Abstract

The intrinsic electrophilic reactivity of linear acenes up to octacene was investigated by using interaction energy potentials and their components, namely Pauli repulsion, orbital interaction, and electrostatic interaction potentials using the Ziegler-Rauk decomposition scheme. We found that the shared regions above C1 and C2 of the outer ring are slightly more attractive towards an incoming electrophile than the inner ring secondary carbon atoms, mostly due to a more advantageous electrostatic interaction. This result agrees with the stability order of prereactive pi complexes formed between anthracene and the electrophilic HCl. Decomposition potentials determined in the bond formation regime indicate that longer acenes become more reactive because of the increasing orbital interaction. In addition, the orbital interaction potentials, which represent the overall effect of the high lying reactive orbital, exhibit straightforward patterns, predicting strongly preferred inner ring secondary carbon atoms over outer ring carbon atoms. Such an orbital-interaction controlled regioselectivity shows up in the total interaction energy potentials when the electrostatic interaction is scaled down, predicting the functionalization of acenes with electrophiles on the inner secondary carbon atoms, in agreement with the experimental regioselectivity. Upon analyzing the stability of transition states for the addition of HCl to anthracene, the Pauli repulsion shows up to be an important factor in controlling the stability of the transition states and favoring functionalization along the central C9 and C10 positions to outer ring carbon atoms. The resonance stabilization concept, which is generally accepted to account for the regioselectivity of higher acenes, could not be evidenced by our analysis.