Knowledge Bank

Evidence from absorption spectra has been given that gaseous benzyne (1,2-dehydrobenzene) is produced when solid films of benzene diazonium 2-carboxylate (I) are $flash-photolyzed.1$ A broad, structureless band ($\lambda max\sim 2450$ {\AA}) was assigned tentatively to benzyne absorption. Further studies have supported this assignment, largely by eliminating other possible sources of the absorption. G. Wittig and coworkers have shown that o-iodophenyl mercuric iodide (II) reacts in a way characteristic of $benzyne.2$ Flash-photolysis of II, supplied by Professor Wittig, gives the same transient absorption as I. Biphenylene, the dimer of benzyne, appears after photolysis of I, but does not appear after photolysis of II unless excess hydrogen is present. Hence biphenylene is probably not responsible for the transient absorption; other possibilities are excluded on trivial chemical grounds. Both I and II shows traces of benzene in their product spectra. No evidence was found for reaction of gaseous benzyne with either, $CO2$ or $BF3$. The intensity and time of appearance of the transient absorption depend on the surface on which the film is deposited. At short times, a concentration gradient is apparent in the cell, due to movement of the transient absorber. Atomic silicon is produced, apparently from the cell walls.