2-and 3-Furylketenes (18 and 20), 2-and 3-thienylketenes (19 and 21), and bis (2, 5-ketenyl) thiophene (24) have been generated as observable reactive intermediates by photochemical Wolff rearrangements. Stabilization energies of the monoketenes 18-21 have been determined by DFT computations of isodesmic energy changes, and these ketenes are predicted to be modestly destabilized relative to phenylketene. Rate constants for reaction ...