A noteworthy feature of<unk>  is the weakness of its four C–Pb bonds. At the temperatures found in internal combustion engines,<unk>  decomposes completely into lead as well as combustible, short-lived ethyl radicals. Lead and lead oxide scavenge radical intermediates in combustion reactions. Engine knock is caused by a cool flame, an oscillating low-temperature combustion reaction that occurs before the proper, hot ignition. Lead quenches the pyrolysed radicals and thus kills the radical chain reaction that would sustain a cool flame, preventing it from disturbing the smooth ignition of the hot flame front. Lead itself is the reactive antiknock agent, and the ethyl groups serve as a gasoline-soluble carrier. When<unk>  burns, it produces not only carbon dioxide and water, but also lead and lead(II) oxide: : : Pb and PbO would quickly over-accumulate and foul an engine. For this reason, 1,2-dichloroethane and 1,2-dibromoethane were also added to gasoline as lead scavengers—these agents form volatile lead(II) chloride and lead(II) bromide, respectively, which flush the lead from the engine and into the air: : :
Tetraethyllead
Tetrachlorodecaoxide
Hydrogen fluoride
Lewis lead
Nickel tetracarbonyl
Selenium disulfide
Dichloroacetic acid
Triphenylchloroethylene