Steric effects and steric inhibition of resonance: structure and ionization of 2-tert-butylbenzoic acid

Abstract

Enhanced acidity of sterically hindered carboxylic acids has been explained by steric inhibition to resonance (SIR) and van der Waals (vdW) tension. These two terms have been quantitatively evaluated on a model comparing two isomers, 2-tert-butylbenzoic acid (3) and 4-tert-butylbenzoic acid (4). Energies and geometries of 3 and 4 and their anions have been calculated using the B3LYP model at a 6-31+G(d,p) or 6-311+G(3df,2pd) basis, both in minimum-energy conformations and with a constrained dihedral angle ϕ between the ring plane and carboxyl plane. From comparison of the two isomers it follows that SIR contributes significantly to the higher energy of 3, but also raises similarly the energy of its anion 3A. Stronger acidity of 3 compared to 4 (both in the gas phase and in solution) cannot be explained by SIR and must be attributed to a stabilizing interaction in the anion 3A; approximately it might be modelled as a pole/induced dipole interaction. The explanation by SIR must be used with more caution, particularly for solution acidities.