A curious case of partial spontaneous resolution on crystallization of a racemate. The crystal and molecular structures of N-Acetyl-DL-alanine methylester and N-Acetyl-L-alanine methylester

Zeitschrift fur Naturforschung - Section B Journal of Chemical Sciences

Volumn 56, Issue 9, 2001, Pages 871-880

  Müller, Gerhard ^a   Lutz, Martin ^a,b  

^a UNIVERSITY OF KONSTANZ   (Germany)

^b UTRECHT UNIVERSITY   (Netherlands)

Author keywords

Amino acids; Chirality; Racemate

Indexed keywords

AMIDES; AMINO ACIDS; ATOMS; CHIRALITY; CRYSTAL STRUCTURE; CRYSTALS; MOLECULES; STEREOCHEMISTRY;

ACHIRAL MOLECULE; CRYSTAL AND MOLECULAR STRUCTURE; CRYSTAL PACKINGS; INTERMOLECULAR HYDROGEN BONDS; MONOCLINIC SPACE GROUPS; ORTHORHOMBIC SPACE GROUPS; RACEMATE; SPONTANEOUS RESOLUTION;

HYDROGEN BONDS;

EID: 0041025114     PISSN: 09320776     EISSN: None     Source Type: Journal    
DOI: 10.1515/znb-2001-0903     Document Type: Article

References (94)

1. R. T. Ingwall, C. Gilon, M. Goodmann, Macromolecules 9, 802 (1976).
2. T. Asakura, M. Kamio, A. Nishioka, Biopolymers 18, 467 (1979).
3. M. Meot-Ner (Mautner), J. Am. Chem. Soc. 110, 3075 (1988); ibid. 110, 3071 (1988); ibid. 106, 278 (1984).
4. M. Meot-Ner (Mautner), J. Am. Chem. Soc. 110, 3075 (1988); ibid. 110, 3071 (1988); ibid. 106, 278 (1984).
5. M. Meot-Ner (Mautner), J. Am. Chem. Soc. 110, 3075 (1988); ibid. 110, 3071 (1988); ibid. 106, 278 (1984).
6. S. U. Kokpol, S. V. Hannongbua, W. Yentongchai, B.-M. Rode, Inorg. Chim. Acta 125, 107 (1986).
7. For n-acetyl-D-alanine methylester it was stated: "The product melted too close to room temperature for the m. p. to be determined in a conventional apparatus. The crystals ... melted on standing at room temperature": J. P. Wolf, C. Niemann, Biochemistry 2, 493 (1963).
8. A. J. Kolar, R. K. Olsen, Synthesis 1977, 457.
9. G. Müller, M. Lutz, Z. Kristallogr., Suppl. 18, 87 (2001).
10. Improper rotations are rotoinversions or rotoreflections. Rotoinversions and rotoreflections are equivalent in pairs making it sufficient to adopt only one kind to represent these symmetry operations. The notation most often adopted by chemists for the description of molecular symmetry uses rotoreflections as is done in the Schoenflies notation. Crystallographers, on the other hand, prefer the use of rotoinversions (International or Hermann-Mauguin notation). It should be noted that inversions and reflections may be regarded as special cases either of rotoinversions or of rotoreflections.
11. Point groups: 1, 2, 222, 4, 422, 3, 32, 6, 622, 23, 432.
12. Symmetry operations of the first kind are concerned with congruent parts of an object. They relate chiral objects to each other without changing their chirality. In addition to proper rotations, simple translations or screw axes are symmetry operations of the first kind. Symmetry operations of the second kind invert the chirality of an object. They convert a chiral object into its mirror image (enantiomer). In addition to improper rotations, glide planes are symmetry operations of the second kind.
13. The term "chiral" has also been used for these space groups but the term "enantiomorphic" seems to be more apt. Apparently, the terms "enantiomorphic" and "enantiomorphous" are often used synonymously in the literature.
14. The 230 space groups are conveniently subdivided into 65 non-centrosymmetric space groups without improper rotations [9], 73 non-centrosymmetric space groups containing improper rotations (point groups: m, mm2, 4, 4mm, 42m, 3m, 6, 6mm, 6m2, 43m), and 92 centrosymmetric space groups (point groups: 1̄, 2/m, mmm, 4/m, 4/mmm, 3̄, 3̄m, 6/m, 6/mmm, m3̄, m3̄m).
15. The following texts have been found particularly useful for providing valuable information on the symmetry aspects of molecules and crystals with regard to the scope of this article: J. D. Dunitz, X-Ray Analysis and the Structure of Organic Molecules, Verlag Helvetica Chimica Acta, Zürich (1995); J. P. Glusker, M. Lewis, M. Rossi, Crystal Structure Analysis for Chemists and Biologists, VCH Publishers, New York, Weinheim (1994); M. F. C. Ladd, Symmetry in Molecules and Crystals, Ellis Horwood, Chichester (1989); G. H. Stout, L. H. Jensen, X-Ray Structure Determination - A Practical Guide, John Wiley, New York (1989); L. V. Azároff, Elements of X-Ray Crystallography, McGraw-Hill, New York (1968).
16. The following texts have been found particularly useful for providing valuable information on the symmetry aspects of molecules and crystals with regard to the scope of this article: J. D. Dunitz, X-Ray Analysis and the Structure of Organic Molecules, Verlag Helvetica Chimica Acta, Zürich (1995); J. P. Glusker, M. Lewis, M. Rossi, Crystal Structure Analysis for Chemists and Biologists, VCH Publishers, New York, Weinheim (1994); M. F. C. Ladd, Symmetry in Molecules and Crystals, Ellis Horwood, Chichester (1989); G. H. Stout, L. H. Jensen, X-Ray Structure Determination - A Practical Guide, John Wiley, New York (1989); L. V. Azároff, Elements of X-Ray Crystallography, McGraw-Hill, New York (1968).
17. The following texts have been found particularly useful for providing valuable information on the symmetry aspects of molecules and crystals with regard to the scope of this article: J. D. Dunitz, X-Ray Analysis and the Structure of Organic Molecules, Verlag Helvetica Chimica Acta, Zürich (1995); J. P. Glusker, M. Lewis, M. Rossi, Crystal Structure Analysis for Chemists and Biologists, VCH Publishers, New York, Weinheim (1994); M. F. C. Ladd, Symmetry in Molecules and Crystals, Ellis Horwood, Chichester (1989); G. H. Stout, L. H. Jensen, X-Ray Structure Determination - A Practical Guide, John Wiley, New York (1989); L. V. Azároff, Elements of X-Ray Crystallography, McGraw-Hill, New York (1968).
18. The following texts have been found particularly useful for providing valuable information on the symmetry aspects of molecules and crystals with regard to the scope of this article: J. D. Dunitz, X-Ray Analysis and the Structure of Organic Molecules, Verlag Helvetica Chimica Acta, Zürich (1995); J. P. Glusker, M. Lewis, M. Rossi, Crystal Structure Analysis for Chemists and Biologists, VCH Publishers, New York, Weinheim (1994); M. F. C. Ladd, Symmetry in Molecules and Crystals, Ellis Horwood, Chichester (1989); G. H. Stout, L. H. Jensen, X-Ray Structure Determination - A Practical Guide, John Wiley, New York (1989); L. V. Azároff, Elements of X-Ray Crystallography, McGraw-Hill, New York (1968).
19. The following texts have been found particularly useful for providing valuable information on the symmetry aspects of molecules and crystals with regard to the scope of this article: J. D. Dunitz, X-Ray Analysis and the Structure of Organic Molecules, Verlag Helvetica Chimica Acta, Zürich (1995); J. P. Glusker, M. Lewis, M. Rossi, Crystal Structure Analysis for Chemists and Biologists, VCH Publishers, New York, Weinheim (1994); M. F. C. Ladd, Symmetry in Molecules and Crystals, Ellis Horwood, Chichester (1989); G. H. Stout, L. H. Jensen, X-Ray Structure Determination - A Practical Guide, John Wiley, New York (1989); L. V. Azároff, Elements of X-Ray Crystallography, McGraw-Hill, New York (1968).
20. J. Jacques, A. Collet, S. H. Wilen, Enantiomers, Racemates, and Resolutions, Krieger, Malabar, Fl. (1994).
21. H. D. Flack, G. Bernardinelli, ActaCrystallogr. A55, 908 (1999).
22. The special cases of racemic (inversion) twinning and the crystallization of pairs of enantiomers in enantiomorphic space groups, which are also encountered in the crystallization of racemates, shall not be discussed here. For details see the following leading references. Racemic twinning: Ref. 15 and: A. Karrer, J. D. Dunitz, Acta Crystallogr. A43, 430 (1987); B. S. Green, M. Knossow, Science, 14, 795 (1981). Pairs of enantiomers in enantiomorphic space groups: Ref. 14.
23. The special cases of racemic (inversion) twinning and the crystallization of pairs of enantiomers in enantiomorphic space groups, which are also encountered in the crystallization of racemates, shall not be discussed here. For details see the following leading references. Racemic twinning: Ref. 15 and: A. Karrer, J. D. Dunitz, Acta Crystallogr. A43, 430 (1987); B. S. Green, M. Knossow, Science, 14, 795 (1981). Pairs of enantiomers in enantiomorphic space groups: Ref. 14.
24. L. Pasteur, Ann. Chim. Phys., 3ème Sér. 28, 56 (1850).
25. E. Eliel, S. H. Wilen, Stereochemistry of Organic Compounds, chap. 7, John Wiley, New York (1994).
26. The spontaneous resolution of a racemate has even been used to produce pure enantiomers on an industrial scale. For this a saturated solution of the racemate is seeded with crystals of one of the pure enantiomers whereupon only crystals of the seeded enantiomer form (preferential crystallization; resolution by entrainment). See ref's 14 and 18.
27. W. S. Kipping, W. J. Pope, J. Chem. Soc. Trans. 73, 606 (1898).
28. T. C. W. Mak, G.-D. Zhou, Crystallography in Modern Chemistry, chap. 2.15, John Wiley, New York (1992), and references therein; B. G. Hyde, S. Andersson, Inorganic Crystal Structures, John Wiley, New York (1989), and references therein.
29. T. C. W. Mak, G.-D. Zhou, Crystallography in Modern Chemistry, chap. 2.15, John Wiley, New York (1992), and references therein; B. G. Hyde, S. Andersson, Inorganic Crystal Structures, John Wiley, New York (1989), and references therein.
30. Hydrogen peroxide: J. M. Savariault, M. S. Lehmann, J. Am. Chem. Soc. 102, 1298 (1980); W. R. Busing, H. A. Levy, J. Chem. Phys. 42, 3054 (1965); S. C. Abrahams, R. L. Collins, W. N. Lipscomb, Acta Crystallogr. 4, 15 (1951). Dibenzoyl peroxide: J. M. McBride, M. W. Vary, Tetrahedron 38, 765 (1982); M. Sax, R. K. McMullan, Acta Crystallogr. 22, 281 (1967); G. A. Jeffrey, R. K. McMullan, M. Sax, J. Am. Chem. Soc. 86, 949 (1964).
31. Hydrogen peroxide: J. M. Savariault, M. S. Lehmann, J. Am. Chem. Soc. 102, 1298 (1980); W. R. Busing, H. A. Levy, J. Chem. Phys. 42, 3054 (1965); S. C. Abrahams, R. L. Collins, W. N. Lipscomb, Acta Crystallogr. 4, 15 (1951). Dibenzoyl peroxide: J. M. McBride, M. W. Vary, Tetrahedron 38, 765 (1982); M. Sax, R. K. McMullan, Acta Crystallogr. 22, 281 (1967); G. A. Jeffrey, R. K. McMullan, M. Sax, J. Am. Chem. Soc. 86, 949 (1964).
32. Hydrogen peroxide: J. M. Savariault, M. S. Lehmann, J. Am. Chem. Soc. 102, 1298 (1980); W. R. Busing, H. A. Levy, J. Chem. Phys. 42, 3054 (1965); S. C. Abrahams, R. L. Collins, W. N. Lipscomb, Acta Crystallogr. 4, 15 (1951). Dibenzoyl peroxide: J. M. McBride, M. W. Vary, Tetrahedron 38, 765 (1982); M. Sax, R. K. McMullan, Acta Crystallogr. 22, 281 (1967); G. A. Jeffrey, R. K. McMullan, M. Sax, J. Am. Chem. Soc. 86, 949 (1964).
33. Hydrogen peroxide: J. M. Savariault, M. S. Lehmann, J. Am. Chem. Soc. 102, 1298 (1980); W. R. Busing, H. A. Levy, J. Chem. Phys. 42, 3054 (1965); S. C. Abrahams, R. L. Collins, W. N. Lipscomb, Acta Crystallogr. 4, 15 (1951). Dibenzoyl peroxide: J. M. McBride, M. W. Vary, Tetrahedron 38, 765 (1982); M. Sax, R. K. McMullan, Acta Crystallogr. 22, 281 (1967); G. A. Jeffrey, R. K. McMullan, M. Sax, J. Am. Chem. Soc. 86, 949 (1964).
34. Hydrogen peroxide: J. M. Savariault, M. S. Lehmann, J. Am. Chem. Soc. 102, 1298 (1980); W. R. Busing, H. A. Levy, J. Chem. Phys. 42, 3054 (1965); S. C. Abrahams, R. L. Collins, W. N. Lipscomb, Acta Crystallogr. 4, 15 (1951). Dibenzoyl peroxide: J. M. McBride, M. W. Vary, Tetrahedron 38, 765 (1982); M. Sax, R. K. McMullan, Acta Crystallogr. 22, 281 (1967); G. A. Jeffrey, R. K. McMullan, M. Sax, J. Am. Chem. Soc. 86, 949 (1964).
35. Hydrogen peroxide: J. M. Savariault, M. S. Lehmann, J. Am. Chem. Soc. 102, 1298 (1980); W. R. Busing, H. A. Levy, J. Chem. Phys. 42, 3054 (1965); S. C. Abrahams, R. L. Collins, W. N. Lipscomb, Acta Crystallogr. 4, 15 (1951). Dibenzoyl peroxide: J. M. McBride, M. W. Vary, Tetrahedron 38, 765 (1982); M. Sax, R. K. McMullan, Acta Crystallogr. 22, 281 (1967); G. A. Jeffrey, R. K. McMullan, M. Sax, J. Am. Chem. Soc. 86, 949 (1964).
36. This is in contrast to the spontaneous resolution of a racemate where on complete crystallization equal amounts of the constituent enantiomorphic crystals of the conglomerate must be formed.
37. D. K. Kondepudi, R. J. Kaufman, N. Singh, Science, 250, 975 (1990); J. M. McBride, R. L. Carter, Angew. Chem. 103, 298 (1991); Angew. Chem. Int. Ed. Engl. 30, 293 (1991).
38. D. K. Kondepudi, R. J. Kaufman, N. Singh, Science, 250, 975 (1990); J. M. McBride, R. L. Carter, Angew. Chem. 103, 298 (1991); Angew. Chem. Int. Ed. Engl. 30, 293 (1991).
39. D. K. Kondepudi, R. J. Kaufman, N. Singh, Science, 250, 975 (1990); J. M. McBride, R. L. Carter, Angew. Chem. 103, 298 (1991); Angew. Chem. Int. Ed. Engl. 30, 293 (1991).
40. The crystals on the right side of the equation constitute the racemic conglomerate.
41. In a number of cases the crystallization of racemic solutions of chiral compounds yields crystals of the enantiomers (spontaneous resolution) or of racemic crystals, depending on the conditions. Examples are Pasteur's sodium ammonium tartrate, which crystallizes only below 28 °C as conglomerate, and glutamic acid (α-L-glutamic acid: M. S. Lehmann, A. C. Nunes, Acta Crystallogr. B36, 1621 (1980); β-L-glutamic acid: M. S. Lehmann, T. F. Koetzle, W. C. Hamilton, J. Cryst. Mol. Struct. 2, 225 (1972); DL-glutamic acid: J. D. Dunitz, W. B. Schweizer, Acta Crystallogr. C51, 1377 (1995); DL-glutamic acid monohydrate: Z. Ciunik, T. Glowiak, Acta Crystallogr. C39, 1271 (1983)). Other cases are given in ref. 18. In some cases even mixtures of several forms are obtained simultaneously (M. S. Dunn, M. P. Stoddard, J. Biol. Chem. 121, 521 (1937)). This suggests that apart from thermodynamic factors (crystallization temperature), also kinetic factors may be operative. Not the least, even tiny amounts of impurities may be decisive for the outcome of the crystallization of racemates.
42. In a number of cases the crystallization of racemic solutions of chiral compounds yields crystals of the enantiomers (spontaneous resolution) or of racemic crystals, depending on the conditions. Examples are Pasteur's sodium ammonium tartrate, which crystallizes only below 28 °C as conglomerate, and glutamic acid (α-L-glutamic acid: M. S. Lehmann, A. C. Nunes, Acta Crystallogr. B36, 1621 (1980); β-L-glutamic acid: M. S. Lehmann, T. F. Koetzle, W. C. Hamilton, J. Cryst. Mol. Struct. 2, 225 (1972); DL-glutamic acid: J. D. Dunitz, W. B. Schweizer, Acta Crystallogr. C51, 1377 (1995); DL-glutamic acid monohydrate: Z. Ciunik, T. Glowiak, Acta Crystallogr. C39, 1271 (1983)). Other cases are given in ref. 18. In some cases even mixtures of several forms are obtained simultaneously (M. S. Dunn, M. P. Stoddard, J. Biol. Chem. 121, 521 (1937)). This suggests that apart from thermodynamic factors (crystallization temperature), also kinetic factors may be operative. Not the least, even tiny amounts of impurities may be decisive for the outcome of the crystallization of racemates.
43. In a number of cases the crystallization of racemic solutions of chiral compounds yields crystals of the enantiomers (spontaneous resolution) or of racemic crystals, depending on the conditions. Examples are Pasteur's sodium ammonium tartrate, which crystallizes only below 28 °C as conglomerate, and glutamic acid (α-L-glutamic acid: M. S. Lehmann, A. C. Nunes, Acta Crystallogr. B36, 1621 (1980); β-L-glutamic acid: M. S. Lehmann, T. F. Koetzle, W. C. Hamilton, J. Cryst. Mol. Struct. 2, 225 (1972); DL-glutamic acid: J. D. Dunitz, W. B. Schweizer, Acta Crystallogr. C51, 1377 (1995); DL-glutamic acid monohydrate: Z. Ciunik, T. Glowiak, Acta Crystallogr. C39, 1271 (1983)). Other cases are given in ref. 18. In some cases even mixtures of several forms are obtained simultaneously (M. S. Dunn, M. P. Stoddard, J. Biol. Chem. 121, 521 (1937)). This suggests that apart from thermodynamic factors (crystallization temperature), also kinetic factors may be operative. Not the least, even tiny amounts of impurities may be decisive for the outcome of the crystallization of racemates.
44. In a number of cases the crystallization of racemic solutions of chiral compounds yields crystals of the enantiomers (spontaneous resolution) or of racemic crystals, depending on the conditions. Examples are Pasteur's sodium ammonium tartrate, which crystallizes only below 28 °C as conglomerate, and glutamic acid (α-L-glutamic acid: M. S. Lehmann, A. C. Nunes, Acta Crystallogr. B36, 1621 (1980); β-L-glutamic acid: M. S. Lehmann, T. F. Koetzle, W. C. Hamilton, J. Cryst. Mol. Struct. 2, 225 (1972); DL-glutamic acid: J. D. Dunitz, W. B. Schweizer, Acta Crystallogr. C51, 1377 (1995); DL-glutamic acid monohydrate: Z. Ciunik, T. Glowiak, Acta Crystallogr. C39, 1271 (1983)). Other cases are given in ref. 18. In some cases even mixtures of several forms are obtained simultaneously (M. S. Dunn, M. P. Stoddard, J. Biol. Chem. 121, 521 (1937)). This suggests that apart from thermodynamic factors (crystallization temperature), also kinetic factors may be operative. Not the least, even tiny amounts of impurities may be decisive for the outcome of the crystallization of racemates.
45. In a number of cases the crystallization of racemic solutions of chiral compounds yields crystals of the enantiomers (spontaneous resolution) or of racemic crystals, depending on the conditions. Examples are Pasteur's sodium ammonium tartrate, which crystallizes only below 28 °C as conglomerate, and glutamic acid (α-L-glutamic acid: M. S. Lehmann, A. C. Nunes, Acta Crystallogr. B36, 1621 (1980); β-L-glutamic acid: M. S. Lehmann, T. F. Koetzle, W. C. Hamilton, J. Cryst. Mol. Struct. 2, 225 (1972); DL-glutamic acid: J. D. Dunitz, W. B. Schweizer, Acta Crystallogr. C51, 1377 (1995); DL-glutamic acid monohydrate: Z. Ciunik, T. Glowiak, Acta Crystallogr. C39, 1271 (1983)). Other cases are given in ref. 18. In some cases even mixtures of several forms are obtained simultaneously (M. S. Dunn, M. P. Stoddard, J. Biol. Chem. 121, 521 (1937)). This suggests that apart from thermodynamic factors (crystallization temperature), also kinetic factors may be operative. Not the least, even tiny amounts of impurities may be decisive for the outcome of the crystallization of racemates.
46. In one thoroughly investigated case the difference in packing potential energy between the racemic crystal and the enantiomorphic crystal was estimated computationally to +7 kcal/mol: A. Uchida, J. D. Dunitz, Acta Crystallogr. B46, 45 (1990).
47. C. P. Brock, W. B. Schweizer, J. D. Dunitz, J. Am. Chem. Soc. 113, 9811 (1991).
48. The relative stabilities of racemic crystals and the respective conglomerate are governed only by enthalpy differences. See ref. 28 for a discussion.
49. There is a notable entropic difference between the fusion of an enantiomorphic crystal (leading to an enantiomerically pure liquid, an one-component system) and a racemic crystal or a racemic conglomerate (leading to a racemic liquid, a two-component system). The racemic liquid has an additional mixing entropy of R ln 2 which amounts to 0.4 kcal/mol at T= 300 K. See ref. 28 for a discussion.
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54. c) J. J. Wolff, Angew. Chem. 108, 2339 (1996); Angew. Chem. Int. Ed. Engl. 35, 2195 (1996), and references therein;
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58. g) J. Bernstein, M. C. Etter, L. Leiserowitz, in H.-B. Bürgi, J. D. Dunitz (eds.): Structure Correlation, Vol. 2, chap. 11, VCH, Weinheim (1994);
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65. o) S. Ramdas, N. W. Thomas, in G. Desiraju (ed.): Organic Solid State Chemistry, chap. 12, Elsevier, Amsterdam (1987);
66. p) J. Bernstein, in G. Desiraju (ed.): Organic Solid State Chemistry, chap. 13, Elsevier, Amsterdam (1987);
67. q) A. I. Kitaigorodskii, Molecular Crystals and Molecules, Academic Press, New York (1973);
68. r) A. I. Kitaigorodskii, Organic Chemical Crystallography, Consultant's Bureau, New York (1961).
69. 1/c. Of these only 21 (0.172%) had three independent molecules. (Two independent molecules: 560 (4.590%); four independent molecules: 6 (0.049 %)) [31f].
70. The very good agreement between the structural parameters of the three crystallographically independent molecules in N-acetyl-DL-alanine methylester (Table 2) including the methyl group conformations (Fig. 3) prompted us to search carefully for overlooked higher crystal symmetry. Apart from the usual checks for higher metrical symmetry as described in the Experimental Section, the intermolecular contacts of the independent molecules were examined and found to be different. For example C2 (Mol. 1)⋯C4 (Mol. 1; 1 - x, 1 - y, 1 - z) 3.572(3) Å. No such distance < 4 Å is found for C8 (= C2 in Mol. 2). Atom C14 (= C2 in Mol. 3)⋯C10 (= C4 in Mol. 2; -0.5 + x, 0.5 - y, 0.5 + z) 3.900(4) Å.
71. Table 2 contains the molecular parameters as based on the refined coordinates. They belong to two molecules in the L-configuration (Mol.'s 1, 2) and one in the D-configuration (Mol. 3) as they are shown in Fig. 3.
72. The angle C2-N1-C3 in N-acetyl-L-alanine methylester (120.1(1)°) differs from that in Mol. 3 of the racemate (121.5(2)°) by 0.5° if the sum of the respective 3 esd's is subtracted from the difference.
73. T. Ashida, Y. Tsunogae, I. Tanaka, T. Yamane, Acta Crystallogr. B43, 212 (1987).
74. This is especially so, because the enantiomorphic (enantiomerically pure) crystals of N-acetyl-L-alanine methylester are in equilibrium with a one-component liquid while the racemic crystals of N-acetylDL-alanine methylester are in equilibrium with an entropically favored two-component liquid at their respective melting points [30]. In other words, enantiomorphic crystals may have a higher melting point than their racemic counterparts even if they are thermodynamically less stable. In our case, they have a lower one, however. See Fig. 6 in ref. 28 for an illustration of these facts.
75. E that of the enantiomer.
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80. acetyl hydrogen bonds. While the individual strands in N-acetyl-L-alanine-N′-methylamide are necessarily homochiral, those in the racemic crystals also seem to be so. In the latter case the crystals must be made up of homochiral strands of opposite chirality in a 1:1 ratio. The low quality of the structure determinations as well as disorder in the racemic crystals make it desirable to redetermine these structures, however: Y. Harada, Y. Iitaka, Acta Crystallogr. B30, 1452 (1974).
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