Efficient Tosylation of Dihydroxybenzenes in a Two-Phase Water–Dichloromethane System

Theoretical and Experimental Chemistry - The effect of the structure of the catalyst and the type of base on the reaction rate and product yield of the tosylation of dihydroxybenzenes in a...     

IR spectra of selenophenopyrylium salts

On the basis of the IR spectra of 16 alkyl-substituted selenopheno[2,3-c]- and selenopheno-[3,2-c]pyrylium salts and a model calculation of the frequencies and the forms of their normal vibrations, an assignment of the frequencies of the skeletal vibrations of the heterocycles is put forward.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1495–1497, November, 1973.     

Studies of reaction heats and structure of complexes formed between strong N-bases and phenols

The heats of reactions between various phenols and two strong N-bases of guanidine-like character in acetonitrile, are determined. The values can be used as a measure of self-assembly abilities of the phenol molecules in the interactions with strong N-bases, playing a very important role in biological systems. In the case of TBD complexes with corresponding nitrophenols, the protonated N-base is hydrogen-bonded to the nitro group excluding the self-assembly process of the phenols. In the case of other phenols, the self-assembly abilities are dependent on pK_a values of phenols. With increasing acidity of phenols their ability to form the hydrogen-bonded chains decreases. The maximum of length of the chains is observed for 4-methylphenol, which has a similar pK_a value to that in the tyrosine residue in biological systems.     

Effect of the Structure of the Acyl Group on the Kinetics of Exchange between O-Nucleophiles

We have studied the rate of 15 reactions of acyl transfer from O-acyl salts of 4-dimethylaminostyryl-4-pyridine N-oxide to 4-morpholinopyridine and 4-dimethylaminopyridine N-oxides in acetonitrile solutions. Analysis of the results based on the Shaik – Pross approach and the Marcus equation shows that if the structure of the acyl group is varied, then the reactivity is determined by such parameters as the resonance interaction in the transition state (B) or the internal barrier (G ₀) of the reaction.     

Loss of isocyanic acid from the internal oxadiazole ring of protonated molecules of some 2,5-diaryl-1,3,4-oxadiazoles

The fragmentation pattern of some protonated 2,5-diaryl-1,3,4-oxadiazoles is discussed. An unusual decomposition consisting of elimination of the isocyanic acid molecule from the internal oxadiazole ring was found. This fragmentation pathway was deduced on the basis of B/E linked scan mass spectra of metastable ions with liquid secondary ion mass spectrometry as the ionization method and also of low-energy CID mass spectra where electrospray was used as the ionization technique. High resolution measurements were also performed.     

Rate and Equilibrium Constants of Dimethylcarbamoyl Transfer between Pyridine N-Oxides

Dimethylcarbamoyl transfer from N-acyloxypyridinium salts to pyridine N-oxides in acetonitrile occurs in one stage by the forced concerted S_N2 mechanism. The rate and equilibrium of the reaction are fairly described by the Brönsted equation. The Marcus equation provides a much higher quality of reactivity predictions.     

Kinetics of acyl exchange of series of N- and O-acyl salts with 4-(p-dimethylamino)styrylpyridine-N-oxide

A study has been mode of the reaction of acyl exchange between a series of N- or O-acyl salts and 4-(p-dimethylamino)styrylpyridine-N-oxide in acetonitrile solution. Values of the rate constant (k₂) and activation characteristics (E_a, G, S, and H) have been obtained for these systems.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 31, No. 2, pp. 86–89, March–April, 1995.This work was performed with financial support from the State Committee of the Ukraine for Matters of Science and Technology.     

Peculiarities of the effect of the electronic nature of substituents on the basic properties of aliphatic amines

Changes in the hybrid state of atomic orbitals of nitrogen and p-character of LEP, which occur under the effect of saturated hydrocarbon radicals and polar substituents in aliphatic amines, differ substantially. Therefore, the effect of the both substituents on the basicity constants of amines cannot be described by the single formal type of interaction. The anomalous changes in the basicity in the series of primary, secondary, and tertiary alkylamines, which are discussed in the literature, and the correlations pK _BH+ = f(*) and G _B = f(*) are, in fact, imaginary, because the alkyl radicals at the N atom do not manifest the electron-donor properties.     

Acetyl Exchange between Pyridine N-Oxides in Acetonitrile Solutions: An Attempt to Apply the Marcus Equation to Acetyl Transfer

Forty-three (including eight identical) reactions of acetyl transfer from N-acetyloxypyridinium salts to pyridine N-oxides in acetonitrile solutions were studied. The rate constants k ₂ vary in the range 10⁷-10^{- 1} l mol^{- 1} s^{- 1}; the equilibrium constants K, in the range 10⁷-10^{- 7}; the activation enthalpy H , in the range 17-30 kJ mol^{- 1}; the activation entropy -S , in the range 60-85 J mol^{- 1} K^{- 1}; and the heat of reaction -H ⁰, within ±50 kJ mol^{- 1}. All reactions occur in a single stage by the concerted SN2 mechanism with a low degree of bond cleavage in the transition state. The rate and equilibrium of the acetyl exchange are satisfactorily described by the Brönsted equation. The quality of predicting the reactivity is substantially improved by introducing into the correlation equation a second parameter, the rates of identical reactions.     

Molecular spectroscopy of symmetric fragmentary exchange in N-oxypyridinium salts

A number of N-acetyloxypyridinium salts has been studied, using molecular spectroscopy methods. These compounds exist in solutions of bipolar solvents as ions and ion pairs of various structure. The association to ion pairs effects the frequences and intensities of characteristic vibrations, the chemical shifts of proton peaks, and the reaction rate of symmetric exchange of acetyl groups. Based on the spectrochemical correlations, data on the structure of the salts, the acetyl exchange reaction mechanism, and the influence of the solvent nature, the reagent structures, the additions of the base electrolyte and crown-ether on its kinetic characteristics have been obtained.