Question

\(2,2^{\prime}-\) Dinitrobiphenyl is to be synthesized. Outline the method of preparation from iodobenzene. No other organic reagent may be used. Any inorganic reagent may be employed.

Short answer

To synthesize 2,2'-Dinitrobiphenyl from iodobenzene, follow these steps: 1. Convert iodobenzene to 2-bromoiodobenzene with Br₂ and AlBr₃ catalyst: \[\text{Iodobenzene} + Br_2 \xrightarrow{AlBr_3} \text{2-Bromoiodobenzene}\] 2. Form biphenyl via Ullmann coupling of two 2-bromoiodobenzene molecules with a copper catalyst and high temperature: \[ 2 \cdot \text{2-Bromoiodobenzene} \xrightarrow{Cu, \Delta} \text{Biphenyl} \] 3. Nitrate biphenyl using a mixture of concentrated HNO₃ and H₂SO₄ at 0°C to yield 2,2'-Dinitrobiphenyl: \[ \text{Biphenyl} + 2 HNO_3 \xrightarrow{H_2SO_4, 0°C} 2,2'^{\prime}-\text{Dinitrobiphenyl} \]

Step-by-step solution

Formation of 2-Bromoiodobenzene

First, we need to convert the iodobenzene molecule into 2-bromoiodobenzene by introducing a bromine atom ortho to the iodine atom. This can be accomplished by treating iodobenzene with bromine (Br₂) in the presence of a Lewis acid catalyst, such as aluminum bromide (AlBr₃): \[ \text{Iodobenzene} + Br_2 \xrightarrow{AlBr_3} \text{2-Bromoiodobenzene}\]

Formation of Biphenyl

Now that we have 2-bromoiodobenzene, we can use the Ullmann coupling reaction to form biphenyl. The Ullmann coupling is a reaction that forms a carbon-carbon bond between two aromatic halides using a copper catalyst. In our case, the reaction will be between two 2-bromoiodobenzene molecules in the presence of copper and high temperature: \[ 2 \cdot \text{2-Bromoiodobenzene} \xrightarrow{Cu, \Delta} \text{Biphenyl} \] After this reaction, we have successfully formed biphenyl from iodobenzene.

Nitration of Biphenyl

The final step is to introduce the nitro groups at the 2 and 2' positions of biphenyl. This can be done using a mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) at low temperature. The concentrated sulfuric acid acts as a dehydrating agent, which makes the nitric acid a better nitrating agent. Since we want to add the nitro groups ortho to the existing aromatic rings, we can control the temperature so that only ortho positions are nitrated: \[ \text{Biphenyl} + 2 HNO_3 \xrightarrow{H_2SO_4, 0°C} 2,2'^{\prime}-\text{Dinitrobiphenyl} \] Now we have successfully synthesized 2,2'-dinitrobiphenyl from iodobenzene using inorganic reagents.

Key concepts

Understanding the Ullmann Coupling Reaction

The Ullmann coupling reaction is a cornerstone of organic synthesis, often used to create biaryl compounds, which are molecules consisting of two aromatic rings. This reaction involves the joining of two aromatic halides, such as bromo- or iodo-substituted benzene derivatives, under the influence of a copper catalyst.

Typically, the reaction conditions require elevated temperatures to promote the formation of the carbon-carbon bonds between the halide-substituted aromatics. A key aspect of the Ullmann reaction is the choice of copper catalyst—copper(I) iodide is commonly used due to its effectiveness in facilitating the biaryl coupling.

One might wonder about the mechanism by which these molecules join. It is generally believed that the copper catalyst helps in the formation of an aryl-copper intermediate, which then undergoes a coupling with another halide-substituted benzene to form the biaryl product. The efficiency of this process can be enhanced by the addition of ligands that stabilize the copper catalyst and help in the transmetalation step.

The Nitration Reaction in Aromatic Chemistry

The nitration reaction is another essential process in organic chemistry, allowing chemists to introduce nitro groups (-NO2) into aromatic compounds. This functional group is crucial for various applications, including the production of dyes, pharmaceuticals, and explosives.

Nitration is typically achieved by treating the aromatic compound with a mixture of concentrated nitric acid and sulfuric acid. The role of sulfuric acid is dual; it acts as a catalyst and also produces the nitronium ion (NO2+), the actual nitrating agent, by protonating nitric acid and removing water from the reaction.

Controlling the reaction conditions, such as temperature and concentration, allows for selective nitration. For instance, lower temperatures favor the addition of the nitro group in the ortho or para positions relative to groups that direct the incoming nitronium ion to those positions on the aromatic ring. An understanding of the directing effects of different substituents on the aromatic ring is crucial for achieving the desired nitration pattern.

Bromination of Aromatic Compounds: A Subtle Art

Bromination is a type of halogenation—an important class of reactions where a halogen atom is introduced into a molecule. When dealing with aromatic compounds, bromination is a reaction that must be carried out with precision to ensure that the bromine atom is added to the right position on the aromatic ring.

To induce bromination, molecular bromine (Br2) is typically used, often in the presence of a catalyst such as iron(III) bromide or aluminum bromide that enhances the electrophilic character of the bromine. These Lewis acid catalysts help to form a bromonium ion, a highly reactive species that can attack the electron-rich aromatic ring.

The choice of catalyst, solvent, and reaction conditions can influence the position where bromine is introduced, given that different substituents already present on the ring can direct the incoming bromine to ortho, meta, or para positions. This selectivity is based on electronic and steric factors, and understanding these will allow chemists to tailor the bromination process to yield the desired substituted bromoaromatic compounds efficiently.