheap, a workup is simple, yields are good, and side products are minimal. However, we need to contain some familiarity with the Hunsdiecker qwerks.

It was also the impetus for the closely related transformation invention, like the Minisci reaction.


Factors Affecting the Rate of Reaction

A few of the factors that affect the rate of reaction can be listed as follows:

Temperature

In most of the cases, the reaction rate in a homogeneous reaction is nearly either doubled or tripled by an increase in temperature of only 100°. Whereas, in some other cases, the rise in the reaction rates is even termed to be higher.

The Concentration of the Reactants

In the absence of a catalyst and at a fixed temperature, the given reaction rate increases with an increased concentration of reactants. With the increasing concentration of the reactant, the molecules count per unit volume is also increased. Therefore, the collision frequency ultimately increases and causes an increased reaction rate.

Nature of Reactants

A chemical reaction includes the arrangement of the atoms between the reacting molecules to the product. Here, the old bonds are broken, and the new bonds are formed. In a consequent way, the strength and nature of the bonds in the reactant molecules greatly influence the rate of its transformation into the products. The reaction which involves the lesser bond rearrangement proceeds faster compared to which involves larger bond rearrangement.

Catalyst

The rate of the chemical reaction increases in the presence of a catalyst, which ultimately enhances a chemical reaction's speed.

Radiation

The rate of the number of chemical reactions increases when the reacting molecules absorb the radiations of particular wavelengths, where such reactions are referred to as the photochemical reactions.


Knowing the Order of Reactions

Changing the concentration of the substances in a reaction changes the reaction rate. A rate equation mathematically exhibits this effect. The reaction orders are a part of the rate equation, and they are found by doing experiments. We cannot deduce anything about the order of a reaction by noticing the equation for the reaction. It can also be implemented to any elementary reaction considered only in one direction and for complex composite reactions. For an elementary reaction taking place in one direction, the order of the reaction is said to be equal to the molecularity, but it describes the kinetics instead of the mechanism.

FAQ (Frequently Asked Questions)

1. Why is Carbon Tetrachloride (CCl₄) Added in the Reaction?

Answer: Carbon tetrachloride is a nonpolar covalent compound. As Bromine is a non-polar compound, it is more soluble in carbon tetrachloride than in water. Carbon tetrachloride provides a neutral nonpolar environment for the reaction. Therefore, it acts as a perfect solvent and is actually used for the smooth conduct of the reaction.

2. Write Any Five Features for the Hunsdiecker Reaction.

Answer: Five features for the Hunsdiecker reaction are given below:

  • This reaction rarely works with unsaturated alkyl groups.

  • This reaction is usually carried out in the presence of carbon tetrachloride.

  • Bromine is often used in this reaction, but the reaction is also possible with chlorine and iodine. 

  • 1:1 ratio of silver carboxylate salt and iodine gives the alkyl halide.

  • An ester (RCOOR) is formed when the reaction is carried in presence of a 2:1 ratio of silver carboxylate and iodine.

3. Explain the SNI Reaction Mechanism?

Answer: A specific but not always noticed nucleophilic aliphatic substitution reaction mechanism stands for Substitution Nucleophilic internal or simply SNi. Also, in 1937, the nucleophilic reactions that occur with the retention of the structure were labelled. However, later these reactions were used to identify different reactions that proceed with identical mechanisms.