ANOMALOUS SIDE-CHAIN CLEAVAGE IN ALKYLAROMATIC THERMOLYSIS

The thermal cracking of alkylaromatics typically leads to appreciable formation of dealkylated aromatic compounds only at high temperatures (T > 550-degrees-C) and in low yield. High temperatures are required because of the need to form hydrogen atoms which then attack the ring at the ipso position. This paper presents an experimental study of the thermolysis (T approximately 400-500-degrees-C) of a compound (1,20-di(1-pyrenyl)eicosane) which results in side-chain cleavage significantly greater than that observed in 1- and 2-ring alkylaromatic systems. The temperature dependence of this cleavage pathway is inconsistent with a hydrogen atom mechanism; i.e., the dealkylation pathway decreases with increasing temperature. Product studies reveal the formation of an internal olefin conjugated to pyrene which is produced in concert with pyrene. This provides evidence for the source of hydrogen needed to effect the side-chain cleavage and constrains the mechanistic possibilities. A thermochemical discussion of possible reaction mechanisms is presented. The cleavage at the ring is preceded by the formation of a highly stable phenalenyl-like radical which is formed by an unusual radical-hydrogen-transfer process. A correlation relating the rates of methyl addition to a series of polycyclic aromatic compounds with the corresponding enthalpies of hydrogen atom addition was developed to estimate the heat of formation of this critical species.