either of two aliphatic organic compounds that have the formula C₄H₆. The term ordinarily signifies the more important of the two, 1,3-butadiene, which is the major constituent of many synthetic rubbers (rubber). It was first manufactured in Germany during World War I from acetylene. During World War II, butenes from petroleum and natural gas were the raw material for 60 percent of American butadiene production, ethyl alcohol for the rest. Butadiene rubber has now completely displaced natural rubber in the manufacture of automobile tires. Nearly all butadiene is made by dehydrogenation of butane or butenes or by high-temperature cracking (breaking up of large molecules) of petroleum distillates.

The 1,3-butadiene is the simplest member of the series of conjugated dienes, which contain the structure C=C−C=C, the C being carbon. The wide variety of chemical reactions peculiar to this system makes butadiene important in chemical synthesis. Under the influence of catalysts, butadiene molecules combine with each other or with other reactive molecules, as acrylonitrile or styrene, to form elastic, rubberlike materials. In uncatalyzed reactions with reactive unsaturated compounds, such as maleic anhydride, butadiene undergoes the Diels-Alder reaction, forming cyclohexene derivatives. Butadiene is attacked by the numerous substances that react with ordinary olefins, but the reactions often involve both double bonds (e.g., addition of chlorine yields both 3,4-dichloro-1-butene and 1,4-dichloro-2-butene).

At atmospheric conditions, 1,3-butadiene exists as a colourless gas, but it is liquefied either by cooling to -4.4° C (24.1° F) or by compressing to 2.8 atmospheres at 25° C (77° F).