Diethyl phthalate (DEP) is a phthalate ester, namely the diethyl ester of phthalic acid. It is a clear substance that is liquid at room temperature and is only slightly denser than liquid water. It has a faint, disagreeable odor and can be transferred from the plastics that contain it. When burned, DEP produces toxic gases.
Diethyl phthalate is produced by the reaction of phthalic anhydride with ethanol in the presence of a catalytic amount of concentrated sulfuric acid. Phthalic anhydride is produced by either the oxo process or the Ald-Ox process from ethanol and the oxidation of naphthalene or o-xylene. The purity of manufactured phthalate esters is reportedly between 99.70% and 99.97% with the main impurities being isophthalic acid, terephthalic acid, and maleic anhydride.