Primary Reactions in the Mercury-Photosensitized Decomposition of 1-Butyne, Propyne, and Acetylene at Low Pressures

The mercury-photosensitized decomposition of 1-butyne, propyne, and acetylene was studied at very low pressures in a flow system attached to a mass spectrometer. The primary decomposition of 1-butyne was found to proceed by the reactionsC4H6*→HC≡C–CH–CH3+H50%→HC≡C–CH2+CH3 25%→1,2 and 1,3 butadiene.7% The primary decomposition of propyne was C3H4*→HC≡C–CH2+H80%→allene.15% The presence of C2H radicals was detected in the decomposition of acetylene. These probably originate from the primary decomposition. The primary quantum yield for acetylene appears very much lower than that of the butyne and propyne. It is concluded that acetylene forms an excited molecule, but that the primary decomposition does not necessarily occur by this process but from reactions involving the formation of HgH and HgC2H.