Ullmann Coupling
The Ullmann reaction, also called Ullmann coupling, is an organic reaction that is used to couple two molecules of aryl halide for forming a biaryl with the help of copper metal and thermal conditions. The mechanism for the Ullmann reaction is not entirely understood, however, there are two popular mechanisms. The radical mechanism includes the single electron transfer from the copper metal to the alkyl halide for forming an aryl radical. Two aryl radicals then react and form the final biaryl product. The second mechanism includes an oxidative addition of the copper to the aryl halide and is followed by a single electron transfer and forms an organocuprate reagent. The organocuprate then performs another oxidative addition on an aryl halide and reductive elimination takes place that results in the final biaryl product. In this article, we will learn about the Ullmann reaction, Ullmann reaction mechanism, and the Ullmann reaction application.
Ullmann Coupling Reaction Mechanism
As mentioned, there are two different Ullmann coupling mechanisms. Both are shown as follows:
Radical mechanism
Image will be uploaded soon
Mechanism involving the aryl copper intermediate
Image will be uploaded soon
Mechanism of Ullmann Reaction in Detail
Let us now take a look at the mechanism in detail.
Step 1:
The mechanism of the Ullmann reaction includes the formation of an active copper(I) species when the aryl halide is introduced to an excess of metallic copper under very high temperatures, above 200C.
Image will be uploaded soon
Step 2:
The resulting copper(I) species further undergoes oxidative addition with another haloarene molecule and links the two molecules.
Image will be uploaded soon
Step 3:
In the final step of the Ullmann coupling mechanism, the copper compound which is formed by the two aryl halide molecules undergoes a reductive elimination and results in the formation of a new carbon-carbon bond between both the aryl compounds.
Image will be uploaded soon
Role of Copper in Ullmann Reaction
The Ullmann reaction is a metal-catalyzed coupling of halogene-benzene derivatives which leads to biaryls (an aryl group is a group obtained by removing a hydrogen atom from and aromatic compound; if the aromatic compound is benzene, the aryl is the phenyl group) and the larger carbon-based structures. This reaction offers an unprecedented opportunity for accessing the molecular functionality by improving the mechanical stability and electron conductance, that is essential for the advancement in the realization of organic-based electronics.
Catalysts, like copper, provide an alternative pathway through which the reaction can proceed, in which the activation energy is slightly lower. It thus increases the rate at which the reaction comes to the equilibrium. The catalyst itself takes part in the reaction without undergoing any permanent chemical change, although it can undergo a physical one. In the classical Ullmann reaction, the oxidation of copper along with the formation of molecular cuprate intermediates and copper halides as side reaction products precedes the cross-coupling reaction. However, there is general agreement that at a certain point of the reaction a copper-coordinated structure is formed, the debromination mechanism, that is the rate-limiting step of the reaction, has been scarcely investigated. At this point, it is not clear whether the formation of radicals or organo-copper complexes or the oxidative addition process precedes the formation of the biaryl compound.
Ullmann Reaction Application
Now that you know about the Ullmann coupling reaction, its nomenclature, the mechanism of the reaction and the importance of copper in the reaction, let us have a look at the applications of the Ullmann reaction.
The Ullmann reaction applications are as follows:
Biphenylenes are obtained from 2, 2- diiodo biphenyl through the Ullmann reaction.
Ullmann reaction can also be used for the closure of the five-membered rings.
An unsymmetrical reaction can be achieved when one of the reactants is provided in excess.
Chiral reactants are coupled into a chiral product through the Ullmann coupling reaction.
Significance of Ullmann Reaction
The Ulmann reaction has its own significance when it comes to the organic chemistry. Let us now look at what it is.
The Ullmann coupling reaction has become a powerful and essential tool in the organic synthesis and drug discovery. Copper-catalyzed Ullmann reactions were very well developed recently by employing the novel ligands and ancillary synthetic tools. Amongst the many exciting and rapid developments of the Ullmann coupling reactions, its is believed that the green synthetic methodologies, such as metal-, ligand-, and additive-free conditions, recyclable heterogeneous catalysts, and microwave-assisted synthesis will continue to have a significant impact on this field.
FAQs on Ullmann Reaction
1. What is the Ullmann reaction as per the CBSE Class 12 syllabus?
The Ullmann reaction, also known as Ullmann coupling, is a chemical reaction that involves the coupling of two molecules of an aryl halide in the presence of finely powdered copper at high temperatures. This process results in the formation of a new carbon-carbon bond, producing a symmetrical biaryl compound. It is a key reaction for synthesising complex aromatic structures.
2. What is a typical example of the Ullmann reaction?
A classic example of the Ullmann reaction is the synthesis of biphenyl from iodobenzene. When two molecules of iodobenzene are heated with copper powder in a sealed tube, they couple to form biphenyl and copper(I) iodide. The reaction is represented as: 2C₆H₅I + 2Cu → C₆H₅-C₆H₅ + 2CuI.
3. What are the essential reagents and conditions for an Ullmann reaction?
The Ullmann reaction requires three main components:
- Aryl Halide: The starting material. The reactivity order is generally I > Br > Cl. Aryl iodides are the most reactive.
- Copper Catalyst: Finely divided copper powder or a copper salt is essential for the coupling to occur.
- Solvent and Heat: The reaction is typically carried out at high temperatures (around 200°C), often in a high-boiling point polar aprotic solvent like DMF (dimethylformamide), pyridine, or nitrobenzene.
4. How is the Ullmann reaction different from the Fittig reaction?
The primary difference lies in the metal catalyst used. The Ullmann reaction uses copper powder to couple two aryl halide molecules. In contrast, the Fittig reaction uses metallic sodium in dry ether to achieve the same type of aryl-aryl coupling. The harsh conditions and different metal give the Ullmann reaction distinct applications, particularly in more complex syntheses.
5. How does the structure of the aryl halide affect the rate of the Ullmann reaction?
The structure significantly impacts the reaction's efficiency. Firstly, the type of halogen is crucial; aryl iodides are the most reactive due to the weaker carbon-iodine bond. Secondly, the presence of an electron-withdrawing group (like -NO₂) on the aromatic ring, especially at the ortho or para position, activates the halide and accelerates the reaction. Conversely, electron-donating groups tend to slow it down.
6. What are the key applications of the Ullmann reaction in organic synthesis?
The main application of the Ullmann reaction is the synthesis of symmetrical biaryls. These compounds are important structural motifs found in:
- Pharmaceuticals: As core structures in various drugs.
- Polymers: Used in the creation of high-performance polymers.
- Agrochemicals: Forms the backbone of certain herbicides and pesticides.
- Liquid Crystals: Biaryl units are common in molecules designed for display technologies.
7. Why is the classic Ullmann reaction generally unsuitable for preparing unsymmetrical biaryls?
Using the classic Ullmann reaction to couple two different aryl halides (e.g., Ar-X and Ar'-X) is inefficient because it leads to a mixture of products. You would get the desired unsymmetrical product (Ar-Ar') along with two symmetrical by-products (Ar-Ar and Ar'-Ar'). Separating these three similar compounds is often difficult and results in a very low yield of the target molecule, making it an impractical method for this purpose.
8. In which chapter of the NCERT Class 12 Chemistry syllabus is the Ullmann reaction included for the 2025-26 session?
For the CBSE academic session 2025-26, the Ullmann reaction is a part of the NCERT Class 12 Chemistry curriculum. You will find it in Chapter 6: Haloalkanes and Haloarenes, under the section covering chemical reactions of haloarenes.
