Enantiomeric 3-Chloromandelic Acid System: Binary Melting Point Phase Diagram, Ternary Solubility Phase Diagrams and Polymorphism

Enantiomeric 3-Chloromandelic Acid System: Binary Melting Point Phase Diagram, Ternary Solubility Phase Diagrams and Polymorphism TAM LE MINH,1 JAN VON LANGERMANN,1 HEIKE LORENZ,1 ANDREAS SEIDEL-MORGENSTERN1,2 1

Max-Planck-Institut fu¨r Dynamik Komplexer Technischer Systeme, Magdeburg, Sachsen-Anhalt, Germany

2

Otto-von-Guericke-Universita¨t, Chemische Verfahrenstechnik, Magdeburg, Sachsen-Anhalt, Germany

Received 8 February 2010; revised 22 March 2010; accepted 25 April 2010 Published online 22 June 2010 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.22234 ABSTRACT: A systematic study of binary melting point and ternary solubility phase diagrams of the enantiomeric 3-chloromandelic acid (3-ClMA) system was performed under consideration of polymorphism. The melting point phase diagram was measured by means of thermal analysis, that is, using heat-flux differential scanning calorimetry (DSC). The results reveal that 3-ClMA belongs to the racemic compound-forming systems. Polymorphism was found for both the enantiomer and the racemate as confirmed by X-ray powder diffraction analysis. The ternary solubility phase diagram of 3-ClMA in water was determined between 5 and 508C by the classical isothermal technique. The solubilities of the pure enantiomers are extremely temperaturedependent. The solid–liquid equilibria of racemic 3-ClMA are not trivial due to the existence of polymorphism. The eutectic composition in the chiral system changes as a function of temperature. Further, solubility data in the alternative solvent toluene are also presented. ß 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:4084–4095, 2010

Keywords: 3-chloromandelic acid; racemic compound; binary and ternary phase diagrams; polymorphism; X-ray diffraction; calorimetry (DSC); chirality; crystallization

INTRODUCTION Chiral substances become increasingly important in many fields, that is, in agrochemistry, food industry, fine chemistry, flavor and fragrance industry, etc. Purification of enantiomers is essential in particular for the pharmaceutical industry. Since chiral substances are used as drugs, it is important to know that only one enantiomer generates often desired effects on organisms. Unexpected or even harmful side effects can be caused by the other enantiomer.1–3 In the period 1983–2002, the demand in single enantiomers in the global pharmaceutical industry increased steadily. Only about 26% of all drugs sold in 1983 were single enantiomers. This number increased up to 35% (1990), 50% (1998), and 55% (2002). In contrast, the worldwide distribution of chiral drugs sold as racemates was 37% in 1983 and decreased then to 33% (1990), 15% (1998), and 6% (2002).4

Correspondence to: Tam Le Minh (Telephone: 49-391-6110-280; Fax: 49-391-6110-524; E-mail: [email protected]) Journal of Pharmaceutical Sciences, Vol. 99, 4084–4095 (2010) ß 2010 Wiley-Liss, Inc. and the American Pharmacists Association

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An important initial milestone in chiral resolution refers back to Louis Pasteur who separated the crystals of two enantiomers from each other by microscope and tweezers.5 Nowadays, various techniques have been reported in literature for enantiomeric purification. Among them, chromatography is known as a highly selective separation technique. Chiral chromatography is useful for determining the enantiomeric purity of drugs using analytical columns or for isolating enantiomers using preparative columns.6,7 Recently, capillary electrophoresis has been used as an analytical alternative to separate enantiomers. This method offers the potential of higher resolution and faster analysis owing to its unique flow profile and efficient heat dissipation.7,8 Besides, kinetic resolution using enzymes is also a powerful technique for enantiomer separation.7 Some other methods such as membrane separation or chiral extraction are also recommended for enantiomer resolution.7,8 Above all, crystallization is appreciated as a very suitable and flexible approach, which often offers a simple and cheap way to resolve enantiomers.2,7,9,10 Different crystallization strategies are applied for specific kinds of racemic mixtures. These mixtures are classified in three categories comprising conglom-

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erate systems (5–10% of chiral substances), racemic compounds (90–95%) and pseudoracemates (rarely).5 A conglomerate is a mechanical mixture of the crystals of two enantiomers, while in the case of a racemic compound two enantiomers coexist in the same unit cell (so called true racemate). Pseudoracemates form solid solutions.5,11 Binary phase diagram (BPD) and ternary phase diagram (TPD) are important to develop and optimize crystallization processes.1,9,10 The different kinds of racemates have distinctive characteristics in the BPD and TPD. Practically, the BPD could be determined by measuring the melting profiles of enantiomeric mixtures. The TPD is constructed from solubility data which could be obtained using classical isothermal and/or polythermal methods. Besides phase diagrams, the investigation of polymorphism is also an objective of this work. Polymorphism is defined as the ability of a substance to exist as at least two crystalline phases that have different arrangements and/or conformations of the molecules in the crystal lattice. Polymorphic forms of compounds may strongly differ in the melting point, the solubility behavior, the crystal habit, the bioavailability, etc. Regarding their relative stability polymorphs of two basic types exist: monotropic and enantiotropic forms.5,11,12 Thermal analysis techniques (e.g., differential scanning calorimetry—DSC, differential thermal analysis— DTA) can be combined with some other characterization techniques (hot-stage microscopy, infrared and Raman spectroscopy, temperature resolved Xray diffraction, etc.) to identify and purify thermodynamically stable and metastable forms at various temperatures.13 Mandelic acid derivatives (including 3-Chloromandelic acid (3-ClMA), C8H7ClO3) are versatile intermediates for cosmetics, antibacterial agents, etc. and are used for the synthesis of target molecules.14,15 However, the knowledge about 3ClMA is still limited. Zhang et al.16 reported that 3ClMA is a racemic compound-forming system and that the eutectic composition measured in TPD (with mixtures of water and isopropanol as solvent) can vary with temperature. Observations regarding polymorphism and solid solutions have not been mentioned. In the present work, the BPD of enantiomeric 3ClMA will be measured by DSC heat flux. The fast cooling rate method will be applied for both the enantiomer and the racemic species. This technique is suitable to evaluate the possibility of different polymorphic forms. Additionally, the TPD of this substance in water will be measured by isothermal and polythermal methods. Finally, toluene as another organic solvent will be used and the corresponding TPD will be determined. DOI 10.1002/jps

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EXPERIMENTAL Materials The enantiomer R (97%) and the racemate (RS) (97%) of 3-ClMA were obtained from Aldrich and Alfa Aesar (Germany). Acetone p.a. (Applichem, Darmstadt, Germany) was used to dissolve and recrystallize powder materials for DSC experiments. Deionized water (Millipore machine, Schwalbach, Germany) was used as solvent in solubility experiments. Toluene p.a. was purchased of Merck (Germany). The substances were used without further purification. Polymorphic forms of the racemate and the enantiomer of 3-ClMA were obtained in the suitable conditions from initial materials. Binary Melting Point Phase Diagram Mechanical blending is considered as an inadequate method to get regular mixtures.17 Therefore, in this work, materials were recrystallized from acetone based on the following steps. Solid mixtures were prepared with a balance (Mettler Toledo, Gießen, the resolution was 0.001 mg). The initial compositions were dissolved in acetone. Then, the solvent was evaporated at room temperature to recrystallize the mixtures. Subsequently, the substance was ground to fine powder using a pestle and mortar. Ten to 20 mg samples were packed in closed aluminum crucibles of 75 mL volume. Thermal analysis was performed with a DSC 131 (Setaram, Diepholz, Germany), which was regularly calibrated using high pure standard metals (indium, tin, and lead) in medium-temperature range. The measurements were carried out with a constant heating rate of 0.5 K/min in the range from 258C up to 1308C, under high pure helium atmosphere (99.999%) at 8 mL/min. The measurements were executed with several samples of the same mixtures. The BPD was depicted by solidus and liquidus curves that were defined by a method described in Ref.18 The melting points of pure enantiomer, racemic species, and eutectic melting of mixtures were taken from the onset-temperature (ton). Otherwise, the peak-temperature (tpeak) corresponded to the end of melting of other enantiomeric mixtures. The possibility of polymorphisms was studied in this work by the rapid cooling method.11 Firstly, mixtures were heated above the melting points to get molten liquids. Then these mixtures were cooled down to 308C (rate 10 K/min) to obtain the recrystallized phase. Subsequently, the same DSC cycle was performed with this crystalline material. The given calorimetric properties were compared with those of previous DSC runs to evaluate the possibility of polymorphism. Additionally, X-ray powder diffraction (XRPD) patterns of cooled material crystallized were compared with the initial ones. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010

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Solubility Measurements Solubility data of 3-ClMA in water were measured using the classical isothermal method.19 Double- or triple-wall jacketed vessels were connected to a thermostat (Lauda, Ko¨ nigshofen, Germany) to ensure isothermal conditions during solubility experiments. The temperature in the jacket vessels was checked with a Pt-100 resistance thermometer (resolution 0.01 K). Small closed vials containing specific compositions of the enantiomer, the racemate and water were immersed in the heat-isolated vessels. These samples were heated up approximately 5 K over the desired temperature and kept these about 30 min under good stirring condition before finally adjusting the temperature. The system was then stirred for 48 h with electromagnetic devices at constant temperature. Subsequently, the liquid portion was separated using a syringe and a filter (PET— membrane, pore size 0.45–25 mm). Amounts of 1– 2 mL clear saturated solution were rapidly placed into two flasks (for double analysis) to avoid mass and heat exchange with the environment. The initial mass of solutions was measured (m1). The solutions were dried by total solvent evaporation (3 days in desiccators and 8 h in a vacuum machine (Heraeus Vacutherm, Langenselbold, Germany) at 508C and pressures of 100 mbar) and weighted (m2). From the masses of solution and dry materials, the solubility was obtained (in mass fraction) using the formula: Wt ¼ m2  100=m1 . In addition, the enantiomeric compositions and concentrations of mixtures were also measured by two other methods: refractometry (Mettler Toledo RE40, Gießen, Germany) and chiral high performance liquid chromatography (HPLC) (Chirobiotic T column, 250 mm  4.6 mm, 5 mm particle size, Supelco, Bellefonte, Pennsylvania; the eluent solution was 1% triethylammonium acetate (TEAA)/methanol (80/20, v/v); pH 4.02). According to their relevance, the measurements were done with three typical enantiomeric compositions (the pure enantiomer, the racemic species and the eutectic mixture) and other enantiomeric mixtures in the range from 5 to 508C. Due to the lack of commercially available (S)-enantiomer, the experiments were executed with mixture compositions of weight fractions wR ¼ 0.5–1.0, actually. However, some crosscheck experiments of the symmetry in the diagram were done with (S)-enantiomer purified within this project.20 To compare with results obtained using the isothermal method, polythermal experiments were undertaken with a Crystal-16TM apparatus (Avantium, Netherlands). Materials were crushed to fine particles and different ratio liquid–solid mixtures were placed in a block of four small closed vials (1 mL). Thermal programs were executed from 10 to 608C JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010

with a heating rate of 0.04 K/min, and stirring of 700 rpm. Clear points corresponding to the saturation temperature were observed. The concentrations at specific temperatures could be obtained from the interpolation of concentrations versus temperatures. Simultaneously, the equilibrated solid phases were analyzed by XRPD method in an X’Pert Pro apparatus (PANanalytical GmbH, Kassel, Germany). The radia˚ ). Samples were tion source was Cu Ka (l ¼ 1.5406 A mounted on a Si holder and scanned from diffraction angles (2u) of 3–408 with a resolution step size 0.0178 and a counting time of 50 s for each step. Finally, toluene was used in additional experiments as an alternative organic solvent to investigate solvent effects and to obtain another TPD. The polythermal method was applied again using the Crystal-16TM apparatus with a suitable program. Effect of Polymorphism on Dissolution Experiment To understand the effect of polymorphism on solubility, a kinetic dissolution experiment of the racemate 3-ClMA was performed. An excess amount of the initial racemate (Alfa Aesar) was dissolved in water at 258C. The concentrations of the solute was followed by HPLC-analyses after a few minutes up to several days. Simultaneously, the residual solid particles were analyzed with XRPD to determine which polymorphic forms could exist in the heterogeneous system. The experiments were repeated two times. Correlating the Phase Equilibria The equations of Schro¨ der–Van Laar (1) and Prigogine–Defay (2)5 have been used in simplified forms to predict the solid–liquid equilibria (SLE) in ideal binary systems at temperature Tf . ! DHAf 1 1  f ln x ¼ (1) T R TAf 2DHRf 1 1  f ln 4x  ð1  xÞ¼ R T TRf

! (2)

where x is the mole fraction of the more abundant enantiomer of the mixture, R the gas constant, TAf ;TRf the melting temperature of enantiomer, racemate, and mixture, and DHAf ; DHRf the enthalpy of fusion of enantiomer and racemate.

RESULTS AND DISCUSSION Binary Melting Point Phase Diagram The obtained results emphasized that it is very important to check carefully the initial material. The racemate provided by Alfa Aesar gave only one sharp DSC melting peak (Fig. 1). A crystalline structure of DOI 10.1002/jps

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Figure 1. DSC measurement for the initial racemate (Alfa Aesar) and the stable polymorph, indicating the same melting behavior.

the racemate was not available in the literature. Therefore this material was considered in the beginning of the project as ‘‘pure’’ material and its XRPD pattern was used as the reference for the racemate (Fig. 2a). However, it was realized later based on XRPD patterns that there was an overlayimage of two different structures (Fig. 2b and c). Their structures represent two polymorphic forms of the racemate as discussed later. Therefore, it had to be concluded that the initial racemate contained two polymorphic forms. A couple of additional DSC experiments were executed under the same conditions with either the

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pure stable polymorph of the racemate or the sample provided from Alfa Aesar to clarify the effect of polymorphic impurity in the initial racemate. The results of six independent experiments revealed that the melting effect of the stable racemate obviously fitted with DSC data from the initial racemate. The same melting temperatures and enthalpies were found (1178C and 27.92 kJ/mol, see Fig. 1a and b). This could be explained by the fact that the relative quantity of the metastable polymorph in the initial mixture was small and the effect of this phase was negligible. The errors in determination of melting data by DSC were evaluated by repeating measurements with different samples of both the pure enantiomer and the racemate. The confidence interval in the determination of the melting temperature was 0.22 K and in measuring the enthalpy of fusion 2.4%. Separate experiments, DSC coupled with thermogravimetry (TG), were performed to define the loss of mass during the melting processes. This loss was below 0.3%, thus can be neglected. The XRPD patterns inferred that there were absolutely different structures of the racemate and the enantiomer of 3-ClMA (compare the stable forms in Fig. 2b and d). They crystallized in various crystal structures, which were distinguished by lattice parameters and with respect to symmetry of their space groups. In addition, the melting temperature of the racemic species was higher than that of the enantiomers (see Tab. 1). Thus, 3-ClMA was defined as a racemic compound forming system.

Figure 2. The XRPD patterns of the initial racemate (a—from Alfa Aesar) was a overlay of the patterns of two polymorphic forms (b and c). Polymorphism was found for both the racemate (b and c) and the enantiomer (d and e) of 3-ClMA. DOI 10.1002/jps

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Table 1. Temperature and Heat of Melting of the Enantiomer, the Racemate of (3)-ClMA and Eutectic Composition Tf (8C)

DHf (kJ/mol)

Composition

Literature16

Present Work

Literature16

Present Work

(RS)-3-ClMA (R)-3-ClMA Eutectic

117.2 105.6 99.6

117.95 103.21 95.18

27.98 26.24 25.84

27.92 22.55 18.62

In Figure 3, DSC profiles present the melting effect of solid samples of various enantiomeric compositions. The DSC traces showed three single sharp peaks at wR ¼ 1.0, 0.89 and 0.5 corresponding to melting processes of the enantiomer, the eutectic mixture and the racemate. The calorimetric properties of these compositions were summarized in Table 1. Those values were similar to the data announced by the manufacture and the literature.16 However, there was a slight difference between the eutectic point found in the present work (wR ¼ 0.89; 95.188C) and literature data (wR ¼ 0.87; 99.68C).16 The heating rates applied (0.5 K/min in the present work and 2 K/min mentioned in literature15) could be accounted for this difference. The other mixtures (between the eutectic composition and the pure enantiomer and between the eutectic and the racemate) showed a melting domain consisting of two peaks, that is, the eutectic fusion and the subsequent dissolution effect. The eutectic temperature was found at approximately 958C. However, the eutectic effect could be very small or even invisible when the enantiomeric mixtures were close to the enantiomer and/or the racemate composition.

Figure 3. Melting profiles of the enantiomeric 3-ClMA system. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010

Figure 4. Tammann’s plot was drawn from enthalpy of eutectics versus composition.

Tammann’s method5 was carried out to define the eutectic composition and to evaluate the possibility of the solid solutions of 3-ClMA. In Figure 4, the melting enthalpy of eutectic amount in enantiomeric mixtures was a first order function versus composition. The eutectic composition was found at wR  0.89 as the intersection point of two straight lines as shown. Moreover, the extrapolated lines may show limitaf tions by partial miscibility in solid state at DHeu ¼ 0. Actually, these points were found at wR approximately 1.0 and 0.5. Thus, no significant solid solutions exist in the case of 3-ClMA. Five consecutive DSC circles were performed with the same samples to evaluate the possibility for polymorphism. The results for the racemate are shown in Figure 5. There is a significant difference between the first and the second DSC curves due to polymorphism existing. That could be explained according to Ostwald’s theory. Crystallization of a

Figure 5. Five DSC cycles (just the melting phase are shown) of enantiomer and racemate 3-ClMA. DOI 10.1002/jps

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Table 2. Temperature and Heat of Melting of 3-ClMA Polymorphs Tf (8C)

DHf (kJ/mol)

Composition

1st Modification

2nd Modification

1st Modification

2nd Modification

(RS)-3-ClMA (R)-3-ClMA

117.95 form (RS)1 103.21 form (R)1

111.54 form (RS)2 91.99 form (R)2

27.92 22.55

17.77 14.62

polymorphic substance from a melt (or solution) yields crystals which firstly exist in the less stable form. Therefore, material obtained in a fast cooling circle (of molten liquid present in the end of a previous circle) should be the metastable polymorph. In the melting profiles, an endothermic effect was typical at 111.548C (the 2nd DSC curve, see Fig. 5g) for the melting of the metastable racemate (namely (RS)2) while the stable form, that is, (RS)1 showed a melting effect at 117.598C (the 1st DSC cycle, see Fig. 5f). Additionally, a phase transition process from (RS)2 to (RS)1 occurred around 1158C (small exothermic effect in Fig. 5g), which demonstrates that the two polymorphs of racemic 3-ClMA were related via monotropy. On another hand, the XRPD results of the metastable racemate (RS)2 obviously differed from the stable one (RS)1 (Fig. 2b and c). This emphasized the existence of polymorphism in the case of the racemate 3-ClMA. Likewise, the polymorph (R)2 of the enantiomer is clearly distinguishable from the stable (R)1 (both the XRPD patterns and thermal properties in Fig. 2d and e and Tab. 2 or Fig. 5a and b), which leads to the same conclusion. Information about decomposition processes could be also obtained from Figure 5. The melting profiles clearly show that the melting temperatures gradually decreased from the 2nd cycle to the last one due to the enrichment of decomposition products after each DSC cycle. That could be seen by comparing Figure 5b–e for the enantiomer and Figure 5g–k for the racemate. Hereby, the 2nd DSC cycle delivered the ‘‘purest’’ metastable form among the last four heating–cooling cycles. Thus, just the second cycle can be used to evaluate the melting behavior concerned to polymorphism. Practically, there were some drawbacks regarding to reproduction of metastable forms. Hereby, just some experiments were successful. The results are summarized in Table 3. The eutectic point of the metastable diagram was experimentally defined at wR0 ¼ 0.85 and Teu0 ¼ 81.528C. The full diagram shown in Figure 6 was obtained by combining the stable and metastable equilibria. As anticipated, a symmetrical diagram was observed around the racemic axis. A new 1:1 compound was formed in the solid state from (S)- and (R)-enantiomers. Obviously, the shapes of the solidus and liquidus lines in the BPD and XRPD data confirmed again that 3-ClMA is a compound-forming system. DOI 10.1002/jps

This conclusion gained a good agreement with literature data.15 However, neither polymorphism nor solid solutions were mentioned. Finally, the results calculated using Eqs. (1) and (2) relatively fitted well with experimental data for both stable and metastable diagrams. The enantiomeric 3-ClMA system shows ideal behavior in the molten state. Polymorphic Transformation of 3-ClMA in Ambient Conditions and in Aqueous Solution Time-resolved XRPD were measured to study the stability of the polymorphs under ambient conditions. Figure 7 presents the variation of the racemic polymorphs within 3 months. As mentioned above, the racemate 3-ClMA could exist in two polymorphic forms. The metastable form (RS)2 will gradually transform into the stable form (RS)1. Nevertheless, the locations of the XRPD reflexes of the less stable (RS)2 did not change after 3 months (see Fig. 7b–g). Thus, the metastable racemate (RS)2 was stable at least 3 months under ambient conditions. Similarly, Figure 8 shows the transformation process of the polymorphs of the (R)-enantiomer. The metastable form (R)2 recrystallized from the melt exhibited a different crystalline structure to the stable one (Fig. 8a and b). The (R)2 was quite stable for 2 months under ambient conditions (Fig. 8b–f). However, after 3 months (see Fig. 8g), some new reflexes appeared at 188, 228, 328, etc. which belong to those of the stable (R)1. That indicated the ongoing polymorphic transformation. The transformations of the polymorphs of 3-ClMA were studied in aqueous solution to investigate the solvent effect as well. Respectively, this transformaTable 3. Liquidus and Solidus Temperature of Metastable Mixtures Melting Temperature Composition xR (%)

Tm (8C)

Teu (8C)

1.00 0.97 0.95 0.92 0.85 0.65 0.60 0.52 0.50

91.99 91.06 n.d. 86.17 — n.d. 107.50 112.02 111.54

— 81.16 80.05 n.d. 81.52 81.05 81.10 n.d. —

n.d., not determinable. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010

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tions of the racemate in water at 508C, and then recrystallizing the samples at 358C for different periods of time. ‘‘New-born crystals obtained in first few minutes (see Fig. 9b) showed the same structure as the less stable form (RS)2 (crystallized from cooled melt, see Fig. 9a). Those crystals were stable for few hours (Fig. 9c) under above conditions. However, the crystals obtained from the second experiment that grew more than 2 days, showed a different structure (RS)1 (Fig. 9d). According to Ostwald’s theory, (RS)1 was the stable form relative to the metastable one (RS)2. This observation illustrated that the less stable polymorph of racemic 3-ClMA was transformed solvent mediated into the stable one when contacted with aqueous solution. Figure 6. Binary melting point phase diagram of 3ClMA. (*, ~)—present melting processes of stable and metastable forms, respectively. Horizontal axis was in weight fraction, liquidus lines were obtained from Eqs. (1) and (2).

tion process was considerably simple for the (R)enantiomer whereas it was more complicated in the case of the racemate. The XRPD results showed in the investigated domain that the (R)-enantiomer was always found as the stable phase (R)1. In contrast, depending on system conditions, both metastable and stable phases of the racemate could be formed (see Fig. 9). In further investigations, we focused on polymorphic transformation of the racemate in water. One observation was based on the following steps: preparing two samples which were saturated solu-

Effect of Polymorphism on the Solubility In Figure 10 the results of kinetic dissolution experiments performed at 258C are shown. Usually a kinetic dissolution curve has the shape of a logarithm function.11 However, in the case of the racemate 3-ClMA a different behavior was found, which can be explained as follows: the initial racemic material contained a mixture of two polymorphic forms (i.e., (RS)1 and (RS)2). In the first period AB (approximately first 30 min in Fig. 10), both forms dissolved into water and the total concentration reached a maximum value (corresponding to point B). Since only one member of a family of polymorphs can be the most thermodynamically stable form under a given set of conditions as explained by the phase rule, the metastable form spontaneously converts to the stable one during the time required to reach on equilibrium state. The dissolution process under the

Figure 7. The variation of the racemate polymorphs under ambient conditions. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010

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Figure 8. The variation of the enantiomer polymorphs under ambient conditions.

influence of phase transition followed path BC (approximately from 30 to 300 min in Fig. 10). Finally, the solubility equilibrium was established after approximately 5 h (line CD). The saturated solution has a concentration of 2.62%. Similar dissolution processes, affected by polymorphic or solvate phases, were reported in the literature, that is, for sulfamethoxydiazine (sulfameter), carbamazepine, etc.11

In Figure 11, the phase transformation was confirmed by XRPD measurements with the residual solid. The solid particles of samples between 5 and 30 min (see Fig. 11c–f) contained two phases, (RS)1 and (RS)2, according to the continuous dissolution processes of these phases into water. Other solid samples (between 30 and 105 min) contained two phases as well, but the amount of the less stable phase

Figure 9. The racemic 3-ClMA was found as two polymorphs, the metastable form (as a, b, and c) and the stable one (d), which corresponding to two distingue crystal structure. DOI 10.1002/jps

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Figure 10. Kinetics of dissolving the racemate in water at a fixed temperature of 258C.

was reduced in favor of the stable one (Fig. 11f–i). Samples taken after 4 h and 8 days showed only one stable phase (RS)1 and the less stable phase (RS)2 was vanished (Fig. 11k and l). Solubility Phase Diagrams of 3-ClMA Species in Water and Toluene Under isothermal experimental conditions, the XRPD results revealed that the residual solid phases in water were always as the stable forms (RS)1 and (R)1

(Fig. 12b and d). Even a eutectic mixture at 508C contains two stable phases of the racemate and the enantiomer (Fig. 12f). Thus, the TPD of 3-ClMA in water in the studied parameter range was related exclusively to the stable forms. Solubilities measured by three techniques (weight, HPLC, and refractometry), were found to be in relatively good agreement. The determined solubility data of the enantiomer and the racemate are summarized in Table 4. There are some deviations between our own results and literature values.16 The reason is probably due to the presence of different polymorph mixtures. As a result of the lack of confirmation regarding the solid phase structures for the equilibria in aqueous solution in Ref.,16 the obtained solids could exist in any polymorphic form and/or as mixtures of different phases. The results of the solubility measurements carried out in our study differed between isothermal and polythermal methods (Tab. 4), even though the temperature ramp in the polythermal measurements was quite shallow (0.048C/min). This difference could be explained partly due to intrinsic errors of the two methods. Another possibility is that the final solid– liquid equilibria (SLE) might not be fully established during polythermal runs. Therefore the solid particles could exist as metastable phases or as mixtures of less and more stable forms. However, this hypothesis is hard to prove due to the fact that a characterization of the solid phases via XRPD during the polythermal method is impossible. Thus, in the present work, the values obtained from the polythermal method are just seen as an orientation. Thus, the data determined

Figure 11. The variation of the racemic polymorphs in aqueous solution. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010

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Figure 12. XRPD patterns of the residual solid phases of typical samples in solutions of water and toluene.

from isothermal method are considered to be more reliably. Figure 13 presents the solubility curves of the pure substances as function of temperature. In the observed range, the solubility curves are almost linear. The solubility of the racemate increases about 4.5 times between 25 and 508C. Furthermore, the solubility of the enantiomers is even more sensitive to temperature changes and possesses higher values at high temperatures. The TPD of 3-ClMA in water as solvent is drawn in the conventional equilateral triangular shape (Fig. 14). The isothermal solubility curves show that 3-ClMA is a racemic compound-forming system. This conclusion is in good agreement with the BPD mentioned in the previous part and with literature data16 as well. The eutectic compositions change from wR ¼ 0.90–0.84 in a temperature range between 5 and 508C. This shows the nonideal characteristics of the

aqueous equilibria of 3-ClMA. The diagram shows the expected mirror-image symmetry with respect to the racemic axis. The TPD of 3-ClMA in toluene as solvent is presented in Figure 15. The results show that the solubility of 3-ClMA in toluene is much lower than in water. The explanation is based on the structure of this substance, which could establish hydrogen bonds with water, but not with toluene. Furthermore, water is known as a strong polar solvent (dipole moment: 0.787 D), while toluene possesses less polarity (0.36 D). Van der Waals forces between two polar molecules are more significant than between less and more polar molecules. Thus, the experiment confirms the expectation that 3-ClMA is better soluble in water than in toluene. Finally, in contrast to the behavior in water, the solid phases of both the enantiomer and the racemate did not change before and after dissolution in toluene.

Table 4. Solubility of the Enantiomer and the Racemic 3-ClMA at Different Temperatures

Temperature, T (8C) 25

(R)-enantiomer Racemate (R)-enantiomer Racemate (R)-enantiomer Racemate

35 50 a

DOI 10.1002/jps

Composition

Solubility (wt% Isothermal Method) 4.97 2.62 34.84 3.76 80.64 12.03

4.75a 2.4a 20.34a 4.3a — —

Solubility (wt% Polythermal Method) 5.32 3.01 31.85 6.89 71.64 11.68

Literature data.16 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010

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LE MINH ET AL.

Figure 13. Solubility of the 3-ClMA in water versus temperature (iso. and poly. for isothermal and polythermal methods, respectively).

This can be seen in Figure 12g and h. Toluene could reduce or even prohibit the polymorphic transformations of 3-ClMA polymorphs.

CONCLUSIONS X-ray diffraction and solubility studies combined with thermodynamic data analysis revealed that enantiomeric and racemic 3-ClMA can be clearly distinguished with respect to crystalline structure and other properties. Hence, racemic 3-ClMA can be considered as a true racemate (racemic compound). Also the analogous shapes of the solubility isotherms in the TPD and the liquidus curves in the BPD

independently confirm that the chiral 3-ClMA belongs to the racemic compound-forming systems. This conclusion is in agreement with previous reports. Fast cooling of the molten phases undertaken in a helium gas atmosphere indicated polymorphism for both the enantiomer and the racemic species. At ambient conditions, the observed metastable polymorphs were found to be quite stable. In the melting point BPD, the eutectic points were obtained approximately at (wR ¼ 0.89, Teu ¼ 95.188C) and (wR0 ¼ 0.85, Teu0 ¼ 81.528C) for the stable and metastable equilibria, respectively. Finally, 3-ClMA showed an ideal behavior in the molten system and no solid solutions were involved. An isothermal method was applied to determine the TPD of 3-ClMA in water. Monitoring the solid phases during the approach to equilibrium revealed that the dissolution process for 3-ClMA is not trivial. In the parameter range studied, the enantiomer was always the stable form, while the racemic species could exist in two polymorphic forms, (RS)1 and (RS)2, depending on system conditions. The polymorphic transformation processes of the racemic polymorphs are not entirely understood, but effects of temperature and time were found to be very important. Furthermore, the solubility of the enantiomers is extremely temperature-dependent. Within the observed temperature range, the compositions of the eutectic points in the TPD in water changed from wR ¼ 0.90–0.84 in the temperature range between 5 and 508C. Using toluene as an alternative solvent did not lead to phase transformations. A polythermal method was also used to measure the TPD of 3-ClMA in toluene as solvent. The solubilities in toluene were found to be considerably lower than in water.

Figure 14. TPD of 3-ClMA in water. Lines are guides to the eye. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010

DOI 10.1002/jps

ENANTIOMERIC 3-CHLOROMANDELIC ACID SYSTEM

Figure 15. TPD of 3-ClMA in toluene. Axis in scale 0–25%.

Based on the achieved understanding, a future task is development of an efficient approach to separate the (S)- and (R)-enantiomers of 3-ClMA. The acquired knowledge regarding the TPD and polymorphism will be useful to design and optimize a selective crystallization process.

ACKNOWLEDGMENTS We thank Jacqueline Kaufmann and Luise Borchert for their help in HLPC and XRPD measurements.

REFERENCES 1. Wesolowski M. 1992. Thermal analytical methods in pharmaceutical technology. J Therm Anal 39:2239–2245. 2. Collet A. 1999. Separation and purification of enantiomers by crystallization methods (overseas publishers association). Enantiomer 4:157–172. 3. Carstensen JT. 2001. Advanced pharmaceutical solids. Drugs and the pharmaceutical science. Decker: New York.

DOI 10.1002/jps

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4. Caner H, Gronner E, Levy L. 2004. Trends in the development of chiral drugs. Drug Discov Today 9:105–110. 5. Jacques J, Collet A, Wilen SH. 1994. Enantiomers, racemates and resolutions. Krieger. Malabar. 6. Rajendran A, Paredes G, Mazzotti M. 2009. Simulated moving bed chromatography for the separation of enantiomers. J Chromatogr A 1216:709–738. 7. Subramanian G. 2007. Chiral separation techniques: A practical approach, 3rd edition. Weinheim: Wiley-VCH. 8. Lu W, Cole RB. 1998. Determination of chiral pharmaceutical compounds, terbutaline, ketamine and propranolol, by on-line capillary electrophoresis-electrospray ionization mass spectrometry. J Chromatography B 714:69–75. 9. Lorenz H, Perlberg A, Sapoundjiev D, Elsner MP, SeidelMorgenstern A. 2006. Crystallization of enantiomers. Chem Eng Process 45:863–873. 10. Chen AM, Wang Y, Wenslow RM. 2008. Purification of partially resolved enantiomeric mixtures with the guidance of ternary phase diagram. Org Process Res Dev 12:271–281. 11. Brittain HG. 1999. Polymorphism in pharmaceutical solid. Drugs and pharmaceutical science. Decker. New York. 12. Marthi K, Acs M, Po¨ kol G, Tomor K, Eross-Kiss K. 1992. DSC studies on the polymorphism of pharmaceutical substance. J Therm Anal 38:1017–1025. 13. Giron D, Piechon P, Goldbronn C, Pfeffer S. 1999. Thermal analysis, microcalorimetry and combined techniques for the study of the polymorphic behaviour of purine derivative. J Therm Anal Calorim 57:61–73. 14. Gro¨ ger H. 2001. Enzymatic routes to enantiomerically pure aromatic a-hydroxy carboxylic acids: A further example for the diversity of biocatalysis. Adv Synth Catal 343:547– 558. 15. Coppola G, Schuster H. 1997. alpha-Hydroxy Acids in Enantioselective Synthesis. Weinheim: Wiley VCH. 16. Zhang Y, Ray A, Rohani S. 2009. Measurement and prediction of phase diagrams of the enantiomeric 3-chloromandelic acid system. Chem Eng Sci 64:192–197. 17. Sapoundjiev D, Lorenz H, Seidel-Morgenstern A. 2005. Determination of solubility data by mean of calorimetry. Thermochim Acta 436:1–9. 18. Leitao MLP, Eusebio ME, Maria TMR, Redinha JS. 2002. (Solid þ liquid) phase diagram for trans-1,2-cyclohexanediol enantiomer mixtures. J Chem Thermodynamics 34:557–568. 19. Lorenz H, Seidel-Morgenstern A. 2004. A contribution to the mandelic acid phase diagram. Thermochim Acta 415:55–61. 20. von Langermann J, Minh TL, Lorenz H, Seidel-Morgenstern A. 2010. Kombination von Biokatalyse und Kristallisation zur Darstellung enantiomerenreiner Mandelsa¨ urederivative. Chem Ing Tech 1–2:93–100.

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