Chemists have invented a method to selectively prepare p-xylene from only ethylene (J. Am. Chem. Soc., DOI: 10.1021/ja307612b). The achievement creates an opportunity to make the commodity chemical terephthalic acid and its blockbuster derivative polyethylene terephthalate (PET) from alternatively sourced p-xylene. p-Xylene is typically produced by catalytic reforming of various crude oil streams in refineries. Using ethanol from biomass or ethane from shale gas would reduce reliance on crude oil as feedstock for p-xylene production. The process, developed by Thomas W. Lyons, Maurice Brookhart, and coworkers at the University of North Carolina, Chapel Hill, starts with chromium-catalyzed ethylene trimerization to form hexene, followed by an iridium-catalyzed dehydrogenation step to make hexadiene. The researchers then couple hexadiene with more ethylene in a Diels-Alder cycloaddition reaction to form 3,6-dimethylcyclohexene. Subsequent dehydrogenation using a platinum catalyst converts 3,6-dimethylcyclohexene to pure p-xylene. Making PET requires costly purification technologies to separate p-xylene from a mixture of benzene, toluene, and o-, m-, and p-xylene. The new synthesis avoids that problem.