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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 21, Number 12
BKCSDE 21(12)
December 20, 2000 

Transmission of Substituent Effects through Five-Membered Heteroaromatic Rings. III. Addition Equilibria of Hydroxide Anion to Benzaldehyde Analogues
Chang Kook Sohn, Young I Chun, Soon Ki Rhee, Chang Kon Kim, Chan Kyung Kim, Ikchoon Lee
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 aldehydes (5MHA). The transmission efficiency (S) in 5MHA(A) increases in the order NH > O > S > PH but the order exactly reverses in 5MHA(T- ). This is originated by the delocalizability of the πlone-pair on Y, n π(Y). A better correlation is obtained with σp than with σp- in the Hammett plots with positive slope, ρZ > 0, indicating that the substituent (Z) effects are not transmitted by a direct conjugation. The magnitude of ρZ for Y=NH is the largest among the heteroaromatic systems, which is consistent with the largest transmission efficiency change ( ΔS). The equilibria for the addition processes are favorable in the gas phase ( ΔG° < 0), which reverses to unfavorable in aqueous solution ( ΔG° > 0) due to the relatively large solvation energy of OH- in the initial state in aqueous solution. The orders of ΔG° and ρZ in the gas phase are almost maintained in solution.
1202 - 1206
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