<unk>  (cis-butenedioic<unk> , toxilic<unk> , dihydro-2,5-dioxofuran) is an organic compound with the formula C4H2O3.  In its pure state it is a colourless or white solid with an acrid odour.<unk>  was traditionally manufactured by the oxidation of benzene or other aromatic compounds.  Due to rising benzene prices, most<unk>  plants now use n-butane as a feedstock.  As of 2006, only few smaller plants continue to use benzene:
Maleic anhydride
Tetrachloroethylene
Methyl tert-butyl ether
Vinyl ether
Trichloroethylene
Pentafluoroethane
Ethotoin
Diethyl ether