Electrophilic aromatic reactivity. Part 27. Protiodetritiation of chrysene

Abstract

All six monotritium-labelled chrysenes have been prepared, and their rates of protiodetritiation measured at 70°, using a mixture of trifluoroacetic acid–chloroform (9 : 1 v/v) as the exchanging medium. These lead to the following partial rate factors (positions in parentheses): 975 (1); 186 (2); 307 (3); 696 (4): 2 790 (5); 12 200 (6) and the corresponding σ⁺ values are –0.342; –0.259; –0.284; –0.325; –0.394; –0.467. Hückel localization energies predict a positional reactivity order, viz. 6 > 1 > 4 > 5 > 3 > 2 and reactivities relative to phenanthrene) close to that observed, only the 5-position being anomalous. Thus as in the case of helicenes, these calculations tend to underestimate the reactivity of the most central position in the molecule, though for chrysene, no localization of electrons at that point through ring distortion can be held responsible. Annelation rules, derived from hydrogen exchange data for other polycyclics, predict that the partial rate factor for the 5-position should be close to that observed. Reactivities in the terminal ring are only half that of the structural isomer benzo[c]phenanthrene (tetrahelicene) which further supports the view that disortion in the latter raises the reactivity through destabilization of the ground state. The relative reactivities of the unhindered positions in naphthalene, phenanthrene, and chrysene in acetylation are the inverse of that predicted by hydrogen exchange, and a possible reason for this is considered.