What reagents and or catalysts are necessary to carry out each conversion: a) benzene to nitrobenzene, b) 1,4-dichlorobenzene to 2-bromo-1,4-dichlorobenzene, c) benzene to 1-bromo-4-chlorobenzene?

To perform the conversions mentioned, specific reagents and conditions are required:

1. Benzene to Nitrobenzene:

   This conversion is achieved through a process known as nitration. The primary reagents used for this reaction are concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄). The sulfuric acid acts as a catalyst to generate the nitronium ion (NO₂⁺), which is the active electrophile that will react with benzene to form nitrobenzene. The reaction is typically conducted at low temperatures to minimize the formation of further nitrated products.

2. 1,4-Dichlorobenzene to 2-Bromo-1,4-Dichlorobenzene:

   This transformation generally requires a bromination reaction. One common method to achieve this is by using bromine (Br₂) in the presence of a Lewis acid catalyst, such as iron(III) bromide (FeBr₃). Under these conditions, the bromine can substitute one of the hydrogen atoms in the aromatic ring, resulting in the formation of 2-bromo-1,4-dichlorobenzene. The presence of the dichloro groups directs the bromination to the meta position relative to the existing substituents.

3. Benzene to 1-Bromo-4-Chlorobenzene:

   For this conversion, a two-step process is typically used. Initially, benzene is reacted with chlorine (Cl₂) in the presence of a catalyst, such as iron(III) chloride (FeCl₃). This results in the formation of chlorobenzene. In the subsequent step, the chlorobenzene undergoes bromination using bromine (Br₂) in the presence of a Lewis acid like FeBr₃. The positional selectivity is influenced by the existing chlorine substituent, which directs the bromine to the para position, yielding 1-bromo-4-chlorobenzene.

In summary, the correct selection of reagents and catalysts is crucial for achieving these specific aromatic substitutions efficiently.