DOI: 10.1021/cg900638h
Spontaneous Transition toward Chirality in the NaClO3 Crystallization in Boiling Solutions
2009, Vol. 9 4802–4806
Zoubir El-Hachemi,*,† Joaquim Crusats,‡ Josep M. Rib o,‡ and Sabino Veintemillas-Verdaguer*,† †
Centro de Astrobiologı´a (CSIC-INTA), Ctra. Ajalvir Km. 4, 28850-Torrej on de Ardoz, Madrid, Spain, anica and Institute of Cosmos Science, Universitat de Barcelona, and ‡Department de Quı´mica Org c. Martı´ i Franqu es 1, 08028-Barcelona Catalonia, Spain Received June 9, 2009; Revised Manuscript Received August 6, 2009
ABSTRACT: The crystallization of NaClO3 from supersaturated boiling solutions leads to a strong bias of enantiomorphic crystals of the same chiral sign, which in the range of the experimental errors cannot be distinguished from that of a homochiral crystal mixture. The crystallization reactor is a closed system but with a temperature gradient between the walls of the reactor and the air/liquid interface that entails an intense recycling of the subcritical nuclei formed during the induction period of the primary nucleation in the bulk. During this period, the evolution of the population of subcritical nuclei takes place without any other noticeable crystal growth process. In these experimental conditions, the fast evolution of a myriad of supercritical nuclei and the immediate separation of the crystals strongly suggest that the evolution toward homochirality should occur during the primary nucleation process. The formation of stationary chiral compositions in the closed system by recycling through the irreversible step of the primary nucleation is discussed in relation to the chiral recognition in the primary nucleation process and the role of the heterogenity created by the phase transition and the nonuniform temperature distribution.
Introduction The crystallization of achiral compounds and chiral compounds in racemic equilibrium with the solution in the form of mixtures of chiral crystals (racemic conglomerates) is of interest in chiral resolutions,1 and is a topic closely related to the spontaneous emergence of chirality and to understanding the robustness of the biological homochirality.2 In this respect, the crystallization of NaClO3 has a long history (see e.g. ref 3) because (a) it shows a remarkable tendency to yield chiral biases by the effect of chiral inductions;3b,j (b) in addition to the kinetic control of the so-called “second order asymmetric transformations”,1a,4 secondary nucleation autocatalytic processes, such as those originated by stirring, can amplify tiny initial chiral excesses of the primary nucleation;3d,g,k and (c) last but not least, the optical isotropy of the NaClO3 crystals facilitates the determination of the chirality of planar deposited crystals (absence of linear birefringence) by observation with a magnifying lens equipped with crossed polarizers.3 The interpretations of the existence of chiral biases in the crystallization of achiral molecules, or of chiral compounds that racemize rapidly compared to their crystallization rate, which crystallize as chiral solids, range from arguments based on the autocatalytic secondary nucleation amplification of a mother crystal,3d,g,k,q,5b to explanations based on bifurcation scenarios at irreversible first stages of the nucleation.3c,5b,c In fact, both processes may lead to a similar chiral outcome, but there is an implicit preference among experimentalists, also based on experimental evidence, for the secondary nucleation autocatalysis. Viedma in 2005 published a seminal experiment6 on the effect of wet grinding of NaClO3 crystals in equilibrium with *To whom correspondence should be addressed. E-mail: elhachemiz@ inta.es (Z.E.);
[email protected] (S.V.-V.). pubs.acs.org/crystal
Published on Web 08/28/2009
their solutions. In contrast with simple gentle stirring, the system under wet grinding evolves to a 100% enantiomeric excess (ee) of (þ)- and (-)-crystals depending on the initial chiral bias or, in the case of initial racemic mixtures, to a 100% ee of random chiral sign. This result is paradoxical, since the input of mechanical energy into the equilibrium crystal/solution system does not modify the character of the closed system (the system exchanging only energy with the surroundings), and the input of more energy, instead of increasing the rate of convergence to the supposedly more stable state, that is, to a racemic mixture of enantiomorphic crystals, leads rather to a collection of crystals of the same chiral sign. This effect has been also detected in achiral compounds yielding separate (þ)- and (-)-crystals7 and in the interesting recent reports on similar spontaneous resolutions of chiral organic compounds when the racemization in the solution is much faster than the crystallization rate.8 These latter systems are analogous to that of NaClO3 in the sense that they are also two component systems (compound plus solvent) in three phases (solution and two enantiomorphic chiral solid phases).9 There are several theoretical reports10 to explain the Viedma experiment. All of them take into account that the supersaturation is locally achieved by the higher solubility of the small crystals eroded by grinding (Ostwald-Freundlich relationship). Most of these models (e.g., refs 10a, c, and d) include chiral recognition pathways. However, in most of the recent reports, the significance of the chiral recognition is underestimated, or even excluded, and the process is being described simply as “continuous- or enhanced-attrition and Ostwald ripening”. Here we present results on the crystallization of NaClO3 in boiling solutions, that reinforces the interpretation by which such chiral resolutions can occur by the strong perturbation of the equilibrium of the enantiomorphic phases with their solution that launches the “recycling” through the primary r 2009 American Chemical Society
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nucleation stage. This means that primary nucleation may lead to a similar chiral outcome as the kinetically controlled crystallization from one “Adam” crystal (second-order asymmetric transformation)11 or as the enhanced secondary nucleation autocatalysis generated by stirring.3d,g,k Results Crystallization from boiling supersaturated aqueous solution is induced by withdrawal of the solvent from the reflux flow. This process can be performed in two ways: (a) Solvent extraction without seeding until the onset of nucleation; until this point the supersaturation continuously increases due to the solvent removal; (b) slow extraction of solvent in the presence of a crystal seed. In this latter case, the solvent extraction rate is controlled in such a way that the supersaturation in the bulk solution is kept approximately constant due to the balancing of the decrease of supersaturation by the crystallization and its increase due to the extraction.12 The first method is used for the preparation of a mass of crystals and the second for the preparation of perfect crystals of centimeter-scale dimensions at low growth rate. In our experiments, we are interested in assessing the chirality of the crystals nucleated in the crystal-free solution, so we will use the approach (a) with a very low rate of solvent extraction (see Experimental Section), but without external seeding. When a critical amount of solvent has been extracted, a sudden nucleation occurs. At this moment, the hot solution was immediately separated from the crystals by filtration (see Experimental Section) and dried under a vacuum. The crystals so obtained are small in size and of irregular and fractured shapes (Figure 1) as consequence of the thermal shock. The same phenomenon has been reported in the case of crystals grown from boiling solution.12c The irregular shape of the crystals (Figure 1) does not allow one to determinate their optical activity [circular birefringence (CB)] using optical polarizers as in the previous reports on NaClO3 crystallizations.3 This is also a difficulty in the case of the Viedma experiment,6 in which a second process of crystal growth is necessary to increase the crystal size in order to determine the chirality sign. Notice that this is not a problem in the case of the corresponding reports on organic compounds,8 because the optical activity of the crystals is measured in the solutions obtained by the crystal redissolution in conditions of no racemization. We have determined the optical activity of the obtained crystal slurry by recording the optical rotary dispersion (ORD) spectra of a Nujol mull of the crystal powders (Experimental Section). This ensures an random orientation of the crystal axis of the crystal powder; that is, it minimizes errors in the determination of the circular birefringence13 (CB), and increases the sensitivity of the measurement in the UV range in which the CB values are much higher than in the visible range, and there is no absorption phenomenon. However, the CB measurements have a low accuracy as the manipulation of the Nujol slurry may lead to errors (e.g., concentration errors due to sedimentation effects) and because the light absorption depends on the size and quality of the crystals.14 For example, even in the case of apparently perfect (þ)- and (-)-NaClO3 monocrystals, the measurement of the Mueller matrix13,15 with the discrimination of the artifactfree linear birefringence contributions to the CB shows ORD spectra of opposite signs, but not perfect mirror images of each other.15 Blank measurements were performed with pure
Figure 1. Optical microscope image of the NaClO3 crystal powder obtained from the boiling solution.
Figure 2. Percentage of the CB values (mdeg mg-1 at 235 nm from the ORD spectra) of the crystal powder outcome of the NaClO3 boiling crystallization with respect to the corresponding values of pure (þ)- and (-)-NaClO3. (a) Aluminum block heater experiments (see text). (b) Electric blanket heater experiments (see text).
(þ)- and (-)-NaClO3 crystals, and the results are normalized to the blank homochiral powder mixtures. As part of our study, we confirmed that the direct crystal powder outcome from wet milling in the Viedma experiment is homochiral within the range of the experimental errors of the CB measurements. The results on the chirality of the crystal powder mixture outcomes, as determined from the ORD spectra are shown in Figure 2. In a total of 32 different crystallizations (no experiment was discarded), 17 were levorotatory, 13 were dextrorotatory, and only 2 experiments yielded a nonoptically active outcome. The experiments correspond to two different experimental set-ups. The results of Figure 2a correspond to experiments performed with an aluminum block heater at a controlled temperature of 180 °C measured at the external part of the aluminum block. The results of Figure 2b correspond to experiments heated with an electric heating blanket operating at its full electrical power, that is, to temperatures in the wall of the flask much higher than those achieved with the aluminum block heater, as evidenced by the stronger boiling and reflux. This indicates the importance in the experiment of the temperature gradient between the flask wall and the boiling solution. More experiments in order to obtain a higher
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statistical significance than that of Figure 2b were not performed due to the increasing hazards due to the high temperatures at the flask walls (see Experimental Section). Discussion One peculiarity of crystal growth from boiling solutions of highly soluble salts such as sodium chlorate is the nature of the saturated solution. In particular, in our case the high concentration attained at saturation at 120 °C implies that there are less than three water molecules available for the solvation of the sodium ion (see Experimental Section), which needs six water molecules to complete the first hydration sphere.16 So, the mother solution is closer to a melt than to an ordinary water solution. Under these conditions the interionic interactions become crucial and high ionic association has been reported in these systems.17 In this respect our experiment is more similar to the previous reports on the crystallization in a melt of 1,10 -binaphtyl18 than to the experimental conditions of NaClO3 crystallization in solution at room temperature. The theory of primary nucleation predicts that under a determined supersaturation only solute clusters larger than a given size (the critical radius) are stable and can develop into a crystal by the incorporation of growth units (isolated ions or molecules or other clusters).19 At low supersaturations the value of the critical radius is relatively high, most of the natural clusters present do not reach this size, and the system remains metastable. As supersaturation increases (by solvent extraction in our case), the critical size decreases and approaches the average cluster size in the solution at the moment when crystallization begins. These nuclei have been observed experimentally by laser scattering ultramicroscopy20 and their formation can be theoretically modeled.21,22 A peculiarity of crystal growth in boiling solutions is the formation of a temperature gradient between the liquid surface and the bottom of the reactor. This is caused by the superheating required by the nucleation of bubbles.23 Under these conditions, and during the period in which the solution remains metastable, the redissolution of crystallites at the bottom and at the walls of the reactor counteracts the primary nucleation process in the bulk and in the air/solution interface. A stationary state is reached which can be maintained for several hours without the appearance of any detectable crystal. Notice that the enantiomeric excesses are higher (compare Figure 2a,b) when this temperature gradient is higher. Our explanation for the onset of homochirality from boiling solutions of NaClO3 is based on phenomena described above and relies on the intense cluster interactions and solute recycling that takes place during the induction period of several hours (see Figure 3a). Notice that in our experimental conditions, that is, relatively high volume of the mother solution (around 230 mL), the soft stirring generated by the bubbles and the high nucleation rate in the surface of the solution due to the evaporation, reduces the possibility that the chirality of the crystal mass was originated by a single “Adam” crystal. The results strongly suggest that cluster-cluster interactions favor the formation of a population of subcritical nuclei of the same chirality. When the system reaches the condition of supersaturation in which the nuclei can grow, the system is already homochiral. Our experimental procedure ensures that the recovered crystals closely correspond to the first crystals formed from the boiling supersaturated solution, because when sudden crystallization occurs, all the system is already seeded by a sea of clusters,
Figure 3. Simplified scheme of the principal processes occurring in (a) boiling solutions; suppression of crystal growth mechanisms after the phase transition to nuclei and their redissolution at the flask walls feeding the irreversible primary nucleation. (b) Solid/ solution equilibrium in stagnant solutions; direct replacement and dissolution of NaClO3 building blocks or their ions.
and thus secondary nucleation and Ostwald Ripening are not relevant. Secondary nucleation in these experimental conditions only implies the production of more crystals but does not explain the origin of the emergent chirality. Therefore, the chirality of the obtained crystal mixture reflects the chirality of the subcritical nuclei generated in the system during the metastable period. In fact, that primary nucleation can lead to a homochiral outcome in NaClO3 has already been proposed by Viedma24 and prior to the experiments of ref 6a. Obviously secondary nucleation and Ostwald ripening can also lead to chirality amplification, but in the experimental conditions described herein a “pseudo equilibrated” steady state among the precritical nuclei favors homochirality, making irrelevant the kinetic phenomenon of secondary nucleation. Figure 3 summarizes the differences between the dynamics of the crystallization in boiling solutions described here with that of the equilibrium of the enantiomorphic phases with the saturated solution. In this latter case (Figure 3b) the solid phases are thermodynamically indistinguishable, as it is inferred from the traditional stability analysis using the Phase Rule:25 two components (compound and solvent) and two phases (solution and one solid phase), that is, two degrees of freedom. In the case of a strong perturbation of the system (either by grinding or by strong temperature gradients) that gives rise to the continuous recycling during the induction period of irreversible primary nucleation (Figure 3a) the solid phases establish a new interaction (chiral recognition) that determines that they are no longer thermodynamically identical; that is, the Phase Rule simply tells us that the system, at fixed P and T, with three phases (solution and two solid phases) is under nonequilibrium conditions and must evolve
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toward a more stable state. In summary, the strong perturbation of the closed system that enables chiral recognition brings the system out of chemical equilibrium. Then, the system evolves toward its most stable state which, according to the previous experimental results6-8 and the results presented herein, for these systems (solutions of achiral or rapidly racemizing chiral compound yielding racemic conglomerates) corresponds to a stable stationary state of homochiral, or near to homochiral, composition. The results presented herein and those previous of refs 6-8 and 24 are of interest to understand the “real” scenarios where spontaneous mirror symmetry breaking may arise.26 In the case of Frank-like reaction networks (enantioselective autocatalysis coupled with a mutual inhibition reaction between the formed enantiomers)27 in closed homogeneous solutions, emergence of chirality may take place, but only when the autocatalytic step is irreversible.28 In this case, when the autocatalysis shows a detectable backward reaction, the final stable state is the racemic one. Furthermore, in other reaction networks showing “recycling” via a different reaction, symmetry breaking is impossible as it is forbidden by the microreversibilty principle.28 However, when the system includes a phase transition and there is a nonuniform temperature distribution (or mechanical energy applied selectively to the crystals but not the solution) equilibrium thermodynamics, and therefore the microreversibility principle, do not apply and symmetry breaking may become possible. A final remark to this point is the importance of identifying the energy flow and thermodynamic forces, that is, the reaction network, that allow the achievement of a such stationary stable chiral state. Experimental Section NaClO3 was purchased from Sigma-Aldrich (ACS reagent) and used as received. Water of Millipore Q quality (18.2 M Ω/cm) was used for the preparation of all the aqueous crystallizations. Optical rotary dispersion spectra (ORD) at room temperature were recorded on Jasco Spectropolarimeter J-810. General Procedure for the Crystallization. The crystallization setup system was similar to that of previous reports on the growing of crystals from boiling solutions.12 It consists of 250 mL glass flasks heated by a heating blanket or by a aluminum block heater (see text) and a distilling head between the flask and a reflux condenser: condensed water vapor is refluxed back to the flask through the distilling head. Part of the refluxed water in the distillation head is allowed to escape from the system through the solvent withdrawal key. The slow rate of the water escape allows one to define the system as a closed system in the discussion of the results. A supersaturated solution of NaClO3 in the crystallization setup system was prepared using 179 g of NaClO3 in 128 mL (mol ratio 1:4.25) of Milli-Q water. A large excess of crystals remains without dissolving at room temperature. The resulting crystals-solution was heated without stirring, but in order to avoid sudden eruptions of the high-viscosity solution 4 g of glass beads (3 mm of diameter) were added to minimize the accumulation of bubbles at the bottom of the flask. The experiments were carried out at a fixed temperature of the heater element (aluminum block heater at 180 °C). After complete crystal dissolution (≈110 °C boiling temperature) heating was continued under these vigorous reflux conditions for 24 h. Supersaturation was attained by withdrawing water from the distilling head: the flow rate was adjusted to 50-60 mL per day. After about 30 h, that is, when about 70 mL had been extracted from the system, sudden (e0.1 s) mass crystallization (≈120 °C boiling temperature) took place throughout the entire flask. There is a risk hazard in the experiments due to the sudden crystallization yielding a thick crystal mixture, which when boiling is continued can lead to ejections out of the system of the strongly oxidating hot NaClO3. The density of the saturated solution at 120 °C is 1.73 g mL-1, which corresponds to a molar ratio H2O/NaClO3 = 2.34. The very thick
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Figure 4. ORD spectra of a Nujol mull (0.1 mm path length; normalized to 100 mg of NaClO3) of powder samples of (-)NaClO3, (þ)-NaClO3, and racemic conglomerate NaClO3 crystals. homogeneous hot solution was immediately filtered under vacuum suction in a hot glass filter holder equipped with a nylon membrane (Whatman, pore size 0.2 μm). Taking advantage of the big amount of crystals formed and with the objective to speed up the filtration process, only a representive part of the slurry was filtrated (leaving the glass beads in the flask). In such a way that a fast drain off of the mother solution could happen, leading to a humid solid which was further dried and stored under vacuum. No microcrystals were observed under the microscope on the crystal surfaces of the final product (Figure 1). Measurement of the Circular Birefringence of NaClO3 Crystal Mixtures. CB was determined by recording the ORD spectra in the range 230-400 nm, where the CB values are much higher than in the visible range. Furthermore, no interferences from circular dichroism occur due to the absence of electronic absorptions. To avoid the contamination of the CB measurement with linear dichroic and linear birefringence contributions,13 the samples were analyzed as a fine powder (slight manual grinding of the dry crystals in an agate mortar; this was only necessary in the case of the blank samples) dispersed in Nujol oil (50-70 mg of NaClO3 in 180 mg of Nujol). These Nujol mulls were sandwiched between two cylindrical plates of a detachable quartz cuvette (path length l = 0.1 mm). This method is used in the detection of CD and CB of chiral crystal mixtures.30 Blank measurements were performed with powder obtained from pure enantiomorphic crystals (Figure 4). The ORD recordings, once the baseline of the pure Nujol blank is discounted (accounting for the different refraction index of the achiral Nujol oil), were normalized to the weight of crystals and to the ORD spectra of pure (þ)- and (-)-NaClO3 crystals (see for example, Figure 3). Reproducibility of the measurement was on the order of (30%, because of the difficulty of placing the same amount of chiral substance in the cuvette, for example, because of the partial sedimentation of the crystals in the mull. Further, the optical properties of the crystals depend on crystal defects.14,15 However, none of this hinders the detection of CB nor obtaining a semiquantitative evaluation of the CB compared to that of pure chiral crystal samples and of racemic mixtures. Notice that the method allows the detection of CB in samples of irregularly shaped microcrystals in contrast to the common evaluation of the chirality of NaClO3 crystals in the polarizing microscope, which needs regular relatively large crystals. The examples of ORD spectra reproduced in Figure 4 correspond to CB values (mdeg mg-1) at 235 nm obtained in the former experimental conditions: The CB values of Figure 3 correspond to the CB values (mdeg mg-1) at 235 nm obtained in the former experimental conditions normalized to the corresponding CB values of pure (þ)- and (-)-NaClO3 blanks measured in the same session.
Acknowledgment. This work has been supported by the Spanish Ministry of Science (MEC) AYA2006-15648-C02-01 and 02 and forms part of the COST Action CM07030 Systems
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Chemistry). We thank Dr. Carlos Escudero for his help in the ORD measurements.
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