Theoretical Studies on the Protonation Equilibria of Benzoyl Derivatives

The effects of ortho- (R = H and CH3) and Y-substituents (Y = OCH3, CH3 H and CN), which are directly at-tached to the carbonyl carbon, on the protonation equilibria of the para-X-substituted benzoyl derivatives, 4-X-2,6-di-R-C6H2-C(=O)-Y, are investigated theoretically using the B3LYP method with 6-31+G * basis set. Structurally, both of the (B) and (BH+) forms in the species with R = H are nearly coplanar regardless of the Y-substituents implying that the steric repulsion between Y-substituent and R = H is relatively small. In the species with R = CH3, the tortional angle (θ) between the carbonyl moiety and aryl ring varies from zero to near right angle depending on the degree of steric repulsion between Y and R = CH3 and the resonance demand. However the reaction energies, ∆Go, for the protonation processes are more favorable for R = CH3 than for R = H due to stronger electron donating effect of R = CH3, although the species with R = CH3 are unfavorable sterically. On the other hand, the Hammett type plots are progressively better correlated with σ+ than with σ values on going from Y = OCH3 to Y = CN for both species with R = H and CH3 indicating that the degree of resonance delocalization between carbonyl moiety and X-substituent is increased for a more electron accepting Y-substituent. Nevertheless the effects of R = CH3 on the magnitude of Hammett type reaction constants (ρ or ρ+) are not much different from those of R = H.