Question

Why is Bromobenzene unreactive?

Answer 1

Hint: In the given question, we are required to state the reason for the unreactivity of Bromobenzene. We should have the knowledge about the activating and deactivating groups with respect to the aromatic benzene ring and leaving ability of ions as they highly influence the reactivity of the ions.

Complete answer:
In the problem, we are given the chemical organic compound bromobenzene.
Bromobenzene is unreactive mostly in Sn1 and Sn2 reactions. This is due to the fact that the carbon bromine bond present in the reaction is very strong as compared to the normal carbon bromine bonds because of the presence of the aromaticity in the benzene ring. Therefore, as bromine has the ability to leave the group it will still remain in the aromatic ring.
Now, we know that in Sn2 reactions, the nucleophile attacks the chemical compound from the backside, opposite to the leaving group. So, in the case of Sn2 nucleophilic reaction, the structure of bromobenzene will not allow for backside attack.
Now, we also know that carbocations are formed in the Sn1 reactions. So, in Sn1 reactions with bromobenzene, the carbocation formed is called phenyl carbocation. Phenyl carbocations are very unstable hence, the reaction does not proceed through the Sn1 mechanism as well.
Since the reaction does not proceed through both the mechanisms. Hence, the compound bromobenzene is highly unstable.

Note:
We should know about both the mechanisms of nucleophilic reactions. One must know the key features of Sn1 and Sn2 reactions in order to provide an explanation to the problem. Aromaticity is a stability feature possessed by resonance delocalization of $\pi$ electrons.