Question

Ullmann reaction involves the use of the following reactants:
A) Iodobenzene and sodium
B) Benzene and copper
C) Iodobenzene and copper powder
D) Benzene diazonium chloride and Cu/HCl

Answer 1

Ullmann reaction, also known as Ullmann biaryl synthesis or Ullmann coupling, is an organic reaction which couples two molecules of aryl halide to produce a biaryl employing copper metal in the presence of thermal conditions.

Complete step by step answer:
The general form of Ullmann reaction is depicted below:

Mechanism of Ullmann reaction: There are two well-known mechanisms for Ullmann reaction.
(i) The radical mechanism initiates with a single electron transfer from the copper metal to the alkyl halide forming an aryl radical. Then, two aryl radicals react to produce the biaryl product.
(ii) The second mechanism initiates with the oxidative addition of copper to the aryl halide which is then followed by a single electron transfer to produce an organocuprate reagent. Another oxidative addition occurs on an aryl halide by this organocuprate followed by reductive elimination, thus resulting in the biaryl product.
Step by step mechanism of Ullmann reaction:
1. Step 1:
When aryl halide is introduced to an excess of copper metal under thermal conditions (>200$^\circ$C), the Ullmann reaction leads to the production of an active copper(I) species.

2. Step 2:
Now, the copper(I) species again undergoes oxidative addition with haloarene molecules thereby, linking the two molecules.

3. Step 3:
Finally, the copper compound produced by the two aryl halide molecules experiences reductive elimination, which results in the formation of a new carbon-carbon bond between the two aryl compounds (shown below).

Therefore, the answer to the above question is C.

Note: Ullmann coupling reaction is known to have relatively low yield that is why palladium-catalyzed coupling reactions (such as the Heck reaction) are preferred over this coupling reaction. Moreover, the reaction conditions for the Ullmann reaction are quite harsh.