Propene molecule. Computer model showing the structure of a molecule of the hydrocarbon propene (propylene). Propene is a flammable gas and is the second simplest of the alkenes (hydrocarbons with carbon-carbon double bonds) and the second most important starting product in the petrochemical industry after ethylene (ethene). It is the raw material for a wide variety of products. Atoms are shown as colour-coded spheres, with the bonds between them as rods. Carbon: black, hydrogen: white.