The thermokinetics for the tautomerization of a series of methylenedihydroacenes to the corresponding methylacenes (toluene to 6-methylpentacene) have been investigated by means of CBS-QB3 calculations. Only for 6-methylpentacene does the methylenedihydro form predominate at room temperature. The obtained equilibrium ratios are consistent with various theoretical methods, but the agreement with the scarce experimental data is only qualitative. The noncatalyzed thermal tautomerization of the methylenedihydroacene in an inert solvent may proceed by means of a reverse radical disproportionation reaction (RRD) as the rate-determining step. The benzylic BDE(C–H)s and the hydrogen atom affinities (HA) of the tautomers have been used to calculate the reaction enthalpy, ΔRRDH. It appears that the Ea,RRD is substantially higher than ΔRRDH. This implies that the opposite reaction (and the tautomer forming step), a radical–radical disproportionation (RD), is an activated process. This is an often ignored or overlooked kinetic feature. The consequence is that although the RRD reaction may be kinetically feasible at elevated temperatures, the products are not the tautomers but rather dimers stemming from radical–radical recombination reactions, with p-isotoluene as a clear exception. It is further shown that the RRD self-reaction of phenalene is too slow at 298 K, despite claims to the contrary.

The keto–enolization of hydroxyl-substituted naphthols and 9-anthrols has been investigated by means of CBS-QB3 calculations. An excellent agreement between experiment and theory is found for the energetics for the anthrone (5) ⇌ anthrol (6) equilibrium, with an enthalpy of tautomerization, ΔtH, of 3.8 kcal mol–1. In contrast, 1-naphthol is the preferred tautomer with a ΔtH = −9.0 kcal mol–1. Substitution of the hydrogens at the adjacent carbons by hydroxyl groups leads to the formation of strong intramolecular hydrogen bonds within a six-membered ring in the enones and the enols. Due to the difference in the intramolecular hydrogen bond enthalpy, ΔHBHintra, the equilibrium shifts further to the enone. Thus, for 1,8-dihydroxy-anthrone (anthralin, dithranol) ΔtH increases to 12.7 kcal mol–1 with an enol/enone ratio of 10–10. The solvent effect on the 5 ⇌ 6 equilibrium has been quantified by considering the formation of intermolecular hydrogen bond(s), leading to an acidity parameter α2H for anthrol of 0.42. It is shown that the hydrogen bond donating ability of bulk methanol is greatly attenuated through the formation of cyclic oligomers. The benzylic and phenolic bond dissociation enthalpies for anthrone up to anthralin suggest some antioxidant potency but the precise (radical) mechanism of action remains uncertain.