tetraethyl lead (tĕtˌrəĕthˈəl) [key], (C2H5)4Pb, viscous, colorless, poisonous liquid. It is an organometallic compound prepared by reacting ethyl chloride with a sodium-lead alloy. When added to gasoline, it improves the combustion characteristics (see octane number). When tetraethyl lead burns in an engine, lead oxide is formed. Ethylene dibromide is usually also added to the gasoline; on burning, the resulting mixture forms products that react with the lead oxide to form lead bromide, a volatile compound that escapes from the engine with other exhausted products. Because the lead bromide is poisonous, lead-free gasolines are required in the United States. An additional reason for lead-free gas is that the lead in the exhaust pipe would poison the catalytic converter that is standard equipment for U.S. cars. Catalytic converters which serve to oxidize unburned hydrocarbons, carbon monoxide, and nitrogen oxides contain platinum group metals that are inactivated by lead.