Electrophilic aromatic substitution. Part XV. Protiodetritiation of the helicene, benzo[c]phenanthrene

Abstract

All six tritium-labelled benzo[c]phenanthrenes have been prepared and their rates of detritiation by anhydrous trifluoroacetic acid at 70° measured; these lead to the following partial rate factors (positions in parentheses) : 1580 (1), 1200 (2), 422 (3), 2050 (4), 8680 (5), and 2465 (6). The order of positional reactivities is in very good agreement with that predicted from localization energies calculated by the Hückel method and the exchange data correctly predict that the 5-isomer should be principally obtained in preparative substitutions. Each position in benzo[c]phenanthrene is more reactive than the corresponding position in phenanthrene, and by a closely similar amount. There is no evidence with this ‘helicene’ that puckering of the rings has any effect upon the positional reactivities nor is there any indication of steric hindrance to exchange of the hydrogen atom at the 1-position, this being the position where interactions cause the rings to pucker.