Jaime D. Gomes de Oliveira

Improved Yukawa–Tsuno equation and the substituent effect on pyridine basicity

An improved Yukawa–Tsuno equation was obtained by inserting the resonance demand parameter r in the tetralinear extension of the Hammett equation constrained by a hyperbolic relation between meta and para substituent effects. The new equation with five adjustable parameters and other Yukawa–Tsuno models were applied to selected literature data for the ionization equilibria of 21 meta- and 20 para-substituted pyridinium ions in water at 25 °C. The data are best described by the new equation using either the sigma-benzoic acid scale with r = 1.35 ± 0.16 or a truncated sigma-plus scale (in which sigma-zero constants are assigned to 4-alkyl substituents) with r = 0.29 ± 0.04. Standard errors were estimated by Monte Carlo simulation. The results for the pyridinium ion demonstrate that the hyperconjugative effect by alkyl groups is inoperative and that through-resonance effects by electron-donating substituents are considerably hindered. The origin of this hindrance is ascribed to the particularities of nitrogen orbitals in heteroaromatic compounds. It is concluded that the pyridine reactivity lies between those of benzoic acid and benzylic cation, being much closer to the former.

Oxidation of benzyl alcohol by pyridinium dichromate in acetonitrile. Using the para/meta ratio of substituent effects for mechanism elucidation

Rate constants were measured for the oxidation reaction of benzyl alcohol and twenty-five ortho-, meta- and para-monosubstituted derivatives in the temperature range 293–323 K at intervals of 10 K. The kinetics were followed spectrophotometrically in dry acetonitrile acidified with trichloroacetic acid (TCA) using pyridinium dichromate (PDC) as oxidising agent under pseudo-first-order conditions with respect to PDC. Benzaldehyde is the only oxidation product and no reaction takes place without TCA. From good linear Eyring plots activation enthalpies Δ‡H° and entropies Δ‡S° are calculated. For ortho-substituted benzyl alcohols high Δ‡H° values and small negative Δ‡S° values point to an ortho effect on the rate-determining step. Using the tetralinear approach to substituent effects, the average value = 1.09 ± 0.05 for the para/meta ratio of inductive or Electra effects is obtained and negative Hammett reaction constants decreasing in magnitude with increasing temperature are found. A mechanism implicating the prior acid-catalysed formation of neutral benzyl hydrogen dichromate ester followed by intramolecular proton transfer is proposed. Modelling of parameter λ in terms of the electrostatic theory showed its experimental value to be consistent with the ratio of electric potentials generated in the immediate vicinity of the nearest chromium atom by dipolar substituents introduced in the aromatic ring on para and meta positions. At a molecular level the oxidative, rate-determining step is suggested to be triggered by the retraction or shrinkage of electron pairs from sigma bonds in Cr2VI species to non-bonding orbitals in unstable CrIV–O–CrVI species. In contrast with past interpretations, an electrochemical approach is used to explain negative values for the Hammett reaction constant.

New evidence for through-space transmission of substituent effects in benzene derivatives

Electrostatic interaction energies between dipolar substituents and dipolar or charged reaction sites are re-examined. At short interaction distances and given orientations, the point-dipole approximation is shown to introduce important errors. Exact expressions are derived for correcting current equations for both interaction types, including the Kirkwood–Westheimer equation. The parameter λ for describing para–meta ratios of substituent inductive or Electra effects in benzene derivatives is modelled in terms of electrostatic interaction energy. Using the new equations it is shown that parameter λ can take values smaller or greater than unity, approaching a value of 2 at very large distances. Experimental λ values are calculated using reliable literature data for the ionisation equilibria of substituted phenylethanoic and 3-phenylpropanoic acids in ethanol–water mixtures. Theoretical λ values for these reactions are obtained taking into account rotational and conformational changes in the corresponding carboxylate anions. From the good agreement between experimental and theoretical λ values, it is concluded that there is through-space transmission of substituent effects in benzene derivatives.