Thin Layer Chromatographic Separation of Ortho,Meta, and Para Substituted Benzoic Acids and Phenols with Aqueous Solutions of α-Cyclodextrin

Abstract

The use of aqueous solutions of α-cyclodextrin as the mobile phase in thin layer chromatography (TLC) is described. A series of eighteen substituted benzoic acid compounds were chromatographed on polyamide thin layer sheets. The R_f values were dependent on the concentration of α-cyclodextrin in the mobile phase as well as the structure and size of the individual molecules. Possible advantages of this technique over those which use pure or mixed solvent systems as the mobile phase are discussed.     

Zur Dünnschicht-Chromatographie von zinnorganischen Verbindungen

With the aid of thinlayer chromatography mixtures of tetraorganotin compounds containing no polar groups can be separated and identified if they differ in the number of phenyl groups per molecule or in the nature of their aliphatic residues (alkyl or allyl). For analytical purposes mixtures of 80% hexane and 20% benzene by volume are suitable as mobile phase. For preparative purposes (requiring multiple development) pure hexane or hexane containing 10 to 20% benzene by volume are best suited as mobile phase, where the amount of hexane depends on the problem of separation at hand. The R_f values of the mobile phases vary linearly with the number of the phenyl groups x and decrease with increasing x. The straight lines connecting the R_f values cross at x=4 (tetraphenyltin). Dithizone and silver nitrate are suitable spray reagents for organotin compounds. By spraying the plates with dithizone tetra-, tri-, and di-organotin compounds can be immediately distinguished on the basis of the ensuing colourations. With silver nitrate allyltin compounds and polystannanes can be distinguished from alkyltin compounds; tetra-phenyltin is not coloured by this reagent.     

Electrogeneration of peroxodicarbonate anion: Synthesis of tetraalkylammonium silylcarboxylate

Oxygen electroreduction in the presence of CO₂ in acetonitrile solutions of tetraalkylammonium salts is a means to produce tetraalkylammonium peroxodicarboxylates. The latter react with triorganosilanes R₃SiH to yield tetraalkylammonium salts of triorganylsilylcarboxylic acids. Diorganylsilanes under the same conditions produce cyclic siloxanes exclusively.     

Salt Effects on Reactivity of Some Fe(II)-Azo Complexes Catalyzing Disproportionation of Hydrogen Peroxide

Summary. Salt effects on kinetics of oxidation and decomposition of novel low-spin Fe(II) complexes of azo amino acids with hydrogen peroxide have been investigated in aqueous medium. The ligands are derived from amino acids (DL-phenylalanine, DL-tryptophane, histidine, and alanine) and p-nitroso aromatic substituted amines (N,N-dimethyl-4-nitrosoaniline and N,N-diethyl-4-nitrosoaniline). Hydrophobic salts of alkali metal and tetraalkylammonium halides, e.g., KBr, tetrabutylammonium bromide (TBAB), tetraethylammonium bromide (TEAB), and tetramethylammonium bromide (TMAB) have been used. Dilute salt solutions exhibit little effects on the reactivities of oxidation and decomposition of the title compounds. This behaviour would be ascribed to a decrease in the activity coefficients of the reacting species in these solutions. Moreover, these effects support the reported pathway in these reactions occurring via the association of dicationic complexes and undissociated H₂O₂. On the other hand, higher salt concentration shows considerable effects on the reactivities. This behaviour is explained in terms of hydrophilicity of the complexes, ion pair formation, salting out, and substituent effects. Hyrophilic nature of the recent complexes enhances reactivity with increasing [KBr] due to salting out effects in the aqueous medium. Increasing [TBAB] retards reactivity via ion pair formation with the reactants. In the presence of R₄N⁺ ions, the reactivity increases with changing R in the following order; R = C₄H₉ < C₂H₅ < CH₃. The bulky R groups in the added tetraalkylammonium salts, exert medium steric hindrance against the approach of reactants.     

Salt Effects on Reactivity of Some Fe(II)-Azo Complexes Catalyzing Disproportionation of Hydrogen Peroxide

Salt effects on kinetics of oxidation and decomposition of novel low-spin Fe(II) complexes of azo amino acids with hydrogen peroxide have been investigated in aqueous medium. The ligands are derived from amino acids (DL-phenylalanine, DL-tryptophane, histidine, and alanine) and p-nitroso aromatic substituted amines (N,N-dimethyl-4-nitrosoaniline and N,N-diethyl-4-nitrosoaniline). Hydrophobic salts of alkali metal and tetraalkylammonium halides, e.g., KBr, tetrabutylammonium bromide (TBAB), tetraethylammonium bromide (TEAB), and tetramethylammonium bromide (TMAB) have been used. Dilute salt solutions exhibit little effects on the reactivities of oxidation and decomposition of the title compounds. This behaviour would be ascribed to a decrease in the activity coefficients of the reacting species in these solutions. Moreover, these effects support the reported pathway in these reactions occurring via the association of dicationic complexes and undissociated H₂O₂. On the other hand, higher salt concentration shows considerable effects on the reactivities. This behaviour is explained in terms of hydrophilicity of the complexes, ion pair formation, salting out, and substituent effects. Hyrophilic nature of the recent complexes enhances reactivity with increasing [KBr] due to salting out effects in the aqueous medium. Increasing [TBAB] retards reactivity via ion pair formation with the reactants. In the presence of R₄N⁺ ions, the reactivity increases with changing R in the following order; R = C₄H₉ < C₂H₅ < CH₃. The bulky R groups in the added tetraalkylammonium salts, exert medium steric hindrance against the approach of reactants.     

The paramagnetic species produced in the reactions of organometallic compounds XIII—the anion radicals of carboxylates

In the reactions of meta- and para-alkyl substituted benzoic acids with aryl Grignard reagents, in tetrahydrofuran solutions and in the presence of nickel salts, the ESR spectra of the anion radicals of carboxylates derived from the benzoic acids were observed which represent the intermediates in the formation of ketyl radicals.     

Ion-pair chromatography with UV absorbing counterions on reversed-phase layers. Part 1: Chromatography with hydroxynaphthalenesulfonic acids

The results of ion-pair chromatography with UV-absorbing counterions are described. Hydroxynaphthalenesulfonic acids are used as ion pair formers, and alkaloids as test samples. The separations are performed on silanized silica gel from Merck and KC₁₈F plates from Whatman. The influence of the counterion on the R_f-value is demonstrated. Some alkaloids exhibit characteristic fluorescence as ion pairs.     

Heterogeneous Platinum‐Catalyzed CH Perfluoroalkylation of Arenes and Heteroarenes

Fluorinated organic compounds are gaining increasing interest for life science applications. The replacement of hydrogen in arenes or heteroarenes by a perfluoroalkyl group has a profound influence on the physical and biological properties of such building blocks. Here, an operationally simple protocol for the direct C? H perfluoroalkylation of (hetero)arenes with R_fI or R_fBr has been developed, using a robust supported platinum catalyst. The ready availability of the starting materials, the excellent substrate tolerance, and the reusability of the catalyst make this method attractive for the synthesis of a variety of perfluoroalkyl‐substituted aromatic compounds. Preliminary mechanistic studies revealed the formation of radicals to be crucial in the reaction system.     

Heterogeneous Platinum‐Catalyzed C?H Perfluoroalkylation of Arenes and Heteroarenes

Fluorinated organic compounds are gaining increasing interest for life science applications. The replacement of hydrogen in arenes or heteroarenes by a perfluoroalkyl group has a profound influence on the physical and biological properties of such building blocks. Here, an operationally simple protocol for the direct C H perfluoroalkylation of (hetero)arenes with R_fI or R_fBr has been developed, using a robust supported platinum catalyst. The ready availability of the starting materials, the excellent substrate tolerance, and the reusability of the catalyst make this method attractive for the synthesis of a variety of perfluoroalkyl‐substituted aromatic compounds. Preliminary mechanistic studies revealed the formation of radicals to be crucial in the reaction system.     

Thermal decomposition of some perfluoroalkyl grignard reagents. Synthesis of trans-1-halo and trans-1-alkylperfluorovinyl compounds

The thermal decomposition of perfluoroalkyl Grignard reagents (R_fMgX), formed from halogen-metal exchange with alkyl Grignard reagents, provides a good synthetic route to trans-1-haloperfluorovinyl compounds.

The decomposition of R_fMgX (where X = Cl, Br, I) in the presence of RMgX generally leads to a mixture of trans-1-alkylperfluorovinyl and trans-1-haloperfluorovinyl compounds. However, the choice of RMgX with respect to both the alkyl group and halogen is critical as regards the formation of the trans-1-alkylperfluorovinyl compounds.Several possible mechanisms are suggested to account for the experimental observations.     

The effects of pH, ionic strength and buffer concentration of mobile phase onR F of acidic compounds in ion-pair TLC

Summary The effects of some factors important in ion-pair, high-performance liquid chromatography were studied in ion-pair, thin-layer chromatography. Tetramethyl and cetyltrimethylammonium salts were used as ion-pairing reagents. As stationary phases, silica gel and chemically bonded reversed-layers (C₁₈) were used. Layers were impregnated with ion-pairing reagent prior to chromatography. In some cases the stationary phase was treated with buffer at different concentrations. The mobile phase contained methanol and water, in one set of experiments buffer, salt for adjustment of ionic strength and ion-pairing reagent were added. The migration behaviour of different benzoic acids was studied. Several problems of ion-pair thin-layer chromatography are discussed. Passed away on 13^th of April, 1998 Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997.     

Cautions regarding the physical interpretation of statistically based separation of the ortho substituent effect into inductive,mesomeric, and steric components—II : Acid catalysed esterification of benzoic acids by methanol and cyclohexanol

In order to attain a better insight into the composition of Taft E_s^o, constants the rate data of hydrion catalysed esterification of both m,p-substituted and o-substiluted benzoic acids by cyclohexanol and methanol were submitted to a statistical analysis using inductive mesomeric and steric substituent constants and various dummy variables differently structured. Furthermore a principal component analysis with subsequent identification of the first principal component via multiple regression analysis was applied. It has been demonstrated that in the reactions of m,p substituted compounds some substituents capable of exerting strong mesomeric effects show peculiar characteristics deviating from the general trend. Since the same result was obtained in the correlations of ortho substituted benzoic acids this effect was taken into account using an appropriate dummy variable which in all cases improved the multiple coefficient of determination. It is concluded that the esterification rates of the ortho substituted compounds depend essentially upon inductive and steric effects (taken away OMe OEt and NO₂) as proposed by Taft. While generally the E_s^o values may be regarded as some measure of a steric effect, this is not true for the methoxy and ethoxy groups.     

Fluoro-ketones IV. Synthesis of phenylperfluoroalkyl ketones - mechanism of reaction between phenyllithium and fluoroesters

Although fluorine containing ketones (R_fC(O)R_f and R_fC(O)R, R_f = perfluoroalkyl) have been prepared from the reaction between organolithium reagents and perfluoroalkyl esters, the reaction has not found general applicability. Variable yields of ketones and co-production of secondary and tertiary alcohol by-products have in most instances been experienced. We have examined in more detail the factors e.g., temperature, mode of addition and perfluoroalkyl ester structure which influence ketone product and by-products formation. By controlling experimental conditions excellent yields of C₆H₅C(O)R_f compounds can be attained. A lithium salt of a hemiketal (II) has been isolated and shown to be the active intermediate in the production of the ketone. The stability of the salt and its potential reaction with the solvent dictates the type of reaction products. Low temperature favors stability of the lithium salt of the hemiketal whereby high yields of ketones are produced on hydrolysis.     

Graphical and visualizing method of identification of selected groups of aliphatic compounds in thin-layer chromatography

A new method of group identification was established for selected aliphatic compounds giving homologous series, based upon constant ΔR_M and directional coefficient “a” values (the “a” values being tangents of the angle between the course of the R_M = f(n_c) function and the “x” axis).The results presented concern identification of higher fatty acids and their ethyl esters, amides of higher fatty acids, higher aliphatic amines, ethylalkyl ketones, and dicarboxylic acids.Simultaneously, another method of identifying groups of aliphatic compounds was established, taking advantage of differentiated visualizing effects.     

Solvent Effects on Ion-Pair Distribution and Dimerization of Tetraalkylammonium Salts

Summary.  The distribution of tetraalkylammonium ions (C _n H _2n+1 )₄N⁺ (R ⁺, TAAn ⁺, n = 4–7) with picrate ion (pic ⁻) and inorganic anions X ⁻, (Cl⁻, Br⁻, ClO⁻ ₄), into various inert organic solvents was studied at 25.0°C. The distribution data were analyzed by taking into consideration the distribution of ion pairs, R ⁺ · X ⁻, and the dimerization of the ion pairs, (R ⁺ · X ⁻)₂, in the organic phase. The ion-pair, distribution constant, K _dist, increases with increasing chain length of the tetraalkylammonium ion and with increasing ionic radius of the anion. The values of K _dist show a good correlation with the E _T value of solvent, i.e. the solvation ability with respect to the anion, and smoothly increase with increasing E _T. The effect of the solvent on the dimerization constants, K _dim, is markedly different between the ion pairs of picrate ion and inorganic anions. In the case of picrate, K _dim significantly decreases with decreasing length of the alkyl chain of the tetraalkylammonium ion, but hardly changes by changing the solvent. On the other hand, in the case of ion pairs of inorganic anions the value of K _dim decreases with decreasing E _T and is almost constant for all anions. These results were reasonably explained by the difference of the solvation of the anion moieties of the monomeric and dimeric ion pairs. Received May 15, 2001. Accepted (revised) July 18, 2001     

Solvent Effects on Ion-Pair Distribution and Dimerization of Tetraalkylammonium Salts

 The distribution of tetraalkylammonium ions (C _n H _2n+1 )₄N⁺ (R ⁺, TAAn ⁺, n = 4–7) with picrate ion (pic ⁻) and inorganic anions X ⁻, (Cl⁻, Br⁻, ClO⁻ ₄), into various inert organic solvents was studied at 25.0°C. The distribution data were analyzed by taking into consideration the distribution of ion pairs, R ⁺ · X ⁻, and the dimerization of the ion pairs, (R ⁺ · X ⁻)₂, in the organic phase. The ion-pair, distribution constant, K _dist, increases with increasing chain length of the tetraalkylammonium ion and with increasing ionic radius of the anion. The values of K _dist show a good correlation with the E _T value of solvent, i.e. the solvation ability with respect to the anion, and smoothly increase with increasing E _T. The effect of the solvent on the dimerization constants, K _dim, is markedly different between the ion pairs of picrate ion and inorganic anions. In the case of picrate, K _dim significantly decreases with decreasing length of the alkyl chain of the tetraalkylammonium ion, but hardly changes by changing the solvent. On the other hand, in the case of ion pairs of inorganic anions the value of K _dim decreases with decreasing E _T and is almost constant for all anions. These results were reasonably explained by the difference of the solvation of the anion moieties of the monomeric and dimeric ion pairs.     

Oxidative cleavage of S‐Arylmercaptoacetic acids by pyridinium chlorochromate: Kinetic and correlation analysis

Kinetics of oxidation of 24 S‐Arylmercaptoacetic acids (SAMA) by pyridinium chlorochromate (PCC) have been studied in acidmedium. The product of oxidation is the corresponding thiophenol. The rate data of meta‐ and para‐substituted acids have been correlated well with σ_I, σ_R^o values and the meta‐compounds correlate well with F,R values. The reaction constants are negative and of smaller magnitudes. Further, the ortho‐substituted acids show a good correlation with triparametric equation involving Taft's σ_I and σ_R^o and charton's steric parameter v. There is no considerable steric contribution to the total orthosubstituent effect. Based on these observations, the mechanism involving the formation of protonated arylsulfinylacetic acid intermediate, followed by an intramolecular rearrangement leading to the product thiophenol has been proposed. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 683–688, 1999     

Regioselectivity of the acidolysis of 2-(chloromethyl)oxirane with aromatic acids in the presence of organic bases

Regioselectivity of the reaction of 2-(chloromethyl)oxirane with aromatic acids in the presence of tertiary amines and tetraalkylammonium halides has been studied. Opening of the oxirane ring follows simultaneously S_N2 and borderline S_N2 mechanisms. The regioselectivity of the acidolysis of substituted oxiranes is determined by acid-base properties of the reactants and catalysts and steric factor. The regioselectivity increases as the contribution of the S_N2 mechanism increases.     

Attempts for the chromatographic separation of D- and L-penicillamine enantiomers

Summary The chromatographic characteristics of the thiazolidine carboxylic acids formed by the reaction of D- and L-penicillamine with various substituted benzaldehydes and heterocyclic aldehydes have been studied on Chiralplate layers. It has been found that the nature and position of substituents strongly affect the R_F values and resolution factors (R_S) of the isomers. An unambiguous relationship has been established between the R_S values of the L- and D-enantiomers and the dipole moment of the aldehydes. The transformation to thiazolidine carboxylic acids with benzaldehyde and substituted benzaldehydes, however, is not complete, thus the reaction with formaldehyde is still more suitable for quantitative determination of penicillamine isomers.