The Photolysis of Acetaldehyde in the Presence of Hydrogen Sulfide

Acetaldehyde has been photolyzed in the presence of hydrogen sulfide over the temperature range from 150 to 360°C. The initial rates of decomposition obtained under various experimental conditions are consistent with a mechanism which involves the reactions CH3+H2S=CH4+HS (6) and HS+CH3CHO=H2S+CH3CO (7) in addition to those established already for the photolysis of acetaldehyde alone. Analysis of the data on this basis gives a value of 4.0 kcal./mol. for the difference in activation energy E4–E6 where E4 refers to the reaction CH3+CH3CHO=CH4+CH3CO (4). This result, when combined with E4 = 6.8 kcal./ mol. found by Ausloos and Steacie, gives E6=2.8 kcal./mol. in excellent agreement with our previous value of 2.6 kcal./mol. obtained in the photolysis of acetone in the presence of hydrogen sulfide. As the temperature is lowered and the ratio [H2S] / [CH3CHO] increased, the importance of the combination of methyl radicals to ethane as the chain terminating step decreases, until it is replaced by the combination of hydrosulfide radicals. This is explained in terms of the relationship E7>E6.