Organic compound, formula C5H8; 2–methyl–1,3–butadiene; it is the major hydrocarbon emitted by vegetation, particularly members of the oak genus. Emission rates depend on the leaf temperature and availability of sunlight. Isoprene is very reactive with the hydroxyl radical and with ozone, and its reactivity is thought to contribute to high levels of ozone found over the rural southeastern United States and other areas.