Reactions of polycyclic alkylaromatics: 5. pyrolysis of methylanthracenes

1‐, 2‐ and 9‐methylanthracene were pyrolyzed neat at temperatures between 350 and 450°C for batch holding times up to 300 min. The pyrolysis proceeded through three parallel primary reaction pathways: one led to anthracene via demethylation; the second to dimethylanthracenes through methyl addition; and the third to methyl‐9,10‐dihydroanthracenes through hydrogenation. The relative importance of these three paths varied for different methylanthracene isomers. The presence of these primary pathways can be rationalized in terms of recently elucidated hydrogentransfer mechanisms and other aspects of the developing free‐radical chemistry of polycyclic alkylarenes. The demethylation rate at 400°C for the methylanthracenes and seven other methylarenes was correlated with Dewar reactivity numbers, which provide a measure of the localization energy, as ln rate (arene yield/min) = 3.7– 7.1 N_ts, where N_ts is the Dewar reactivity number for the perpheral aromatic carbon atom bearing the methyl substituent. This correlation may be useful in molecular‐based reaction models for the conversion of heavy hydrocarbon resources such as coals and heavy oils.