A mixture of benzene and bromine solution remains unchanged for indefinite period of time, but if an iron nail is added to the solution, bromination of benzene occurs rapidly-explain.

Explanation

Benzene is an aromatic compound having no ethylenic unsaturation. So benzene does not participate in addition reaction with bromine. Again, substitution of benzene does not take place with the poor electrophile bromine alone.
So a solution of bromine in benzene remains stable (i.e., unchanged ) for indefinite period of time.
However, when an iron nail is added to the solution , bromination fo benzene occurs to yield bromobenzene because iron then acts as a halogen carrier. The red solution of bromine becomes colourless.
`2Fe+3Br_2to2FeBr_3,Br_2+FeBr_3toBr^(oplus)FeBr_4^(Theta)`
`C_6H_6+Br^(oplus)FeBr_4^(Theta) toC_6H_5Br+HBr+FeBr_3`