Stopped-flow Gas Chromatography Research Articles

Stereointegrity of thalidomide: gas-chromatographic determination of the enantiomerization barrier

Enantioselective chromatographic methods, representing the most commonly used techniques for the determination of enantiomeric ratios, can also be used for the evaluation of stereochemical integrity. In the present study, two chromatographic methods, dynamic gas chromatography (DGC) and stopped-flow gas chromatography (SFGC) were used to determine the enantiomerization barrier of thalidomide. In the presence of a chiral stationary phase (CSP), the enantiomers of thalidomide produced characteristic elution profiles exhibiting plateaus and/or peak broadening which were observed between 190 and 220 °C in DGC. To obtain the enantiomerization barrier of thalidomide from experimental data, the fast and efficient simulation program ChromWin was used to simulate the elution profiles and obtain kinetic activation parameters. From temperature-dependent measurements the rate constants k 1 and k −1 and the kinetic activation parameters Δ G # , Δ H # and Δ S # of the enantiomerization of thalidomide were obtained by DGC. The enantiomerization barrier Δ G # was determined to be 154±2 kJ/mol with DGC and 150±3 kJ/mol with the sfGC technique at 200 °C, respectively. The concept of the retention increment R′ has been applied to separate the enantiomerization barrier of thalidomide in the dissolved and complexed state of the CSP.

Dynamic phenomena involving chiral dimethyl-2,3-pentadienedioate in enantioselective gas chromatography and NMR spectroscopy

The chiral allene dimethyl-2,3-pentadienedioate 1 undergoes enantiomerization during gas-chromatographic enantiomer separation on the two polymeric chiral stationary phases Chirasil-Nickel 2a and Chirasil-Dex 3 at ambient temperatures. The enantiomerization barrier of 1 is determined by dynamic gas chromatography via computer simulation of interconversion elution-profiles and by multidimensional stopped-flow gas chromatography. The previously reported deracemization of 1 in the presence of a chiral paramagnetic lanthanide shift reagent by NMR spectroscopy is confirmed.

On the enantiomerization barrier of atropisomeric 2,2′,3,3′,4,6′-hexachlorobiphenyl (PCB 132)

Abstract The rotational barrier of chiral, tri-ortho-substituted 2,2′,3,3′,4,6′-hexachlorobiphenyl (PCB 132) was determined. After semipreparative HPLC separation of PCB 132 on polysiloxane-anchored permethyl-β-cyclodextrin (Chirasil-Dex), one single enantiomer was subjected to an enantiomerization kinetic and the enantiomeric ratio was determined by chiral gas chromatography. Furthermore, on-column inversion of PCB 132 was investigated in the presence of Chirasil-Dex via stopped-flow gas chromatography.

Diffusion coefficients from stopped-flow gas chromatography

Mutual diffusion coefficients of two gases A and B can be determined in an empty gas chromatographic column by letting component B enter at an intermediate position of the column and continuously flow through a part only of it, as a carrier gas. The other component A is injected in a small amount instantaneously at the closed end of the column with the detector placed at the other end. By repeatedly stopping and then restoring after a short time the flow of B, narrow extra peaks are produced on the chromatographic elution curve, owing to diffusion of A into B. An equation is derived giving the area under the curve of each stop-peak as a function of time of the corresponding stop. Plotting the experimental data according to this equation permits the determination of the diffusion coefficient of A into B. Some results obtained by this method show negligible variations with changes in the experimental parameters.

Conversions in catalytic deamination calculated by stopped-flow gas chromatography

By calculating absolute values for the intercepts of the rate equation that describes the deamination of aminocyclohexane on aluminium oxide, the components of a weighted mean rate constant can be found. From these the conversion of the reactant amine to product(s) can be computed, and this is in good agreement with the conversion determined directly by separate experiments. This is an indication of the internal consistency of the theoretical rate equation, derived on the basis of a particular reaction model with many kinds of active sites. The conversions are much less than 100%, showing that a considerable fraction of the reactant is adsorbed on sites which transform it to irreversibly adsorbed products. The conversion to gaseous cyclohexene increases with increasing flow-rate of the carrier gas. Finally, it was found that the effective duration of the stops is very different from the real duration.

Kinetics of n-butene isomerization studied by stopped-flow gas chromatography

The kinetics of isomerization of n-butenes on molecular sieve 13X, silica gel and alumina were studied by stopped-flow gas chromatography. The results are explained by assuming a reaction scheme with two kinds of active sites and a common intermediate. Based on this scheme, analytical expressions are derived describing the various rate laws. These are used to calculate rate constants of isomerization on the three catalytic surfaces, at various temperatures. Activation energies and entropies for each kind of active sites and for each reaction have been determined.

Catalytic deamination on solid surfaces. Part 1.—Kinetic studies by stopped-flow gas chromatography

Stopped-flow gas chromatography was used to study the detailed kinetics of formation of unsaturated hydrocarbons by deamination on aluminium oxide and on porous glass of aminocyclohexane, 2-aminopropane, 1-aminobutane, 2-aminobutane, 1-amino-2-methylpropane and 2-amino-2-methylpropane. The results are consistent with a proposed mechanism in which the reacting amine is rapidly adsorbed on two or three kinds of active sites of aluminium oxide or porous glass, respectively, leading to unsaturated hydrocarbons through the formation of an intermediate adsorbed product on each site. Based on this mechanism, theoretical equations are derived which adequately describe the experimental reaction rate as a function of time. Analysis of the data by means of these equations permits the calculation of the various rate constants for the surface processes. True activation energies and frequency factors for the deamination of aminocyclohexane on aluminium oxide have also been determined.

Ideal stopped-flow gas chromatography with chemical reactions in the stationary phase

By assuming a general mechanism for surface reactions on a chromatographic column, equations are derived from which can be calculated the elution curves of the reactant, the products and the peaks produced by stopping the carrier-gas flow for a short time. Particularly, the area under the curve of these stop-peaks can be calculated as a function of the time of cessation of the gas flow, and this can be used to determine, from experimental values, the rate constants of the surface reactions. Application of the method requires specification of the mechanism assumed in each case. As an example, the equations derived are applied for a simple first-order reaction on one kind of active sites, with no intermediate. In this limiting case, two of the final equations coincide with the results of earlier static derivations.

Theory of non-equilibrium stopped-flow gas chromatography

An equation is derived that gives the area under the curve of the stop-peaks, as a function of the time of the stop and of the carrier gas flow-rate, in non-equilibrium stopped-flow gas chromatography. Adsorption-desorption is assumed to take place at two kinds of reactive sites. It is shown that, with certain approximations, the equation derived can be used to determine the rate constants for desorption and the partition coefficients for both kinds of sites. Experiments at various temperatures can then yield desorption activation parameters, as well as enthalpies and entropies of adsorption. Furthermore, the stop-peaks provide a simple method for finding ideal retention times in linear non-ideal ( i.e. non-equilibrium) chromatography.

The study of catalysis by novel gas chromatographic techniques

Studies of catalytic reactions have been made using differential (sample-vacancy) and stopped-flow gas chromatography. Reactions are often very sensitive to trace materials and in particular it has been necessary to devise a technique for removing oxygen from carrier-gas streams down to very low levels. The techniques and the results are often relevant to the more normal analytical applications of gas chromatography.

Catalytic dehydrohalogenation of bromobutanes studied by stopped-flow gas chromatography

Dehydrohalogenation reactions of bromobutanes on alumina modified with potassium bromide were studied by the stopped-flow technique. Isomerization and polymerization reactions of the products were kept to a minimum. The reactions were first-order with respect to the reactant. From 1-bromobutane all four possible alkenes were produced, 1-butene being the main product. From 1-bromo-2-methyl-propane almost exclusively 2-methylpropene was obtained, whereas 2-bromobutane gave cis- and trans-2-butene in about equal amounts. The heat of adsorption of the first two bromides was determined and this was correlated with the Arrhenius parameters to interpret the compensation effect observed. From this correlation certain conclusions about the mechanism of the reactions are drawn. The results are consistent with a two-step mechanism, the second step being rate determining.

The use of stopped-flow gas chromatography to study the catalytic dehydration of alcohols

The dehydration of alcohols on modified aluminas wag studied by using the same material simultaneously as catalyst and as stationary phase in a gas-solid Chromatograph. In such a system, periodic stopping of the gas flow through the column leads to the reaction products appearing as sharp chromatographic peaks for each stopped-flow interval, so that the rate of reaction is effectively followed throughout the whole course of the reaction. The reliability of comparative measurements is enhanced by carrying out a standard dehydration (e.g., of n-heptanol) together with the dehydration of another alcohol. The results support a trans-elimination mechanism and suggest that there are two distinct types of catalytic site. Autocatalysis (by water) and polymerization of olefin products were also investigated and allowance was made for these effects in the kinetic analysis.