Question

Biphenyl is formed when (a) bromobenzene is heated with Na in ether. (b) iodobenzene is heated with copper powder in a sealed tube. (c) chlorobenzene is heated with benzene in presence of anhydrous aluminium chloride. (d) phenyl magnesium bromide is treated with bromobenzene in the presence of a small amount of \(\mathrm{CoCl}_{2}\).

Short answer

Short Answer: The formation of biphenyl occurs in options (a) bromobenzene is heated with sodium in ether, (b) iodobenzene is heated with copper powder in a sealed tube, and (d) phenyl magnesium bromide is treated with bromobenzene in the presence of a small amount of cobalt chloride (CoCl2).

Step-by-step solution

Understand the structure of biphenyl

Biphenyl is an organic compound where two benzene rings are attached through a C-C single bond. Its molecular formula is C12H10 and its chemical structure is shown below: \[ \ce{ C_6H_5-C_6H_5 } \]

Analyse the reaction conditions in option (a)

In this option, bromobenzene is heated with sodium (Na) in ether. This reaction involves the reduction of the bromobenzene to form phenyl sodium, followed by coupling of two phenyl sodium molecules to form biphenyl: \[ \ce{ 2 C_6H_5Br + 2 Na -> 2 C_6H_5Na + 2NaBr } \] \[ \ce{ 2 C_6H_5Na -> C_6H_5-C_6H_5} \] Considering the given reaction conditions, it is evident that this option leads to the formation of biphenyl.

Analyse the reaction conditions in option (b)

In this option, iodobenzene is heated with copper powder in a sealed tube. This reaction is known as the Ullmann reaction, which involves the coupling of two iodobenzene molecules to form biphenyl in the presence of copper: \[ \ce{2 C_6H_5I + Cu -> C_6H_5-C_6H_5 + CuI_2} \] This reaction also leads to the formation of biphenyl.

Analyse the reaction conditions in option (c)

In this option, chlorobenzene is heated with benzene in the presence of anhydrous aluminium chloride. This reaction conditions are typical for Friedel-Crafts alkylation, which would lead to the formation of alkylbenzene rather than the desired biphenyl: \[ \ce{C_6H_5Cl + C_6H_6 + AlCl_3 -> C_6H_5-C_6H_6 + HCl + AlCl_3} \] This option does not lead to the formation of biphenyl.

Analyse the reaction conditions in option (d)

In this option, phenyl magnesium bromide is treated with bromobenzene in the presence of a small amount of cobalt chloride (\(\mathrm{CoCl_2}\)). This reaction is a type of Grignard reaction that would lead to the formation of biphenyl, as the phenyl magnesium bromide acts as a nucleophile and attacks the bromobenzene to eliminate the bromide ion: \[ \ce{ C_6H_5MgBr + C_6H_5Br -> C_6H_5-C_6H_5 + MgBr_2 } \] This option also leads to the formation of biphenyl. To conclude, the formation of biphenyl occurs when: (a) bromobenzene is heated with sodium in ether. (b) iodobenzene is heated with copper powder in a sealed tube. (d) phenyl magnesium bromide is treated with bromobenzene in the presence of a small amount of \(\mathrm{CoCl}_{2}\).

Key concepts

Ullmann Reaction

The Ullmann reaction is a classic method in organic chemistry for the synthesis of biphenyls through an aromatic coupling process. It specifically involves the coupling of aryl halides, typically iodobenzene, in the presence of copper, which acts as a catalyst.

During this process, the aryl halide undergoes oxidative addition with copper, followed by a reductive elimination step that couples two aryl groups together to form biphenyl. The general reaction can be illustrated as follows:
\[ \ce{2 Ar-X + Cu -> Ar-Ar + CuX_2} \]
where 'Ar' represents the aryl group and 'X' is the halide.

Students may encounter the Ullmann reaction in their textbook exercises, as it's a pivotal reaction for understanding the synthesis of complex organic compounds, particularly those with biaryl structures.

Grignard Reaction

The Grignard reaction is a prominent tool in organic synthesis, named after the French chemist Victor Grignard. It involves the use of Grignard reagents, typically alkyl or aryl magnesium halides (\( R-Mg-X \)), which act as nucleophiles.

In the context of biphenyl formation, phenyl magnesium bromide (\(C_6H_5MgBr\)) is reacted with an aryl halide like bromobenzene. The Grignard reagent attacks the electrophilic carbon where the halide is attached, leading to the formation of a new C-C bond and the elimination of the magnesium halide salt: \[ \ce{R-Mg-X + Ar-X -> R-Ar + MgX_2} \]
Keep in mind that Grignard reactions require strict anhydrous conditions; moisture can deactivate the Grignard reagent.

Organic Compound Synthesis

Synthesizing organic compounds, particularly complex aromatic structures like biphenyl, is a fundamental practice in both academic and industrial chemistry. The construction of these molecules can involve a range of chemical reactions, including the Ullmann and Grignard reactions.

Organic synthesis often requires a deep understanding of reaction mechanisms, as well as the ability to predict and manipulate the course of chemical reactions to achieve a desired product. Techniques like these are not only academically interesting but are also crucial in the development of pharmaceuticals, plastics, and many other materials that are essential in our daily lives.

Aromatic Coupling Reactions

Aromatic coupling reactions are types of chemical reactions that result in the direct bonding of two aromatic rings. They are key processes in the construction of polycyclic aromatic compounds. There are several types of aromatic coupling reactions, like the Ullmann reaction and those involving Grignard reagents.

These reactions are important for students to understand because they are widely utilized in the synthesis of dyes, pharmaceuticals, and agrochemicals. The formation of biphenyl through aromatic coupling reactions serves as a foundational example for understanding the principles governing the synthesis of more complex polycyclic aromatic compounds.