which compound decolourize brmine water in the dark?

Dear student
Cyclohexene is that compound.
Bromine breaks the double bond of cyclohexene (and all alkenes), making the molecular structure change and therefore the molecule's properties change.
Bromine is very reactive because it can form free radicals, which means there’s one molecule of Br with an uneven number of electrons. As a result, one electron is going to be unpaired. Since this is very unstable, the Br radical will look for other electrons everywhere.
The result? Electrophile reaction. Double bonds have a big electron cloud because of the sigma bound + pi bond. The unpaired electron loves it, so it tries to fit and breaks the doble bound.
Regards
Arun (askIITians forum expert)