When bromine reacts with benzene in the presence of sunlight, what will happen?

Benzene (C₆H₆) is an aromatic compound that generally undergoes electrophilic substitution reactions rather than addition reactions because addition would destroy its stable aromatic ring system. However, under specific conditions such as the presence of sunlight or ultraviolet (UV) light, benzene can participate in addition reactions with halogens like bromine (Br₂).

In normal laboratory conditions, when benzene is treated with bromine in the presence of a Lewis acid catalyst (such as FeBr₃ or AlCl₃), the reaction proceeds by substitution, forming bromobenzene (C₆H₅Br) and hydrogen bromide (HBr). This occurs because the catalyst helps generate the electrophile (Br⁺), which substitutes a hydrogen atom on the ring.

However, when the reaction occurs in the presence of sunlight or UV radiation, no catalyst is needed. The high-energy light breaks the bromine molecule into two bromine radicals (•Br). These radicals are highly reactive and attack the π-electron system of benzene, disrupting its aromaticity and leading to an addition reaction. The result is the formation of hexabromocyclohexane (C₆H₆Br₆), also known as benzene hexabromide.

This addition reaction is a photochemical process, meaning it requires light energy to proceed. The benzene ring loses its aromatic character as the π-bonds are converted into σ-bonds during the addition of bromine atoms.

Therefore, under sunlight, benzene reacts with bromine through an addition mechanism, not substitution. This reaction demonstrates how reaction conditions (light vs. catalyst) can significantly change the behavior of aromatic compounds in organic chemistry