Rapid catalytic synthesis of 11 C‐labelled aromatics

11 C‐Labelled benzene derivatives were prepared catalytically by trimerizing acetylene‐ 11 C with acetylene and propyne carriers over 1.5 grams of silica‐alumina support activated by potassium chromate (0.2% by weight). Optimized yields of the 11 C‐labelled aromatics were obtained when the reactants were exposed to the catalyst for 10 minutes at 24°C. A minimum addition of 50 μmole of alkyne carrier was necessary before the trimerization proceeded to give appreciable yields of 11 C‐labelled aromatics. In the competitive trimerization of acetylene‐ 11 C with acetylene and propyne carriers, three 11 C‐labelled aromatic products were generated: benzene, toluene, and xylene. From these studies acetylene was observed to be 1.6 times more reactive than propyne. In addition, the observed distribution of 11 C‐labelled xylenes (3.4% ortho, 10.6% meta, and 86% para) indicates an apparent regiospecificity in the trimerization of 11 C‐acetylene with propyne.