Hunsdiecker Reaction Explained: Mechanism, Steps & Examples

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.

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.

The Hunsdiecker reaction occurs when a benzene molecule, which has two adjacent hydrogens on opposite sides of the ring, is reacted with an alkyl halide. The result can be a cyclopropane or a cyclobutane. It was discovered by Adolf von Hunsdiecker in 1912 and is an example of a metal-halogen exchange reaction

Hunsdiecker Reaction Mechanism 

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 reactants to produce the desired product.

To break down this particular process further, the reaction starts by heating silver carboxylate in CCl4, including 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 quirks.

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 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 molecule 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 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.

Hunsdiecker Reaction is Important Because of Various Reasons

1. It is used to make a carbocyclic ring which is not possible by using only a Grignard reaction.
2. It can produce cyclopropane rings from alcohols and alkynes, as the other methods to form these types of rings are very difficult.
3. In certain cases, it can selectively produce two different types of alkyl halides from the same benzene compound.
4. This reaction enables chemists to synthesise complex organic compounds through an efficient methodology with a high atom economy and low cost.
5. The metal-halogen exchange is a key step in many organic reactions such as Suzuki Coupling, Kumada Coupling, and Heck Reaction.
6. Cyclopropanes or Cyclobutanes produced through Hunsdiecker Reaction can be utilized in the synthesis of steroids and alkaloids.

Conclusion

This is all about Hunsdiecker’s Reaction and its mechanism. Follow the features of this reaction and understand how it is conducted. Understand the changes occurring in the reactants to form products at every step to develop your conceptual foundation.