petrochemical, any one of a large group of chemicals derived from a component of petroleum or natural gas. The cracking processes for manufacturing gasoline produce vast quantities of gaseous hydrocarbons. Originally considered waste products suitable only for use as illuminants and fuels, the gases today are manufactured into petrochemical substances widely employed in industry. Important petrochemical compounds are alcohols and aldehydes, butylene, butadiene, ethylene, propylene, toluene, styrene, acetylene, benzene, ethylene oxide, ethylene glycol, acrylonitrile, acetone, acetic acid, acetic anhydride, and ammonia. Materials made from the gases include carbon black, synthetic rubber, polystyrene, polypropylene, and polyethylene. Petrochemicals are widely used in agriculture, in the manufacture of plastics, synthetic fibers, and explosives, and in the aircraft and automobile industries.