Hexachlorobenzene
Hexachlorobenzene is an aromatic compound. The molecule structure  resembles benzene, except that hydrogen atoms are replaced by chlorine. It is a colorless powder that is produced using catalysis from Iron(III) chloride or from Hexachlorocyclohexane.  Hexachlorobenzene was used as a fungicide, mostly to treat cereals. The Stockholm Convention on Persistent Organic Pollutants forbids its production and use. The main reason for this restriction was that the outbreaks of rare diseases. Certain people eating food that had been treated became ill, and the illness could be linked to this substance.