Quantum-Chemical Study of Alkyl Carbenium Ions in 100% Sulfuric Acid

A quantum-chemical study of neutral and protonated monoalkyl sulfates RHSO₄and [RH₂SO₄]⁺(where R = CH₃, C₂H₅, iso-C₃H₇, and tert-C₄H₉) is carried out. Calculations are performed using the Hartree–Fock method in the 6-31G** and 6-31++G** basis sets taking into account electron correlation according to the Müller–Plesset perturbation theory MP2/6-31+G*//6-31+G*. Protonated tert-butyl sulfate was also calculated by the DFT B3LYP/6-31++G** method. It was found that monoalkyl sulfates are covalent compounds, and the complete abstraction of alkyl carbenium ions from them has substantial energy cost: 196.4, 161.7, 150.8 and 136.0 kcal/mol, respectively. Protonated methyl and ethyl sulfates are also covalent compounds according to the calculation. They have lower but still high energies of heterolytic dissociation (65.0 and 33.5 kcal/mol, respectively). The energy of R⁺abstraction from protonated isopropyl sulfate is much lower: 23.6 kcal/mol. The main covalent state and the ion–molecular pair, which is a carbenium ion [C(CH₃)₂H]⁺solvated by the H₂SO₄molecule, have about the same energy. The ground state of protonated tert-butyl sulfate corresponds to the ion–molecular complex [C(CH₃)⁺ ₃H₂SO₄] with still lower energy of carbenium ion [C(CH₃)₃]⁺abstraction, which is equal to 10.0 kcal/mol. Calculation according to the DFT B3LYP/6-31++G** method shows the absence of a minimum for the protonated tert-butyl sulfate with a covalent structure on the potential energy surface.     

Electroreduction of Molybdate Ions in the Presence of Hydrofluoric Acid

The electroreduction of molybdate ions in the presence of hydrofluoric acid was studied. The effective activation energy of the electroreduction of molybdate ions was determined using the Arrhenius equation.     

ChemInform Abstract: Quantum-Chemical Study of the Electronic Structure and Stability of Aluminum Fluorides.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

CC Bond Formation by Addition of Carbenium Ions to Alkenes: Kinetics and Mechanism

The addition of carbenium ions to CC double bonds, a key step in many syntheses in organic and macromolecular chemistry, is analyzed using the Lewis acid promoted reactions of alkyl chlorides with alkenes as an example. Stereochemical and kinetic experiments suggest that the transition state is slightly bridged and product-like. Rearrangements of the carbenium ions that result from the electrophilic attack can be minimized by adding salts with nucleophilic counter ions. The thermodynamics of the addition reactions are analyzed, and the conditions necessary in order to observe the back reaction (i.e. the Grob fragmentation) are discussed. Multiparameter equations that predict rate constants are derived from kinetic studies on the reactivities of carbenium ions and alkenes. Reactivity-selectivity relationships over a reactivity range that covers eight orders of magnitude show that the structure of the transition state is only changed by variation of substituents in the immediate vicinity of the reaction center.     

Nucleophilicity of Alkyl Zirconocene and Titanocene Precatalysts,and Kinetics of Activation by Carbenium Ions and by B(C6F5)3

Kinetics of activation of methyl and benzyl metallocene precatalysts by benzhydrylium ions, tritylium ions, and triarylborane B(C₆F₅)₃ were measured spectrophotometrically. The rate constants correlate linearly with the electrophilicity parameter E of the benzhydrylium and tritylium ions employed, allowing us to determine the σ‐nucleophilicities of the metal–carbon bond of several zirconocenes and titanocenes. Bridging, substitution, metal, and ligand effects on the rates of metal–alkyl bond cleavage (M=Zr, Ti) were studied and structure–reactivity correlations were used to predict the kinetics of generation of metallocenium ions pairs, which are active catalysts in polymerization reactions and are highly electrophilic Lewis acids in frustrated Lewis pair catalysis.     

Practical and General Method for Direct Synthesis of Alkyl Fluorides from Alcohols under Mild Conditions

Summary. A variety of alcohols were treated with Ph₃P and KF in CCl₄-DMF at room temperature to afford the corresponding fluorides in very good yields.     

Quantum-Chemical Study of the Adsorption of Bi3+ Ions on Au(111)

Russian Journal of Electrochemistry - The interaction between the Bi3+ ion and gold is studied using the cluster metal surface model and the density functional method. The geometric and energy...     

Practical and General Method for Direct Synthesis of Alkyl Fluorides from Alcohols under Mild Conditions

A variety of alcohols were treated with Ph₃P and KF in CCl₄-DMF at room temperature to afford the corresponding fluorides in very good yields.     

Effect of the Electronic Structure of Carbenium and Silylium Ions on Their Chemical Behavior

Being isoelectronic analogs, silylium and carbenium ions exhibit quite a different reactivity toward nucleophiles. This is explained by their different electronic structures and charge distributions: In silylium ions the positive charge is almost completely concentrated on the silicon atom, and hydrogen atoms on the cationic center are hydride in nature, wheteas in carbenium ions, the positive charge is uniformly distributed between the carbenoid center and hydrogen atoms.     

Electrophoretic Mobility Study of the Adsorption of Alkyl Xanthate Ions on Galena and Sphalerite

The adsorption of ethyl and amyl xanthate ions on galena and sphalerite fines has been studied using electrophoretic light-scattering (ELS) measurements. It was performed on galena and sphalerite (<2&mgr;m) in aqueous solution at different potassium ethyl xanthate (PEX) and potassium amyl xanthate (PAX) concentrations. It has been observed that the presence of PEX or PAX caused the isoelectric points (IEP) of galena and sphalerite fines to shift and the electrophoretic mobility to reverse in sign, indicating that the xanthate ions chemisorbed on galena and sphalerite surfaces. This adsorption markedly broadened the electrophoretic mobility distribution of the mineral fines, suggesting that the populations of the particles have quite different adsorption densities of xanthate ions, and therefore the particle hydrophobicity was different. This phenomenon might be attributable to the effect of the hemimicelle adsorption of the xanthate ions on the minerals, the nonuniform distribution of active sites and their degree of activity, the effect of particle size and shape, etc. The nonuniform adsorption has been found to increase with increasing PEX or PAX concentration, reaching a maximum at a medium concentration followed by a decline. Also, experimental results have demonstrated that the nonuniform adsorption of the xanthate ions is much stronger on sphalerite than on galena, which may explain why sphalerite has a worse flotation response than galena when alkyl xanthates are used as collectors in flotation systems. Copyright 2001 Academic Press.     

Radical Nature of Pathways to Alkene and Ester from Thermal Decomposition of Primary Alkyl Diacyl Peroxide

Thermal decomposition of a primary alkyl diacyl peroxide 2 is investigated. Dependence of product yields on temperature, viscosity, and solvent polarity is examined in a variety of media. The excess of the alkene disproportionation product 4 and the presence of ester 3 and acid 5 is argued to demonstrate the existence of a discrete acyloxy-alkyl geminate radical pair. Stereoselective deuterium labeling of 2 and subsequent (1)H-NMR analysis of the resulting isotopomers of 4 confirm the radical nature of detected decomposition products.     

Kinetics and Mechanism of Oxidation of l‐Histidine by Permanganate Ions in Sulfuric Acid Medium

The reaction kinetics for the oxidation of l ‐histidine by permanganate ions have been investigated spectrophotometrically in sulfuric acid medium at constant ionic strength and temperature. The order with respect to permanganate ions was found to be unity and second in acid concentration, whereas a fractional order is observed with respect to histidine. The reaction was observed to proceed through formation of a 1:1 intermediate complex between oxidant and substrate. The effect of the acid concentration suggests that the reaction is acid catalyzed. Increasing the ionic strength has no significant effect on the rate. The influence of temperature on the rate of reaction was studied. The presence of metal ion catalysts was found to accelerate the oxidation rate, and the order of effectiveness of the ions was Cu²⁺ > Ni²⁺ > Zn²⁺. The final oxidation products were identified as aldehyde (2‐imidazole acetaldehyde), ammonium ion, manganese(II), and carbon dioxide. Based on the kinetic results, a plausible reaction mechanism is proposed. The activation parameters were determined and discussed with respect to a slow reaction step.     

Evaluation of Acid Dissociation Constants in DMSO and DMF by Quantum-Chemical Methods

We have calculated total electronic energies (E) and Gibbs energies (G) of a large number of acids and their anions in water, dimethylsulfoxide, and dimethylformamide using the hybrid B3LYP functional DFT method in the 6-31++G(d,p) basis set, taking into account the solvent effect by the conductor-like polarizable continuum model method. A linear correlation has been found between the experimental values of acid dissociation constants (pK_a) of different nature and the difference between anion and acid E values, and between pK_a and the difference between anion and acid G values. The obtained correlations allowed us to evaluate the pK_a values of both inorganic and organic acids. Such an evaluation is of special importance for nonaqueous solvents as it is quite problematic to determine these dissociation constants.     

Quantum-Chemical Study of Organophosphorus Ligands Coordinated to Chromium and Nickel Carbonyls

A comparative analysis of the electronic structure of free and coordinated nitrogen- and phosphorus-containing ligands was made. The geometric structures of NR3 and PR3 ligands and carbonyl complexes Cr(CO)₅(XR3) and Ni(CO)₃(XR3) (X = N or P) were optimized by quantum-chemical calculations. Replacement of H atoms in the coordinated NH₃ molecule by organic radicals R decreases the energy of the M-NR3 bond, and replacement of H atoms in PH₃ increases the ligand binding energy. A correlation was found between the metal-phosphine bond energy and the extent of the electron density transfer from the ligand. According to the calculations, the -acceptor power of triphenylphosphine is associated with the population of ^* orbitals of the aromatic system, rather than of phosphorus d orbitals.     

Reactions of Charged Substrates. 6. The Methoxymethyl Carbenium Ion Problem. 1. A Semiempirical Study of the Kinetic and Thermodynamic Stabilities of Linear and Cyclic Oxo- and Thiocarbenium Ions Generated from Pyridinium and Dimethylanilinium Ions

AM1-calculated energy profiles for dissociation of (methoxymethyl)pyridinium and dimethylanilinium ion substrates show that the methoxymethyl carbenium ion is not sufficiently stable to exist as an intermediate on the reaction coordinate for this model reaction. [(Thiomethoxy)methyl]pyridinium ion, however, has a distinct transition state because of the stability of the resulting ion-neutral complex. The complete potential energy surfaces for water displacement on the methoxymethyl substrate with either pyridine or dimethylaniline as the leaving group show distinct transition states and very flat surfaces for the ion-neutral complexes in which interaction of the carbenium ion with both leaving group and nucleophile is stabilizing. Secondary systems studied, including linear methoxy and thiomethoxy substrates, 5- and 6-membered cyclic oxo and thio substrates, and ribosyl-, xylopyranosyl-, and glucopyranosylpyridinium ions yield ion-neutral complexes with sufficient intrinsic stability to exist as intermediates. Comparison with solution data, primarily activation entropy and Br?nsted coefficients, suggests that the sugar oxocarbenium ions, either as distinct, solvent-equilibrated intermediates or elements of ion-neutral complexes, are formed by unimolecular dissociation of the respective substrates in solution.