Rates Of Acid-catalyzed Reactions Research Articles

Maximum Impact of Ionic Strength on Acid-Catalyzed Reaction Rates Induced by a Zeolite Microporous Environment.

The intracrystalline ionic environment in microporous zeolite can remarkably modify the excess chemical potential of adsorbed reactants and transition states, thereby influencing the catalytic turnover rates. However, a limit of the rate enhancement for aqueous-phase dehydration of alcohols appears to exist for zeolites with high ionic strength. The origin of such limitation has been hypothesized to be caused by the spatial constraints in the pores via, e.g., size exclusion effects. It is demonstrated here that the increase in turnover rate as well as the formation of a maximum and the rate drop are intrinsic consequences of the increasingly dense ionic environment in zeolite. The molecularly sized confines of zeolite create a unique ionic environment that monotonically favors the formation of alcohol-hydronium ion complexes in the micropores. The zeolite microporous environment determines the kinetics of catalytic steps and tailors the impact of ionic strength on catalytic rates.

Fast predictions of liquid-phase acid-catalyzed reaction rates using molecular dynamics simulations and convolutional neural networks.

The rates of liquid-phase, acid-catalyzed reactions relevant to the upgrading of biomass into high-value chemicals are highly sensitive to solvent composition and identifying suitable solvent mixtures is theoretically and experimentally challenging. We show that the complex atomistic configurations of reactant–solvent environments generated by classical molecular dynamics simulations can be exploited by 3D convolutional neural networks to enable accurate predictions of Brønsted acid-catalyzed reaction rates for model biomass compounds. We develop a 3D convolutional neural network, which we call SolventNet, and train it to predict acid-catalyzed reaction rates using experimental reaction data and corresponding molecular dynamics simulation data for seven biomass-derived oxygenates in water–cosolvent mixtures. We show that SolventNet can predict reaction rates for additional reactants and solvent systems an order of magnitude faster than prior simulation methods. This combination of machine learning with molecular dynamics enables the rapid, high-throughput screening of solvent systems and identification of improved biomass conversion conditions.

Quantifying the Stability of the Hydronium Ion in Organic Solvents With Molecular Dynamics Simulations.

The solution-phase stability of the hydronium ion catalyst significantly affects the rates of acid-catalyzed reactions, which are ubiquitously utilized to convert biomass to valuable chemicals. In this work, classical molecular dynamics simulations were performed to quantify the stability of hydronium and chloride ions by measuring their solvation free energies in water, 1,4-dioxane (DIOX), tetrahydrofuran (THF), γ-valerolactone (GVL), N-methyl-2-pyrrolidone (NMP), acetone (ACE), and dimethyl sulfoxide (DMSO). By measuring the free energy for transferring a hydronium ion from pure water to pure organic solvent, we found that the hydronium ion is destabilized in DIOX, THF, and GVL and stabilized in NMP, ACE, and DMSO relative to water. The distinction between these organic solvents can be used to predict the preference of the hydronium ion for specific regions in aqueous mixtures of organic solvents. We then incorporated the stability of the hydronium ion into a correlative model for the acid-catalyzed conversion of 1,2-propanediol to propanal. The revised model is able to predict experimental reaction rates across solvent systems with different organic solvents. These results demonstrate the ability of classical molecular dynamics simulations to screen solvent systems for improved acid-catalyzed reaction performance.

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.

Proton proximity – New key parameter controlling adsorption, desorption and activity in propene oligomerization over H-ZSM-5 zeolites

The proximity of protons (∼5Å) derived from the distribution of Al atoms in the H-ZSM-5 framework has been shown to substantially affect the rate of propene oligomerization. Two sets of H-ZSM-5 zeolites (Si/Al 13–30) greatly differing in the occurrence of Al pairs (AlSiSiAl sequences in 6MRs) or distant Al atoms in the framework (both located prevailingly at the channel intersections) were synthesized to analyze the effect of the proton proximity. Propene adsorption and formation of oligomeric alkoxides is faster over distant isolated protons, while the close (5Å) positive charges of proximate protons result in faster olefin(s) release from H-bonded propene and oligomeric alkoxides. As the steady-state TOF of propene oligomerization to C4–C9 olefins at 473–533K over close protons exceeds 2–10 fold (with increasing Si/Al) those over isolated protons, the release of olefins from alkoxides is suggested to be an important reaction step. The distribution of framework Al atoms determining the mutual distances of protons represents a new parameter for evaluating rates of acid-catalyzed reactions and individual steps over H-zeolites.

Accelerated bimolecular reactions in microdroplets studied by desorption electrospray ionization mass spectrometry

Functional group derivatization reactions occur in the course of microdroplet/surface collisions in the ambient ionization process of desorption electrospray ionization (DESI). The unique environment in the microdroplet causes rate enhancements of as much as several orders of magnitude in typical bimolecular reactions that proceed through either cationic or anionic intermediates. The environment in the evaporating charged microdroplet differs from that of the bulk: (i) the pH of the solution moves towards the extremes, (ii) the concentrations of the reagents increase, (iii) the relative surface area increases and (iv) collision frequencies increase. The rates of acid-catalyzed reactions, such as the reaction of Girard T reagent with ketosteroids, increase with decreasing pH in positively-charged microdroplets compared to the bulk solution rates. Similarly, the increased pH in evaporating negatively-charged microdroplets contributes to an increase in the rates of base-catalyzed Michael reactions over those recorded under bulk solution conditions. The amount of product formed depends on the reaction time and the droplet size. Nanoelectrospray ionization generates larger droplets than the secondary droplets of DESI so it does not show significant product formation in the analysis period and can be used to analyze products of the DESI experiments. When secondary microdroplets (ca. 1 micron diameter) are generated either by spraying a homogeneous solution of both reagents against an inert surface (reactive DESI) or when a solution of Girard T reagent is sprayed against a solid surface bearing the ketosteroid significant amounts of product are generated. In the case of the Michael reaction with cinnamic acid an alternative dehydrogenated reaction product is formed under microdroplet conditions. Some parallels between the phenomenon reported here and the rate acceleration seen in sonochemistry are noted. The potential value of mass spectrometry in establishing conditions that enhance reaction rates is also indicated. It is possible that these observations will assist in the selection of reaction conditions involving the use of charged microdroplets to enhance the rates of ordinary bulk chemical reactions, especially those involving strong steric hindrance.

The origin and development of the acidity function

The acidity function is a thermodynamic quantitative measure of acid strength for non-aqueous and concentrated aqueous Bronsted acids, with acid strength being defined as the extent to which the acid protonates a base of known basicity. The acidity function, which was developed, both theoretically and experimentally, by Louis P. Hammett of Columbia University during the 1930s, has proven useful in the area of physical organic chemistry where it has been used to correlate rates of acid-catalyzed reactions and to quantitate the acidity of superacids, acids with protonating abilities greater than pure sulfuric acid. All Bronsted acids can now be compared using a common measure. Karl Popper’s seminal idea of theory falsification does not apply here because of the many successful applications of the acidity function. Likewise, Thomas Kuhn’s idea of a paradigm shift does not apply here, even though the acidity function concept was revolutionary, because the acidity function is commensurate with classical concepts of acidity.

Hydrolysis kinetics of biopolymers in subcritical water

The unique properties of water at elevated temperatures and pressures, namely the alterability of ionic product, dielectric constant, and density, make it an interesting and promising reaction medium. Several investigations have been made in order to obtain valuable products from biopolymers or, more general, biomass. Biopolymers can react with high-temperature water in very short residence times and with high rates of conversion. The achievable products are manifold and can be varied to a large extent by changing the operating conditions. The possibilities of sub- and supercritical water can even be significantly expanded by adding carbon dioxide since it reacts to carbonic acid, which serves as a catalyst. This work deals with the hydrolysis kinetics of different kinds of biopolymers, namely starch, cellulose (polysaccharides), and proteins (polypeptides). Kinetics was conducted over a broad range of experimental conditions using a continuous-flow reactor. In all three cases, the obtained experimental results could be described by a reaction model according to the approach of a single consecutive reaction following first order kinetics. The rate constants of the hydrolytic conversion were determined for the resulting monomers (glucose and amino acids), and the values were found to strongly depend on the type of bond. The peptide bonds in proteins exhibited a much higher stability compared to the β-1,4- and β-1,6-glycosidic linkages in cellulose and starch, respectively. The stability of the resulting monomers and their conversion to further degradation products were determined. The addition of carbon dioxide to water under hydrothermal conditions resulted in a significant increase in acid catalyzed reaction rates, which could be confirmed by the obtained rate constants.

Recent advances in acid- and base-catalyzed organic synthesis in high-temperature liquid water

High-temperature (200– 350 ∘ C) liquid water (HTW) is a promising reaction medium for conducting acid- and base-catalyzed organic synthesis reactions in an environmentally responsible manner. This article provides a summary of recent advances made concerning acid- and base-catalyzed organic synthesis in HTW. One advance is demonstrating that rates of acid-catalyzed reactions conducted in HTW can be accelerated while maintaining the solvent benignity by using CO 2 as an additive. A second advance is showing that additional commercially significant chemical products can be synthesized in HTW without catalyst. A third advance is demonstrating that product selectivity can be controlled by process variables such as temperature, water density, and heat-up time. A fourth advance is the emergence of mechanistic insight regarding acid- and base-catalyzed reactions in HTW. For example, we discuss the possibility that protons or hydroxide ions resulting from the dissociation of water may not be responsible for the occurrence of some classically acid- or base-catalyzed reactions in HTW without catalyst.

Intramolecular acid-catalyzed reactions of o-carbomethoxy-?-diazoacetophenones

1. A variety of new o-carbomethoxy-ω-diazoacetophenones and isochromane-1,4-diones were synthesized. 2. The decomposition of o-carbomethoxy-ω-diazoacetophenones in methanol-H2SO4, in constrast to reactions of their o-unsubstituted analogs, takes place via the A-SE2 mechanism. The intramolecular nucleophilic assistance of the o-CO2CH3 group increases the decomposition rate of ω-diazoacetophenones by 13–25 times. 3. A correlation was observed between the acid-catalyzed reaction rates of o-carbomethoxy-ω-diazoacetophenones and Hammet's σ constants for benzene ring substituents.

Phosphorus amides X. Rate acceleration due to solvation effects in the acid-catalyzed cleavage of phosphinamides. Application to acid-catalysis at enzymic active sites.

Phosphinamides undergo acid-catalyzed reaction with enhanced rates as the amount of water is decreased in acetonitrile from about 20 M to about 1 M. This phenomenon appears to be due to increased activity of acid and increased basicity of substrate as the concentration of water is decreased. The same effects may cause enhanced rates of acid-catalyzed reactions at enzymic active sites.

Solvation energies in acid catalyzed processes

Rates of acid catalyzed reactions have been correlated with various parameters, such H o or log c H + (Zucker-Hammett hypothesis), or log a H 2O (Bunnett's w and w* values and Yates's r parameter), in order to clarify whether the reaction mechanism may be classified as A-1 or A-2, or A SE-2. If one considers the general equations for acidcatalyzed processes: ▪ it becomes apparent that, after correlation for the amount of protonated substrate ([SH +]/[S] st), the variation in rates is caused only by variation in the activity coefficient ratios of the protonated substrate and the transition state. Therefore, this, rather than the classification into a defined mechanistic scheme, is the only information that can be obtained by studying the rate vs. acidity profiles. This piece of information is however of paramount importance since activity coefficients of the type reported in eqn. (3) represent the free energy of transfer of the indicated species from the reference aqueous state to the acid solutions, i.e. their solvation requirements. Since solvation of charged species in aqueous acid media depends on structural factors, such as the site protonation, the charge localization, etc., a knowledge of the solvation requirements of the transition state offers several hints on its structures, and hence on the reaction mechanism. Derivation of the analytic expression for the ratio ƒ SH + /ƒ ≠ and the results obtained in perchloric and sulphuric acid will be discussed for some exemplicative reactions.

The rates of acid-catalyzed reactions in water + deuterium oxide mixtures

The rates of acid-catalyzed reactions in water + deuterium oxide mixtures