Water was irradiated with 1 MHz ultrasound (about 2 W/cm2) under mixtures of argon and acetylene of various compositions. A few experiments were performed using deuterated acetylene. Acetylene is rapidly consumed, the maximum rate occurring at a solution concentration of C2H2 of 2 × 10-3 M. The products are H2, CO, CH4, a great number of hydrocarbons containing two to about eight C atoms, formic and acetic acids, formaldehyde and acetaldehyde, and insoluble soot. Some larger product molecules are benzene, isomers of benzene, phenylacetylene, styrene, and naphthalene. The products are similar to the ones observed in the pyrolysis and combustion of acetylene. The relative abundancies of the products change with acetylene concentration, which is in part attributed to the varying temperature of the adiabatically compressed cavitation bubbles. All products are initially formed proportional to the irradiation time, even at times in the 10-s range. Volatile products are, however, consumed in longer irradiations. It is concluded that all products are formed in single cavitation events and not by stepwise formation and subsequent sonolysis of intermediate compounds in different cavitation bubbles. A mechanism is proposed according to which water vapor decomposition is the main primary process at low C2H2 concentration, the OH radical and H and O atoms formed attacking acetylene molecules. At higher C2H2 concentrations, the direct pyrolysis of acetylene is the principal primary process, C4H2 and C4H4 being formed as the most important precursors of the higher C atom number products. Among the latter, even C atom numbers are more abundant than odd numbers. Small soot molecules form a colloid absorbing uniformly at all wavelengths in the UV/vis region, and larger molecules mainly scatter light. The results are discussed in terms of mechanisms developed in combustion chemistry. © 1990 American Chemical Society.
