Solvent Kinetic Effect Research Articles

Mechanism and solvent kinetics of acid catalysed Hydrolysis of Propyl Methanoate in Binary-Solvent system of water and Propanol-2

The kinetic solvent effect on the rate of reaction for the acid catalysed hydrolysis of propyl methanoate has been studed in different compositions (30 to70%) of aqueous mixture of prpanol-2(v/v) over the different ranges of temperature(20 to 400 c). The reaction followed second order kimnetics. With addition of solvent the rate of reaction decreases progressively. The energy of activation (Ec) is calculated with the help of Arrhenius plots with reciprocal of absolute temperature. Linear relation has been observed for the plots of logk against different dielectric values of propanol-2. The effect of solvent on thermodynamic parameter has also been discussed and calculated with wynne Jones and Eyring equation. The nature of solvent-solute interaction has been explained by calculation Iso-kinetic temperature.

Computing Kinetic Solvent Effects and Liquid Phase Rate Constants Using Quantum Chemistry and COSMO-RS Methods.

Many industrially and environmentally relevant reactions occur in the liquid phase. An accurate prediction of the rate constants is needed to analyze the intricate kinetic mechanisms of condensed phase systems. Quantum chemistry and continuum solvation models are commonly used to compute liquid phase rate constants; yet, their exact computational errors remain largely unknown, and a consistent computational workflow has not been well established. In this study, the accuracies of various quantum chemical and COSMO-RS levels of theory are assessed for the predictions of liquid phase rate constants and kinetic solvent effects. The prediction is made by first obtaining gas phase rate constants and subsequently applying solvation corrections. The calculation errors are evaluated using the experimental data of 191 rate constants that comprise 15 neutral closed-shell or free radical reactions and 49 solvents. The ωB97XD/def2-TZVP level of theory combined with the COSMO-RS method at the BP-TZVP level is shown to achieve the best performance with a mean absolute error of 0.90 in log10(kliq). Relative rate constants are additionally compared to determine the errors associated with the solvation calculations alone. Very accurate predictions of relative rate constants are achieved at nearly all levels of theory with a mean absolute error of 0.27 in log10(ksolvent1/ksolvent2).

Unexpected Kinetic Solvent Effects Enhance Activity and Selectivity in Biphasic Systems

Biphasic dehydration of fructose to 5-hydroxymethylfurfural (HMF) has shown unprecedented increases in productivity, but a mechanistic understanding is lacking. Herein, we couple fast experimental reaction kinetics, multiscale modeling (phase behavior, classical molecular dynamics(MD), and quantum mechanics/molecular mechanics MD), in situ sampling, and IR and 13C-NMR spectroscopy to elucidate the complex effects of nonpolar extracting organic solvents on the kinetics of fructose dehydration. We show that these organic solvents can reach significant mutual solubility with water at reaction temperatures, enabling the partition of the sugar and catalyst into the extracting phase. In the organic-rich environment, the dehydration of fructose proceeds faster and more selectively than in water due to increased relative abundance of the reactive furanose isomer, enhanced water–catalyst–substrate interactions driven by nanophase separation, and higher product stability stemming from preferential solvation. We demonstrate that these solvent effects impact other critical biphasic reactions in biomass upgrading and provide qualitative principles for solvent selection.

Kinetic Study of Solvent Effect on the Hydrolysis of Mono-3, 5-Dimethylaniline Phosphate

The hydrolysis of phosphate esters is one of the most fundamental chemical and biochemical reaction. The kinetic solvent effect on the hydrolysis of mono-3, 5-dimethylaniline phosphate has been studied in aqueous mixtures of varying compositions (0-40% v/v) of some protic and aprotic solvents at four different temperatures. The rate of reactions increases with increasing proportion of solvents. Activation parameters (Ea, ΔH≠, ΔG≠, -ΔS≠) have been evaluated. The significance of these parameters have been explained on the basis of solvent-solute interaction, solvent of the transition state of the medium.

An Expanded SET Model Associated with the Functional Hindrance Dominates the Amide-Directed Distal sp3 C-H Functionalization.

The mechanistic understanding of catalytic radical reactions currently lags behind the flourishing development of new types of catalytic activation. Herein, an innovative single electron transfer (SET) model has been expanded by using the nonadiabatic crossing integrated with the rate-determining step of 1,5-hydrogen atom transfer (HAT) reaction to provide the control mechanism of radical decay dynamics through calculating excited-state relaxation paths of a paradigm example of the amide-directed distal sp3 C-H bond alkylation mediated by Ir-complex-based photocatalysts. The stability of carbon radical intermediates, the functional hindrance associated with the back SET, and the energy inversion between the reactive triplet and closed-shell ground states were verified to be key factors in improving catalytic efficiency via blocking radical inhibition. The expanded SET model associated with the dynamic behaviors and kinetic data could guide the design and manipulation of visible-light-driven inert bond activation by the utilization of photocatalysts bearing more or less electron-withdrawing groups and the comprehensive considerations of kinetic solvent effects and electron-withdrawing effects of substrates.

Hydrogen Atom Transfer from HOO. to ortho-Quinones Explains the Antioxidant Activity of Polydopamine.

Melanins are stable and non‐toxic biomaterials with a great potential as chemopreventive agents for diseases connected with oxidative stress, but the mechanism of their antioxidant action is unclear. Herein, we show that polydopamine (PDA), a well‐known synthetic melanin, becomes an excellent trap for alkylperoxyl radicals (ROO., typically formed during autoxidation of lipid substrates) in the presence of hydroperoxyl radicals (HOO.). The key reaction explaining this peculiar antioxidant activity is the reduction of the ortho‐quinone moieties present in PDA by the reaction with HOO.. This reaction occurs via a H‐atom transfer mechanism, as demonstrated by the large kinetic solvent effect of the reaction of a model quinone (3,5‐di‐tert‐butyl‐1,2‐benzoquinone) with HOO. (k=1.5×107 and 1.1×105 M−1 s−1 in PhCl and MeCN). The chemistry disclosed herein is an important step to rationalize the redox‐mediated bioactivity of melanins and of quinones.

Hydrogen Atom Transfer from HOO. to ortho‐Quinones Explains the Antioxidant Activity of Polydopamine

AbstractMelanins are stable and non‐toxic biomaterials with a great potential as chemopreventive agents for diseases connected with oxidative stress, but the mechanism of their antioxidant action is unclear. Herein, we show that polydopamine (PDA), a well‐known synthetic melanin, becomes an excellent trap for alkylperoxyl radicals (ROO., typically formed during autoxidation of lipid substrates) in the presence of hydroperoxyl radicals (HOO.). The key reaction explaining this peculiar antioxidant activity is the reduction of the ortho‐quinone moieties present in PDA by the reaction with HOO.. This reaction occurs via a H‐atom transfer mechanism, as demonstrated by the large kinetic solvent effect of the reaction of a model quinone (3,5‐di‐tert‐butyl‐1,2‐benzoquinone) with HOO. (k=1.5×107 and 1.1×105 M−1 s−1 in PhCl and MeCN). The chemistry disclosed herein is an important step to rationalize the redox‐mediated bioactivity of melanins and of quinones.

Kinetic solvent effects in organic reactions

AbstractThis article reviews prior work studying reaction kinetics in solution, with the goal of using this information to improve detailed kinetic modeling in the solvent phase. Both experimental and computational methods for calculating reaction rates in liquids are reviewed. Previous studies, which used such methods to determine solvent effects, are then analyzed based on the reaction family. Many of these studies correlate kinetic solvent effect with one or more solvent parameters or properties of reacting species, but it is not always possible, and investigations are usually done on too few reactions and solvents to truly generalize. From these studies, we present suggestions on how best to use data to generalize solvent effects for many different reaction types in a high throughput manner.

Vanillin Hydrodeoxygenation: Kinetic Modelling and Solvent Effect

Vanillin hydrodeoxygenation was investigated using Pt/C catalyst in the temperature and total pressure ranges of 80–200 °C and 20–30 bar in several solvents, such as tetrahydrofuran, 2-propanol, water and in solventless conditions using 1:1 mass ratio of vanillin to guaiacol. The results revealed that the rate increased with increasing solvent polarity as follows: tetrahydrofuran < 2-propanol < water. The main product was p-creosol with 66% selectivity at complete vanillin conversion in HDO under 30 bar total pressure at 100 °C after 4 h using water as a solvent. In a solventless experiment with 1:1 mass ratio of vanillin–guaiacol as a feedstock only vanillin was transformed to p-creosol with 91% conversion in 4 h at 200 °C under 30 bar total pressure, while guaiacol did not produce any HDO products. Both thermodynamic analysis and kinetic modelling were performed. Vanillin hydrodeoxygenation resulted in formation of p-creosol over Pt/C catalyst using an optimum vanillin initial concentration in water solution. From the industrial point of view vanillin hydrodeoxygenation proceeded rapidly giving high yields of p-creosol in solventless hydrodeoxygenation of vanillin-guaiacol mixture, while guaiacol was not deoxygenated.

Correction: Universal kinetic solvent effects in acid-catalyzed reactions of biomass-derived oxygenates

Correction for ‘Universal kinetic solvent effects in acid-catalyzed reactions of biomass-derived oxygenates’ by Theodore W. Walker et al., Energy Environ. Sci., 2018, 11, 617–628.

Universal kinetic solvent effects in acid-catalyzed reactions of biomass-derived oxygenates

Experiments and molecular simulations are combined to understand organic solvent effects, enabling prediction of acid-catalyzed reaction rates for biomass conversion.

Kinetic solvent effects in the reduction of alkyl halides by {Sm[N(SiMe3)2]2(THF)2}

The rate of reduction of two representative alkyl halides, 1-bromododecane and 1-chlorododecane by {Sm[N(SiMe3)2]2(THF)2} was examined in five different solvents. Reductions were several orders of magnitude faster in nonpolar, non-coordinating solvents than in polar coordinating solvents. Good correlations between the rate of reduction and coordination index of each solvent was obtained whereas excellent correlations were obtained between rates and dielectric (as a measure of solvent polarity). The basis of this effect is proposed to be a consequence of solvent coordination to the inner sphere of Sm(II) leading to a deceleration of ET in coordinating solvents and possible transition state stabilization by nonpolar solvents.

Potential antioxidants for biodiesel from a softwood lignin pyrolyzate

Softwood lignin was pyrolyzed to afford a “bio-oil” distillate in which phenols were major products. Extraction with alkali gave a range of lignin-related phenols having molecular weights (MWs) from 110 to 344amu. The total phenolic extract was at least equal in antioxidant activity to the synthetic compound 2,6-di(tert)butyl-4- methylphenol (BHT). The extract was separated by chromatographic techniques to characterize components potentially responsible for the activity. A series of phenols in the extract exhibited MWs of 274, 302, 316, 330, and 344 and are likely to be dimers responsible for the majority of the observed antioxidant properties. Several of these compounds appear by mass spectrometry to be bifunctional phenolic compounds containing catechol or guaiacol groups. We hypothesize that a type of effective antioxidant for fatty acid esters will contain one phenolic hydroxyl group hydrogen-bonded to the ester and another at a radical-quenching site; monophenols are likely to be ineffective in this system due to the predominance of hydrogen bonding (kinetic solvent effect).

Role of Hydrogen Bonding in Photoinduced Electron-Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand.

Electron-proton transfer (EPT) from phenols to a triplet metal-to-ligand charge transfer (MLCT)-excited Ru polypyridine complex containing an uncoordinated nitrogen site, 1(T), can be described by a kinetic model that accounts for the H-bonding of 1(T) to phenol, 1(T) to solvent, and phenol to solvent. The latter plays a major role in the kinetic solvent effect and commonly precludes simultaneous determination of the EPT rate constant and 1(T)-phenol H-bonding constant. A number of these quantities previously reported for similar systems are shown to be in error due to inconsistent data analysis. Control experiments replacing either 1(T) by its structural isomer with a sterically screened nitrogen site or phenol by its H-bonding surrogate, trifluoroethanol, and the observation of negative activation enthalpies for the overall reactions between 1(T) and phenols lend support to the proposed model and provide evidence for the formation of a precursor H-bonded complex between the reactants, which is a prerequisite for EPT.

A comparative study on the hydrolysis of acetic anhydride and N,N-dimethylformamide: Kinetic isotope effect, transition-state structure, polarity, and solvent effect

A comparative study on the hydrolysis of acetic anhydride and N,N-dimethylformamide: Kinetic isotope effect, transition-state structure, polarity, and solvent effect

Hydroperoxyl Radicals (HOO. ): Vitamin E Regeneration and H-Bond Effects on the Hydrogen Atom Transfer.

Hydroperoxyl (HOO. ) and alkylperoxyl (ROO. ) radicals show a different behavior in H-atom-transfer processes. Both radicals react with an analogue of α-tocopherol (TOH), but HOO. , unlike ROO. , is able to regenerate TOH by a fast H-atom transfer: TO. +HOO. →TOH+O2 . The kinetic solvent effect on the H-atom transfer from TOH to HOO. is much stronger than that observed for ROO. because noncovalent interactions with polar solvents (Solv⋅⋅⋅HOO. ) destabilize the transition state.

Reactivity of (Pyridine-Diimine)Fe Alkyl Complexes with Carbon Dioxide

The reaction of CO2 with (PDI)FeMe (1), (PDI)Fe(Me)PMe3 (1-PMe3) and [(PDI)FeMe][BPh4] (2, PDI = 2,6-(2,6-iPr2–C6H3–N═CMe)2–C5H3N) generates (PDI)FeOAc (3), (PDI)Fe(OAc)PMe3 (3-PMe3), and [(PDI)FeOAc][BPh4] (4), respectively. Kinetic data and solvent effects provide evidence that these reactions occur by precoordination of CO2 to the Fe center regardless of the charge state and thus favor an insertion mechanism for carboxylation. Carboxylation of 1-PMe3 requires initial dissociation of PMe3 to generate 1, which reacts with CO2; 1-PMe3 itself does not react directly with CO2. CO2 reacts 5 times faster with neutral 1 than with cationic 2 (at 0 °C), which is ascribed to the higher nucleophilicity of the Fe–Me group in 1.

A Continuous Visible Light Spectrophotometric Approach To Accurately Determine the Reactivity of Radical-Trapping Antioxidants.

Inhibited autoxidations-monitored either by O2 consumption or hydroperoxide formation-are the most reliable way to obtain kinetic and stoichiometric information on the activity of radical-trapping antioxidants (RTAs). While many comparatively simple "antioxidant assays" (e.g., the DPPH assay) have supplanted these in popularity, they are generally very poor substitutes since they often do not employ peroxyl radicals as the oxidant and do not account for both the kinetics and stoichiometry of the radical-trapping reaction(s). In an effort to make inhibited autoxidations as simple as the most popular "antioxidant assays", we have developed a spectrophotometric approach for monitoring reaction progress in inhibited autoxidations. The approach employs easily prepared 1-phenylbutadiene-conjugated or styrene-conjugated BODIPY chromophores (PBD-BODIPY or STY-BODIPY, respectively) as signal carriers that co-autoxidize along with a hydrocarbon substrate. We show that inhibition rate constants (kinh) are accurately determined for a range of phenolic and diarylamine RTAs using this approach and that mechanistic experiments, such as kinetic isotope effects and kinetic solvent effects, are equally easily carried out. Moreover, synergistic interactions between RTAs, as well as the unconventional activity of diarylamine RTAs, are captured using this methodology. Lastly, we show that the approach can be employed for monitoring reactions in aqueous solution.

Use and Abuse of the DPPH(•) Radical.

The 2,2-diphenyl-1-picrylhydrazyl (DPPH(•)) radical is approaching 100 years from its discovery in 1922 by Goldschmidt and Renn. This radical is colored and remarkably stable, two properties that have made it one of the most popular radicals in a wide range of studies. First, there is the evaluation of the antioxidant abilities of phenols and other natural compounds (A-H) through a "test" that-at a closer look-is utterly inappropriate. In fact, the test-derived EC50, that is, the concentration of A-H able to scavenge 50% of the initial DPPH(•), is not a kinetic parameter and hence its purported correlation with the antioxidant properties of chemicals is not justified. Kinetic measurements, such as the second-order rate constants for H-atom abstraction from A-H by DPPH(•), in apolar media, are the only useful parameters to predict the antioxidant ability of A-H. Other applications of DPPH(•) include kinetic and mechanistic studies, kinetic solvent effects, EPR spectroscopy, polymer chemistry, and many more. In this review these applications are evaluated in detail by showing the usefulness of some and the uselessness of others. The chemistry of DPPH(•) is also briefly reviewed.

Kinetic aspects of [3+2] cycloaddition reactions between (E)-3,3,3-trichloro-1-nitroprop-1-ene and ketonitrones

Kinetic measurements show that despite their polar nature, cycloaddition reactions of (E)-3,3,3-trichloro-1-nitroprop-1-ene with ketonitrones take place according to a mechanism that proceeds without intervention of zwitterionic intermediate. This is indicated by the values of activation parameters (activation enthalpy, activation entropy) and the nature of the kinetic solvent effect.