Hunsdiecker Reaction

Introduction

Hunsdiecker Reaction is an organic named reaction proposed by the scientist Alexander Borodin in the year 1861. It is named Borodin reaction, Hunsdiecker degradation, and Hunsdiecker–Borodin reaction because of his contribution. Alexander Borodin synthesized methyl bromide by the reaction of silver acetate.

But this is further demonstrated by Cläre Hunsdiecker and her husband Heinz Hunsdiecker for developing a new general method for the preparation of alkyl halides. And named Hunsdiecker reaction because of their contribution. There are many reviews and publications of this reaction as it involves the production of alkyl halides. The reaction includes the halogenation and the decarboxylation process. The catalytic process of the Hunsdiecker reaction is also present.

History

Alexander Borodin in 1861 held a reaction between silver acetate and bromine where he observed that the produced carbon compound has one carbon less in number. Methyl bromide is first synthesized. So, he concluded that it is a decarboxylation process. Then Adolf Lieben held a reaction of silver acetate with iodine which resulted in the formation of an alkyl iodide. It is just a modification of the discovery made by Alexander Borodin. Later Cläre Hunsdiecker and Heinz Hunsdiecker in the year 1930 introduced a new general method for the preparation of alkyl halide. And named Hunsdiecker reaction after them.

What is the Hunsdiecker Reaction?

Hunsdiecker reaction is one of the important organic chemistry named reactions. It involves the synthesis of alkyl halides by the reaction of silver salts of carboxylic acids with respective halogens. The reaction involves the decarboxylation thermally and it will lead to the formation of an alkyl halide with one carbon less than the starting material. Carbon tetrachloride, CCl₄ is used as a solvent during the reaction. The reaction that happens here is shown below,

Hunsdiecker Reaction Mechanism

A free radical mechanism is followed in the Hunsdiecker reaction with carboxyl radicals as intermediate. The steps involved in the reaction are pointed out below.

• The first step of this reaction involves the formation of acyl hypobromite by the reaction of carboxylic acids with bromine. The reaction is,

• Then the homolytic cleavage taking place at the acyl hypobromite results in the breaking of O-Br and forms a carboxyl radical and a Bromo radical.

• The carboxyl radical formed undergoes decarboxylation to produce alkyl radical and carbon dioxide gas.

• The obtained alkyl radical reacts further with another acyl hypobromite resulting in the formation of alkyl halide. The reaction is,

Some Variations of the Reaction

There has been some verification or modifications in the case of this reaction by the contributions given by some scientists. They are,

• Cristol-Firth modification − In this modification 1-bromo-3-chlorocyclobutane is prepared by the reaction of bromine and mercuric oxide.

• Kochi reaction − The variation is done by Jay Kochi in which lithium chloride or lithium bromide and lead acetate are used for the decarboxylation and halogenation reaction.

Factors Affecting the Rate of Reaction

Some factors can influence the rate of Hunsdiecker reaction. They are generally of five types,

• The concentration of reactant − The increase in the concentration of reactants during chemical reactions has an important role. Since the increased concentration will increase the number of molecules available in a unit volume. And thereby making more collisions between molecules. The proper collision of molecules is responsible for the formation of respective products. So, the rate of reactions will increase when we rise the number of reactants.

• Temperature − The rise in temperature of about 100°C will increase the rate of formation of products in the Hunsdiecker reaction.

• Nature of reactants − The nature of the corresponding reactant involved in the formation of the product greatly influences the rate. Since a proper orientation of reactants will lead to the formation of products. It is always necessary to know that the bond breaking of reactants results in the formation of new bonds in products. So, selecting reactants is very necessary so that one can break the bonds that must be selected.

• Catalyst − The use of catalysts will always increase the rate of reaction. So, reaction will be faster when a catalyst is also incorporated.

• Radiation − The incorporation of radiation in this reaction results in the absorption of radiation by reactant molecules. Since this reaction is radical intermediate forming, this will faster the rate of reactions.

Importance of Hunsdiecker Reaction

There are many important reasons for this reaction. Some of them are,

• This reaction helps in the synthesis of an alkyl halide with a carbon atom numbered 2 to 18.

• It is applicable in the halogens chlorine and bromine.

• This mechanism helped in the arylation, alkylation, and acylation process.

• Only mild conditions are needed for the reaction to proceed.

• The starting material used in the reaction is less expensive.

• The reaction produces a limited number of by-products only.

• It is the first radical intermediate forming decarboxylative reaction.

• In some conditions, this reaction selectively produces two types of alkyl halides at the same time.

• This reaction helps to synthesize cyclopropanes from alkynes and alcohols.

Conclusion

Hunsdiecker reaction is one of the important reactions in organic chemistry which involves the synthesis of an alkyl halide by the reaction of silver salts of carboxylic acids and halogens. It involves both halogenation and decarboxylation reactions. A free radical mechanism is followed in the reaction. That involves the formation of an intermediate. This is a breakthrough in the organic chemistry field since it helped in the synthesis of several organic compounds that have complex reaction procedures required. This reaction is very simple and easy to handle organic synthesis reactions. The rate of this reaction relies on several other external factors such as temperature, use of catalyst, concentration, etc. Some modifications are also present in this reaction.

FAQs

1. When iodine is used in the Hunsdiecker reaction what will happen?

The ratio between iodine and carboxylate is important in the case of iodine. Alkyl iodide will be obtained when the ratio is 1:1 but an ester is obtained when the ratio is 2:1. So the ratio is important in this case.

2. Why is fluorine not used in the Hunsdiecker reaction?

Fluorine is a very reactive chemical element so it will result in a violent reaction and forms hydrogen fluoride and carbon. No alkyl fluoride will be obtained by this reaction.

3. Why do we use CCl₄ in the Hunsdiecker reaction?

Carbon tetrachloride, CCl₄ doesn't contain any C-H bond so it will not undergo any free radical formation since the bond in CCl₄ is strong. And also, as a solvent, it will help in the smooth working of this synthesis.

4. What Colour is silver acetate?

Silver acetate is a white-coloured crystal and has high light sensitivity.

5. What is metal halogen exchange?

A chemical reaction in which the bond present between carbon and halogen is exchanged with an atom that contains metal and another atom is a metal halogen exchange reaction.