Transmission of substituent effects through five-membered heteroaromatic rings. III. Addition equilibria of hydroxide anion to benzaldehyde analogues

Transmission of substituent effects through 5-membered heteroaromatic rings is investigated theoretically at the RHF/6-31+G* and B3LYP/6-31+G* levels using the equilibria for the OH- addition to five-membered heteroaromatic a ldehydes (SMHA). The transmission efficiency (S) in 5MHA(A) increases in th e order NH > O > S > PH but the order exactly reverses in 5MHA(T-). This is originated by the delocalizability of the pi lone-pair on Y, n(pi)(Y). A b etter correlation is obtained with sigma (p) than with sigma (-)(p) in the Hammett plots with positive slope, rho (Z)>0, indicating that the substitue nt (Z) effects are not transmitted by a direct conjugation. The magnitude o f rho (Z) for Y=NH is the largest among the heteroaromatic systems, which i s consistent with the largest transmission efficiency change (DeltaS). The equilibria for the addition processes are favorable in the gas phase (Delta G degrees < 0), which reverses to unfavorable in aqueous solution (<Delta>G degrees > 0) due to the relatively large solvation energy of OH- in the in itial state in aqueous solution. The orders of DeltaG degrees and rho (Z) i n the gas phase an almost maintained in solution.