Dioxin in its original usage denoted the organochemical
heterocyclic compound with the general chemical formula C4H4O2 in which two CH groups of a benzene ring were replaced (
endocyclic substitution) by oxygen atoms, of which two isomers were possible, the ortho (o) and the para (p) variety: 1,2-dioxin and 1,4-dioxin. This name then extended to the family of compounds derivable from the parent compounds by
exocyclic substitution, and especially to the dibenzo substitution of the para variety.
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