<unk>  is the common name for the organic compound 2-chloro-1,3-butadiene, which has the chemical formula C4H5Cl.  The chemical structure is shown at right.  It is used as monomer for the production of the polymer polychloroprene, a type of synthetic rubber.  Polychloroprene is better known to the public as Neoprene, the trade name DuPont gave it when the company first developed it and currently used by DuPont Dow.
Chloroprene
Dimethylphthalate
Trichloroethylene
Tetrachloroethylene
Nitrazine
Methylene diphenyl diisocyanate
Coal tar
Benzopyrene