1,3-Butadiene is a simple conjugated diene with the formula C4H6. It is an important industrial chemical used as a monomer in the production of synthetic rubber. When the word butadiene is used, most of the time it refers to 1,3-butadiene.
The name butadiene can also refer to the isomer, 1,2-butadiene, which is a cumulated diene. However, this allene is difficult to prepare and has no industrial significance. This diene is also not expected to act as a diene in a Diels–Alder reaction due to its structure. To effect a Diels-Alder reaction only a conjugated diene will suffice.
Although butadiene breaks down quickly in the atmosphere, it is nevertheless found in ambient air in urban and suburban areas as a consequence of its constant emission from motor vehicles. The EPA lists it as the "mobile source air toxic" with the highest normalized risk factor, exceeding that of formaldehyde, the second riskiest air toxic emitted by motor vehicles, by a factor of more than 20.