Disubstituted Benzene Isomers Research Articles

Chromatography of ionizable positional isomers with methanol–water–carbon dioxide mixtures

The separations of five groups of ionogenic disubstituted benzene isomers were achieved by taking advantage of the pH variation caused by proportions of CO2 in the methanol–water–CO2 mobile phases without the addition of an aqueous buffer. The retention behavior of acidic, basic, and zwitterionic isomers was predominantly affected by the dissociation of these analytes within methanol–water–CO2 mobile phases of different pH. A comparison with previously published reversed-phase high-performance liquid chromatography (HPLC) of these isomers revealed that resolution improvement for anisic acid, toluic acid and aminobenzoic acid isomers was achieved with methanol–water–CO2 enhanced-fluidity liquids. Significant reductions in the retention factors were also obtained. Meanwhile, the detrimental effect of the presence of an aqueous buffer on the stationary phase was avoided which made fast repetitive analyses possible.

Preparation of insoluble β-CD polymer and its application to enantiomers and isomers separation

An insoluble β-cyclodextrin polymer cross-linked with epichlorohydrin was prepared, and its structure was identified with infrared spectrum. Colloid stationary phase was prepared by dissolving the polymer in the mixed solvent of diisopropyl ether, methylene dichloride and benzene and treated for 0.5 h by ultrasonication, and then was coated on a fused silica capillary column. The optimun reaction conditions are as follows: the mole ratio of epichlohydrin to β-cyclodextrin is 12.1:1, reacting at 65 °C for 24 h. The Chromatographic performance such as column efficiency, thermal stabilities and polarity were studied, two kinds of disubstituted benzene isomers and eight pairs of enantiomers were separated on the capillary column. The results show that the β-cyclodextrin polymer is suitable for use as a capillary gas chromatographic stationary phase, and possess excellent chromatographic properties in separating enantiomers and position isomers.

The study of interaction of disubstituted benzenes and water by using Monte Carlo simulations

Abstract Monte carlo simulations have been carried out to study the interaction of water molecules with different isomers of disubstituted benzenes in the isothermal-isobaric ensemble (NPT) at 298.15 K temperature and 1 atm pressure. The study of the first hydration shell of the solutes has shown that the hydration features of the disubstituted benzene isomers were different and depended on the kind of substitution. The results are discussed here in terms of radial distribution functions, electrostatic and van der Waals interactions of isomers with water molecules. The most soluble isomers show low free energies of solvation. The absolute partition coefficients of compounds in Water/Chloroform system, log PCW, were computed.

Structures, Energetics, Relative Stabilities, and Out-of-Plane Distortivities of Skeletally Disubstituted Benzenes, (CH)4X2 (X = N, P, C-, Si-, O+, and S+): An ab Initio and DFT Study

The positional isomers of disubstituted benzenes, (CH)4X2 (X = N, P, C-, Si-, O+, and S+), are studied using ab initio molecular orbital theory at Hartree−Fock (HF), MP2, and CCSD(T) levels and als...

Insoluble β-cyclodextrin polymer for capillary gas chromatographic separation of enantiomers and isomers

An insoluble β-cyclodextrin polymer cross-linked with identified epichiorohydrin has been prepared; its structure was identified by means of infrared and 13C NMR spectra. Three capillary columns have been coated with the polymer treated by ultrasonication by three coating methods. Chromatographic characteristics such as column efficiency, thermal stability and polarity, have been studied, and two kinds of disubstituted benzene isomers and eight pairs of enantiomers have been separated on the three capillary columns. The results show that the β-cyclodextrin polymer is suitable for use as a capillary gas chromatographic stationary phase, and that the column, prepared by loading β-cyclodextrin polymer stationary phase with 50% methyl–50% phenylsilicone (OV-17), shows good chromatographic properties in separating enantiomers and positional isomers.

Gas chromatographic studies employing p-(2?-methyltridecyl-2?)calix[8]arene diluted in a polysiloxane as stationary phase

p-(2′-Methyltridecyl-2′)calix[8]arene was synthesized and applied as selector in capillary gas chromatography. The modification of the calixarene skeleton with a long alkyl chain exhibits an improved solubility in common gas chromatography (GC) stationary phases. Employing the concept of the retention increment R′, the selective interaction of several aromatic analytes on the stationary phase containing 20% (w/w) p-(2′-methyltridecyl-2′)calix[8]arene diluted in the polysiloxane SE-54 was studied. The investigated calixarene derivative selectively interacts with the positional isomers of disubstituted benzenes. Large molecules such as naphthalene derivatives as well as quinoline and isoquinoline show a strong interaction with the calixarene selector as expressed by increased R′ values. ©1999 John Wiley & Sons, Inc. J Micro Sep 11: 313–317, 1999

Gas-phase reactivity of [arene—Cr]+adduct ions: Differentiation of disubstituted benzene isomers

The gas-phase reactivity of isomeric [arene-Cr+] adduct ions was studied by tandem mass spectrometry. Ortho, meta and para-disubstituted benzene-chromium ions were generated by electron impact-induced loss of three CO groups from corresponding (ρ6-arene)tricarbonylchromium complexes. Six series of substituted toluene-chromium ions [RC6H4CH3-Cr]+ (R = Cl, F, OCH3, CO2CH3, COCH3 and CONH2) and a series of ethylanisoles [C2H5C6H4OCH3-Cr]+ were considered. Comparison of metastable decomposition and/or collision-induced dissociation spectra allows the characterization of the ortho isomer through specific fragmentations, often owing to the ability of Cr+ to form a bridge between the two substituents, and differentiation of the meta from the para isomer. The method described here relies on the abundance of a diagnostic fragment ion which is present in the spectrum of one isomer but is absent or shows different relative intensities in the spectra of the other isomers. Deuterium labelling was used to demonstrate some specific fragmentation mechanisms.

Gas‐phase reactivity of [arene—Cr]adduct ions: Differentiation of disubstituted benzene isomers

The gas-phase reactivity of isomeric [arene-Cr+] adduct ions was studied by tandem mass spectrometry. Ortho, meta and para-disubstituted benzene-chromium ions were generated by electron impact-induced loss of three CO groups from corresponding (ρ6-arene)tricarbonylchromium complexes. Six series of substituted toluene-chromium ions [RC6H4CH3-Cr]+ (R = Cl, F, OCH3, CO2CH3, COCH3 and CONH2) and a series of ethylanisoles [C2H5C6H4OCH3-Cr]+ were considered. Comparison of metastable decomposition and/or collision-induced dissociation spectra allows the characterization of the ortho isomer through specific fragmentations, often owing to the ability of Cr+ to form a bridge between the two substituents, and differentiation of the meta from the para isomer. The method described here relies on the abundance of a diagnostic fragment ion which is present in the spectrum of one isomer but is absent or shows different relative intensities in the spectra of the other isomers. Deuterium labelling was used to demonstrate some specific fragmentation mechanisms.

Cross-Linked Hydroxy-Al-Montmorillonite as a Stationary Phase in Liquid Chromatography

AbstractQuasi-spherical particles (7-µm mean diameter) were prepared from cross-linked hydroxy-Al-montmorillonite (basal spacing =15.3 and 18.6 Å) by spray-drying. These particles (SP-CLM) were used as a packing material for columns in high-pressure liquid chromatography (HPLC). Aromatic phosphate esters, chlorosubstituted phenyl-ureas, monosubstituted benzenes, and the o-, m-, and p-isomers of disubstituted benzenes were separated on the columns, using isopropanol and hexane. Alkyl and alkoxy groups in the solute molecules sterically hindered the interaction of these molecules with the solid phase. The retention of the isomers of disubstituted benzenes that possess one electronegative (or dipolar) and one nonpolar substituent increased in the order: o- < m- < p-. In contrast, for all investigated disubstituted benzenes containing two electronegative (or dipolar) substituents, the o-isomer was retained on the 18.6-Å SP-CLM more strongly than the m- and p-isomers. The chromatographic data suggest that the strength of the interaction of phenolic groups with the solid phase increased with the acidity of these groups. The o-isomers of phenols that contained an additional electronegative (or dipolar) substituent displayed an exceptionally strong adsorption. The capacity of the o-isomers of disubstituted benzenes containing two electronegative (or dipolar) substituents to form chelates with Al probably was responsible for the strong retention of such o-isomers compared with the retention of the corresponding m- and p-isomers.Cross-linked hydroxy-Al-montmorillonite acted as a strong and selective adsorbent for many families of organic compounds. It was used successfully as the solid phase in HPLC separations with eluents ranging from nonpolar (e.g., hexane) to highly polar (e.g., water).

Prospectives of polymeric liquid crystal stationary phases for capillary column gas chromatographic separations

The liquid crystalline polysiloxane substrate offers unique selectivity when used in capillary column gas chromatography. The separation and identification of priority pollutants along with other compounds is very difficult but important. In particular, the separation of isomeric organic compounds having similar volatilities could not be obtained on capillary columns using conventional stationary phases. The complete separation of ortho, meta, and para isomers of certain disubstituted benzenes as well as polycyclic aromatic compounds was achieved on a capillary column using a liquid crystalline polysiloxane stationary phase. The complete separation of octachlorodibenzodioxin from octachlorodibenzofuran was also obtained on this column which is not possible on columns of identical lengths using conventional stationary phases. The separation of polycyclic aromatic compounds in a complex sample of airborne particulate matter extract on the liquid crystal column is demonstrated.

Electronic spectra and ionization potentials of rotational isomers of several disubstituted benzenes

The electronic spectra of several disubstituted benzenes ( o-fluorophenol, m-fluoroanisole, hydroquinone, and p-dimethoxybenzene) were investigated in supersonic free jets. For all the molecules studied, the existence of rotational isomers was established, and the ionization potentials of the individual rotational isomers were determined by two-color multiphoton ionization spectroscopy.

Methylated cyclodextrins as mobile phase additives in liquid chromatography

Both 2,3,6- and 2,6-methylated derivatives of α and β cyclodextrins were added to the mobile phases in liquid chromatography using an octadecylsilyl stationary phase. The 2,3,6-methyl-substituted derivative of α-cyclodextrin improves the separation of the ortho, meta and para isomers of disubstituted benzenes and also decreases their retention compared with the unmodified α-cyclodextrin. By the addition of 2,3,6-methylated β-cyclodextrin, the three isomers of nitrobenzoic acid can be completely separated, and the separation of trans-nitrocinnamic acid is greatly improved. In contrast, 2,6-dimethylation of both α- and β-cyclodextrins did not bring about such significant improvements in the retention of the solutes investigated.

Properties of single and mixed liquid-crystalline stationary phases : I. Single liquid-crystalline stationary phases

The polarities and selectivities of twelve liquid-crystalline stationary phases have been tested as well as their column efficiencies. The effect of these properties on the separating ability of the liquid-crystalline stationary phases with respect to di-substituted benzene isomers is discussed. The separating properties of the individual stationary phases have been predicted, and experimentally confirmed by carrying out separations of mixtures of isomers of disubstituted benzene derivatives.

Retention behaviour of disubstituted benzene isomers on acetylated cyclodextrin stationary phases

Chemically bonded α- and β-cyclodextrin stationary phases were treated with acetic anhydride. The resulting acetylated stationary phases exhibit selectivity in the separation of disubstituted benzene isomers by liquid chromatography, as do the unmodified, parent cyclodextrin stationary phase. However, the acetylated β-cyclodextrin stationary phase is superior to the unmodified one and can completely separate the o-, m- and p-isomers of toluidine or dinitrobenzene, which cannot be done on the umnodified stationary phase. Acetylation of the α-cyclodextrin stationary phase does not necessarily bring about a similar improvement in the separation.

Separation of disubstituted benzene isomers on chemically bonded cyclodextrin stationary phases

α- and β-cyclodextrins have been immobilized on polyacrylamide gel beads via two spacers. The resulting stationary phases containing α-cyclodextrin exhibit selectivity in the liquid chromatographic separation of disubstituted benzene isomers as models: the retention order is o <m <p in all cases. Except for aminobenzoic acid, the o-, m- and p-isomers of each solute tested can be completely separated on the α-cyclodextrin stationary phases. The β-cyclodextrin stationary phases also interact more strongly with the p- than with the o- or m-isomers. In the case of dinitrobenzene, the o-isomer is retained most strongly.

Retention Behaviour of Benzene Derivatives on Chemically Bonded β-Cyclodextrin Phases

Abstract β-Cyclodextrin is first immobilized on the gel supports of polyacrylamide and agarose. The resulting stationary phase containing β-cyclodextrin chemically bonded on polyacrylamide shows novel selectivity in the liquid chromatographic separation of disubstituted benzene isomers. Thep-isomers are retained more strongly compared with the o-or m-isomers.

Properties of liquid crystalline cyanoazoxybenzene alkyl carbonates as stationary phases in gas chromatography

Seven liquid crystalline azoxybenzene p-cyano- p′-alkyl carbonates have been tested as stationary phases, using Chromosorb W as the support and xylene isomers as the solutes. The data obtained on the effect of the properties of the liquid crystal-support system on column selectivity and efficiency were utilized in the separation of mixtures of disubstituted benzene isomers.

Use of Liquid Crystals for the Separation of Position Isomers of Disubstituted Benzenes

Liquid crystals used as stationary phases for GC columns have been reported for the separation of a number of meta- and para-position isomers of disubstituted benzenes. Few of the ortho isomers have been reported and little information concerning the effect of the substituent type upon the order of elution of the isomers has been published. This paper summarizes work undertaken to add some of these data to the knowledge concerning the behavior of liquid crystal materials when used as stationary phases in gas chromatography. Three liquid crystal materials were studied: 4,4′-azoxydianisole, 4,4′-azoxydiphenetole, and 4,4′-biphenylylene bis[p-(heptyloxy)benzoate]. The behavior of the ortho, meta, and para isomers of twenty-five disubstituted benzenes was determined with one or more of these liquid crystals. Data are tabulated.