Aromaticity-induced basicity.

The molecules below were discussed in the previous post as examples of highly polar but formally neutral molecules, a property induced by aromatisation of up to three rings. Since e.g. compound 3 is known only in its protonated phenolic form, here I take a look at the basicity of the oxygen in these systems to see if deprotonation of the ionic phenol form to the neutral polar form is viable.

The equilibrium being considered is shown below for compound 2:

The energetics of this equilibrium shown below, computed at the ωB97XD/Def2-TZVPPD/SCRF=water level and for which the FAIR data DOI is 10.14469/hpc/4073

For 1: X=Cl, the energy is shown below as a function of the O….H distance. Proton abstraction from HCl is exothermic by ~25 kcal/mol.

For 2: X=Cl, the exothermicity increases by only ~5 kcal/mol , despite the apparent aromatisation of a further ring. It is also worth noting that this is greater basicity than that of e.g. water, where around 4-5 water molecules acting in concert are required to deprotonate HCl.For 1: X=OH, the proton abstraction from water is mildly endothermic by about 13 kcal/mol; indeed there is no energy minimum for carbonyl protonation and instead a relatively strong hydrogen bond to the water is formed instead.

For 2: X=OH the endothermicity is reduced to ~9 kcal/mol.

For 1: X=CH₃, deprotonation of methane is now strongly endothermic by ~40 kcal/mol.

So the molecules 1 – 2 above are clearly not superbases, which perhaps augers well for being able to deprotonate the ionic phenols into these neutral but highly polar molecules.