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Substituent effects in solvolysis of 1,1-diphenylethyl p-nitrobenzoates. Symmetrically disubstituted and monosubstituted systems
Fujio, Mizue,Uddin, Md. Khabir,Kim, Hyun-Joong,Tsuno, Yuho
, p. 544 - 549 (2007/10/03)
The rates of solvolysis of 1,1-diarylethyl p-nitrobenzoates and chlorides were determined conductimetrically at 25 °C in 80% (v/v) aqueous acetone. Applying the Yukawa-Tsuno (Y-T) equation, the symmetrical (X = Y) subseries gave a precise additivity relationship for the whole substituent range with a rsym value of -3.78 and an rsym value of 0.77. While any Y subsets gave statistically less reliable Y-T correlations, the apparent ρ value changed significantly depending on the fixed Y substituents; the ρ value decreases with the more electron-donating fixed substituents Y, which is compatible with the Hammond shift of the transition state coordinate. Nevertheless, the concave correlations of the More O'Ferrall non-linearity relationship for any Y subsets are not in line with what is expected from the reactivity-selectivity relationship suggesting an anti-Hammond shift of transition state. However, we found a precise extended Bronsted relationship between the pKR+ values of 1,1-diarylethylenes and solvolysis rate process with a constant slope of α = 1.03 ± 0.03. This is direct evidence indicating that there is no significant shift of the transition-state coordinate over the whole range of substituent change. Copyright
