Electrophilic aromatic substitution. Part 19. Protiodetritiation of 1,2-diphenylethane and 9,10-dihydrophenanthrene: effect of strain on aromatic reactivity

Abstract

Rates of protiodetritiation of 1,2-diphenylethane and 9,10-dihydrophenanthrene in anhydrous trifluoroacetic acid at 70 °C have been measured and yield the following partial rate factors (positions in parentheses) : diphenylethane, 100(2); 5.73(3); 200(4); 9,10-dihydrophenanthrene, 100(1); 2 840(2); 189(3); 1 970(4); the corresponding σ⁺ values are –0.23, –0.085, –0.26, –0.23, –0.395, –0.26, and –0.375. The overall reactivity of 9,10-dihydrophenanthrene relative to that of fluorene is compatible with the differences in coplanarity between the molecules. The ratio of the reactivities of the positions α and β to the central ring in 9,10-dihydrophenanthrene is higher than in fluorene and confirms that the low reactivity of the α-positions of the latter arises from an increase in strain produced on going to the transition state for α-substitution. The bond-strain theory accounts for the anomalously low reactivity of the 7-position, and of the 3-bromo-substituent effect in detritiation of fluoranthene. A linear free energy correlation exists between molecular chlorination and detritiation of 9,10-dihydrophenanthrene, fluorene, biphenyl, naphthalene, and benzene.