Is the Hammett’s Constant Free of Steric Effects?

Sigma Hole Potentials as Tools: Quantifying and Partitioning Substituent Effects

Empirical substituent constants, such as the Hammett parameters, have found important utility in organic and other areas of chemistry. They are useful both in predicting the impact of substitutions on chemical processes and in rationalizing after-the-fact observations on chemical bonding and reactivity. We assess the impact of substitutions on monoiodinated benzene rings and find that the modifications that substituents induce on the electrostatic potentials at the sigma hole on the terminal I center correlate strongly with established trends of common substituents. As an alternative to the experimental procedures involved in obtaining empirically based substituent constants, the computationally determined constants based on induced electrostatic potentials offer a model for quantifying the influence of mono- and polyatomic, neutral, and ionic substituents on their compounds. A partitioning scheme is proposed that allows us to discretely separate σ and π contributions to generate quantitative measures of substituent effects.

The Reactivity and Reaction Pathway of Fenton Reactions Driven by Substituted 1,2-Dihydroxybenzenes

Fenton systems are interesting alternatives to advanced oxidation processes (AOPs) applied in soil or water remediation. 1,2-Dihydroxybenzenes (1,2-DHBs) are able to amplify the reactivity of Fenton systems and have been extensively studied in biological systems and for AOP applications. To develop efficient AOPs based on Fenton systems driven by 1,2-DHBs, the change in reactivity mediated by different 1,2-DHBs must be understood. For this, a systematic study of the reactivity of Fenton-like systems driven by 1,2-DHBs with different substituents at position 4 was performed. The substituent effect was analyzed using the Hammett constant (σ), which has positive values for electron-withdrawing groups (EWGs) and negative values for electron-donating groups (EDGs). The reactivity of each system was determined from the degradation of a recalcitrant azo dye and hydroxyl radical (HO·) production. The relationship between these reactivities and the ability of each 1,2-DHB to reduce Fe(III) was determined. From these results, we propose two pathways for HO· production. The pathway for Fenton-like systems driven by 1,2-DHBs with EDGs depends only on the Fe(III) reduction mediated by 1,2-DHB. In Fenton-like reactions driven by 1,2-DHBs with EWGs, the Fe(III) reduction is not primarily responsible for increasing the HO· production by this system in the early stages.

Mechanistic Information from Nonstationary Points

The thermal cyclization of enyne-carbodiimides substituted at both the alkyne and carbodiimide terminus showed two curved Hammett correlations (log k/k0 against σp) that were fully reproduced by DFT (density functional theory) computational results. The latter suggest a concerted mechanism, but the transition state (TS) analysis failed to reveal any mechanistic insight about the reason for a curved Hammett correlation. Instead a preTS inspection, i.e., examination of the electronic and steric details on route between reactant and TS, furnished a detailed picture of the mechanism.

From information theory to quantitative description of steric effects

In this study, further evidences of the efficiency of informational theory for a quantitative description of steric effects are showcased.

Substituent effects on the kinetics of bifunctional styrene SAM formation on H-terminated Si

Self-assembled monolayers (SAMs) on metal and semiconductor surfaces are of interest in electronic devices, molecular and biosensors, and nanostructured surface preparation. Bifunctionalized molecules, where one functional group attaches to the surface while the other remains free for further modification, allow for the rational design of multilayer chemisorbed thin films. In this study, substituted styrenes acted as a model system for SAM formation through an alkene moiety. Substituents ranging from activating to strongly deactivating for aromatic reactions were used to probe the effect of the electronic properties of functionalizing molecules on the rate of SAM formation. Substituted styrene SAMs were formed on hydrogen-terminated p-type Si(100) and n-type Si(111) via sonochemical functionalization. Monolayers were characterized via ellipsometry, IR spectroscopy, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). Initial rates of reaction for molecules that selectively attached through the alkene were further studied. A linear relationship was observed between the initial rates of surface functionalization and the substituent electron donating/withdrawing ability for the substituted styrenes, as described by their respective Hammett constants. This study provides precedent for applying well quantified homogeneous chemical reaction relationships to reactions at the solid-liquid interface.

In vitro activity of natural phenolic compounds against fluconazole-resistant Candida species: a quantitative structure-activity relationship analysis

AIMS

To evaluate the antifungal activity and to analyse the structure-activity relationship of eleven natural phenolic compounds against four Candida species which are resistant to fluconazole.

METHODS AND RESULTS

Four different species of Candida isolates were used: Candida albicans, Candida krusei, Candida tropicalis and Candida dubliniensis. The phenolic compound carvacrol showed the highest anti-Candida bioactivity, followed by thymol and isoeugenol. The obtained minimum inhibitory concentration (MIC) values obtained were used in a quantitative structure-activity relationship (QSAR) analysis where the electronic, steric, thermodynamic and topological descriptors served as dependent variables. According to the descriptors obtained in this QSAR study, the antifungal activity of phenols has a first action specific character which is based on their interaction with plasma or mitochondrial membranes. The second action is based on a steric descriptor-the maximal and minimal projection of the area-which could explain the inability of some phenolic compounds to be biotransformed to quinones methylene by Candida species.

CONCLUSIONS

According to the descriptors obtained in this QSAR study, the anti-Candida activity of ortho-substituted phenols is due to more than one action mechanism. The anti-Candida activity of phenolic compounds can be predicted by their molecular properties and structural characteristics.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results could be employed to predict the anti-Candida activity of new phenolic compounds in the search for new alternatives or complementary therapies to combat against candidiasis.