The dimeric condensation product of lactic acid, namely (S,S)-2-[(2-hydroxypropanoyl)oxy]propanoic acid, C6H10O5, (I), crystallizes with two independent molecules in the asymmetric unit, which both have an essentially planar backbone. The trimeric condensation product, namely (S,S,S)-3-hydroxybut-3-en-2-yl 2-[(2-hydroxypropanoyl)oxy]propanoate, C9H14O7, (II), has one molecule in the asymmetric unit and consists of two essentially planar parts, with the central C-O bond in a gauche conformation. Both molecules of the dimer are involved in intermolecular hydrogen bonds, forming chains with a C(8) graph set. These chains are connected by D(2) hydrogen bonds to form a two-dimensional layer. The trimer forms hydrogen-bonded C(10) and C22(6) chains, which together result in a two-dimensional motif. The Hooft method [Hooft, Straver & Spek (2008). J. Appl. Cryst. 41, 96-103] was successfully applied to the determination of the absolute structure of (I).
Supporting information
CCDC references: 790641; 790642
(S,S)-Lactoyllactic acid, (I), was prepared as follows.
(S,S)-Lactide (401 g) was mixed with demineralized water (100 g)
and acetone (161 g). The mixture was refluxed for 5 h and afterwards cooled to
about 313 K. The excess water, the acetone and some monomeric lactic acid were
then removed by means of a short-path distillation (KDL-4, UIC). The resulting
product contained about 90wt% of (S,S)-lactoyllactic acid. The
crude product (398 g) was mixed with diisopropyl ether (112 g), cooled to 288 K and seeded. After 2.5 h at 288 K the crystals were separated off using a
filtering centrifuge. The crystals were dried in vacuo at room temperature.
The yield was 177 g of dried solids (differential scanning calorimetry m.p.
318.7 K, heat of fusion 93 kJ kg-1).
(S,S,S)-Lactoyllactoyllactic acid, (II), was prepared as
follows. (S,S)-Lactide (637 g) was mixed with (S)-lactic acid
(230 g). The mixture was heated to 423 K and allowed to react for 1 h, then
cooled rapidly to 353 K. Further overnight cooling to room temperature
resulted in crystallization of the excess lactide. The viscous suspension was
separated by means of a filtering centrifuge, resulting in a viscous filtrate
containing lactic acid and oligomers up to a degree of polymerization of about
12, and some lactide. The viscous liquid was separated by means of a
short-path distillation at low pressure (1 mbara; 1 bar = 100000 Pa) to remove
lactic acid, lactide and some of the lactoyllactic acid. After distillation,
the residue, containing about 25% of trimer, was used for further processing
by means of preparative chromatography (Sepacor, Buchi). Separation of the
oligomer mixture was achieved with a solvent gradient from water–methanol
85:15 (v/v) to methanol–acetonitrile 40:60 (v/v).
The fractions containing the (S)-lactic acid trimer were combined and
evaporated to dryness at 303 K and 20 mbara. Each 10 g of crude oligomer
mixture resulted in 2.2 g of pure (S)-lactic acid trimer after
chromatography. The product crystallized slowly after concentration and had a
purity of 95–97% [differential scanning calorimetry m.p. 346.2 K (onset); heat
of fusion 83 kJ kg-1].
Friedel pairs were merged prior to refinement. H atoms were located in
difference Fourier maps. Thereafter, hydroxy H atoms were refined freely with
isotropic displacement parameters and all other H atoms were refined using a
riding model, with C—H = 0.98 for methyl H atoms or 1.00 Å for the other H
atoms, and with Uiso(H) = 1.5Ueq(C) for methyl H atoms or
1.2Ueq(C) for the other H atoms.
For both compounds, data collection: COLLECT (Nonius, 1999); cell refinement: PEAKREF (Schreurs, 2005); data reduction: EVAL15 (Schreurs et al., 2010) and SADABS (Sheldrick, 2008a); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008b); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008b); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: manual editing of CIF from SHELXL97 (Sheldrick, 2008b).
(I) (
S,
S)-2-[(2-Hydroxypropanoyl)oxy]propanoic acid
top
Crystal data top
C6H10O5 | F(000) = 688 |
Mr = 162.14 | Dx = 1.376 Mg m−3 |
Orthorhombic, P212121 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2ac 2ab | Cell parameters from 62696 reflections |
a = 7.94437 (7) Å | θ = 1.7–35.0° |
b = 11.71472 (17) Å | µ = 0.12 mm−1 |
c = 16.82348 (12) Å | T = 150 K |
V = 1565.70 (3) Å3 | Block, colourless |
Z = 8 | 0.38 × 0.36 × 0.24 mm |
Data collection top
Nonius KappaCCD area-detector diffractometer | 3856 independent reflections |
Radiation source: rotating anode | 3674 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.026 |
ϕ and ω scans | θmax = 35.0°, θmin = 2.1° |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | h = −12→12 |
Tmin = 0.703, Tmax = 0.747 | k = −18→18 |
69491 measured reflections | l = −27→27 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.026 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.076 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.07 | w = 1/[σ2(Fo2) + (0.0528P)2 + 0.0671P] where P = (Fo2 + 2Fc2)/3 |
3856 reflections | (Δ/σ)max < 0.001 |
219 parameters | Δρmax = 0.31 e Å−3 |
0 restraints | Δρmin = −0.20 e Å−3 |
Crystal data top
C6H10O5 | V = 1565.70 (3) Å3 |
Mr = 162.14 | Z = 8 |
Orthorhombic, P212121 | Mo Kα radiation |
a = 7.94437 (7) Å | µ = 0.12 mm−1 |
b = 11.71472 (17) Å | T = 150 K |
c = 16.82348 (12) Å | 0.38 × 0.36 × 0.24 mm |
Data collection top
Nonius KappaCCD area-detector diffractometer | 3856 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | 3674 reflections with I > 2σ(I) |
Tmin = 0.703, Tmax = 0.747 | Rint = 0.026 |
69491 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.026 | 0 restraints |
wR(F2) = 0.076 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.07 | Δρmax = 0.31 e Å−3 |
3856 reflections | Δρmin = −0.20 e Å−3 |
219 parameters | |
Special details top
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell e.s.d.'s are taken
into account individually in the estimation of e.s.d.'s in distances, angles
and torsion angles; correlations between e.s.d.'s in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.
planes. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
O11 | 0.82164 (10) | 0.42378 (6) | 0.10719 (5) | 0.03150 (16) | |
O21 | 0.89807 (9) | 0.27881 (6) | 0.18537 (5) | 0.02953 (14) | |
H21O | 0.983 (3) | 0.3293 (17) | 0.2025 (12) | 0.054 (5)* | |
O31 | 0.49795 (7) | 0.32710 (6) | 0.10479 (3) | 0.02104 (11) | |
O41 | 0.49862 (12) | 0.34578 (10) | 0.23757 (4) | 0.0457 (2) | |
O51 | 0.15854 (8) | 0.41356 (7) | 0.21850 (4) | 0.02722 (13) | |
H51O | 0.200 (2) | 0.4338 (15) | 0.2641 (11) | 0.042 (4)* | |
C11 | 0.79636 (9) | 0.33017 (6) | 0.13505 (4) | 0.01769 (11) | |
C21 | 0.64675 (10) | 0.25651 (6) | 0.11113 (4) | 0.01838 (12) | |
H21 | 0.6286 | 0.1944 | 0.1510 | 0.022* | |
C31 | 0.67689 (11) | 0.20615 (7) | 0.02930 (5) | 0.02385 (14) | |
H31A | 0.6902 | 0.2680 | −0.0094 | 0.036* | |
H31B | 0.5807 | 0.1585 | 0.0142 | 0.036* | |
H31C | 0.7792 | 0.1595 | 0.0302 | 0.036* | |
C41 | 0.43711 (11) | 0.36720 (9) | 0.17371 (5) | 0.02484 (15) | |
C51 | 0.28349 (10) | 0.44303 (8) | 0.16164 (5) | 0.02380 (15) | |
H51 | 0.2373 | 0.4288 | 0.1072 | 0.029* | |
C61 | 0.33359 (14) | 0.56804 (10) | 0.16788 (7) | 0.03360 (19) | |
H61A | 0.2332 | 0.6161 | 0.1626 | 0.050* | |
H61B | 0.4134 | 0.5867 | 0.1254 | 0.050* | |
H61C | 0.3864 | 0.5819 | 0.2196 | 0.050* | |
O12 | 0.26701 (9) | 0.46502 (5) | 0.37050 (4) | 0.02352 (12) | |
O22 | 0.38102 (9) | 0.36903 (7) | 0.47259 (4) | 0.02930 (14) | |
H22O | 0.464 (2) | 0.4222 (16) | 0.4669 (11) | 0.048 (5)* | |
O32 | −0.01762 (7) | 0.33292 (5) | 0.38147 (3) | 0.02008 (10) | |
O42 | −0.04579 (9) | 0.44065 (7) | 0.49146 (4) | 0.03008 (15) | |
O52 | −0.35923 (8) | 0.50074 (6) | 0.45163 (4) | 0.02718 (13) | |
H52O | −0.294 (2) | 0.5170 (16) | 0.4907 (11) | 0.046 (5)* | |
C12 | 0.26684 (9) | 0.38520 (6) | 0.41660 (5) | 0.01832 (12) | |
C22 | 0.13758 (10) | 0.28996 (7) | 0.41478 (5) | 0.02043 (13) | |
H22 | 0.1173 | 0.2610 | 0.4699 | 0.025* | |
C32 | 0.19469 (13) | 0.19297 (7) | 0.36180 (7) | 0.03069 (18) | |
H32A | 0.2182 | 0.2224 | 0.3084 | 0.046* | |
H32B | 0.1058 | 0.1351 | 0.3588 | 0.046* | |
H32C | 0.2970 | 0.1587 | 0.3839 | 0.046* | |
C42 | −0.09660 (9) | 0.40899 (7) | 0.42739 (5) | 0.01950 (12) | |
C52 | −0.25657 (10) | 0.45373 (7) | 0.39000 (5) | 0.02163 (13) | |
H52 | −0.3180 | 0.3889 | 0.3644 | 0.026* | |
C62 | −0.21641 (15) | 0.54314 (10) | 0.32719 (7) | 0.0359 (2) | |
H62A | −0.3212 | 0.5777 | 0.3082 | 0.054* | |
H62B | −0.1577 | 0.5069 | 0.2826 | 0.054* | |
H62C | −0.1444 | 0.6024 | 0.3504 | 0.054* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O11 | 0.0341 (3) | 0.0207 (2) | 0.0397 (4) | −0.0083 (2) | −0.0153 (3) | 0.0088 (3) |
O21 | 0.0240 (3) | 0.0263 (3) | 0.0383 (3) | −0.0023 (2) | −0.0122 (3) | 0.0091 (3) |
O31 | 0.0167 (2) | 0.0304 (3) | 0.0160 (2) | 0.0013 (2) | −0.00005 (17) | −0.0028 (2) |
O41 | 0.0386 (4) | 0.0800 (7) | 0.0185 (3) | 0.0281 (5) | −0.0068 (3) | −0.0092 (3) |
O51 | 0.0161 (2) | 0.0440 (4) | 0.0215 (2) | −0.0047 (2) | −0.0009 (2) | −0.0040 (3) |
C11 | 0.0169 (3) | 0.0180 (3) | 0.0182 (3) | −0.0002 (2) | −0.0006 (2) | −0.0005 (2) |
C21 | 0.0173 (3) | 0.0191 (3) | 0.0187 (3) | −0.0022 (2) | 0.0016 (2) | −0.0001 (2) |
C31 | 0.0253 (3) | 0.0239 (3) | 0.0224 (3) | −0.0042 (3) | 0.0036 (3) | −0.0069 (3) |
C41 | 0.0186 (3) | 0.0386 (4) | 0.0173 (3) | 0.0040 (3) | −0.0014 (2) | −0.0058 (3) |
C51 | 0.0160 (3) | 0.0389 (4) | 0.0165 (3) | 0.0026 (3) | −0.0010 (2) | −0.0046 (3) |
C61 | 0.0281 (4) | 0.0392 (5) | 0.0334 (4) | −0.0015 (4) | 0.0070 (4) | 0.0002 (4) |
O12 | 0.0267 (3) | 0.0233 (2) | 0.0205 (2) | −0.0041 (2) | −0.0009 (2) | 0.0022 (2) |
O22 | 0.0186 (2) | 0.0362 (3) | 0.0331 (3) | −0.0004 (2) | −0.0068 (2) | 0.0082 (3) |
O32 | 0.0157 (2) | 0.0224 (2) | 0.0221 (2) | 0.00249 (19) | 0.00024 (18) | −0.00292 (19) |
O42 | 0.0240 (3) | 0.0446 (4) | 0.0217 (3) | 0.0080 (3) | −0.0034 (2) | −0.0093 (3) |
O52 | 0.0150 (2) | 0.0355 (3) | 0.0310 (3) | 0.0033 (2) | 0.0005 (2) | −0.0106 (3) |
C12 | 0.0154 (3) | 0.0211 (3) | 0.0185 (3) | 0.0025 (2) | 0.0019 (2) | −0.0006 (2) |
C22 | 0.0164 (3) | 0.0198 (3) | 0.0251 (3) | 0.0014 (2) | 0.0015 (2) | 0.0034 (2) |
C32 | 0.0285 (4) | 0.0192 (3) | 0.0443 (5) | 0.0056 (3) | 0.0015 (4) | −0.0035 (3) |
C42 | 0.0147 (3) | 0.0237 (3) | 0.0201 (3) | 0.0010 (2) | 0.0013 (2) | −0.0017 (2) |
C52 | 0.0155 (3) | 0.0255 (3) | 0.0239 (3) | 0.0018 (2) | −0.0027 (2) | −0.0051 (3) |
C62 | 0.0357 (5) | 0.0390 (5) | 0.0331 (4) | 0.0080 (4) | −0.0004 (4) | 0.0091 (4) |
Geometric parameters (Å, º) top
O11—C11 | 1.2093 (10) | O12—C12 | 1.2148 (10) |
O21—C11 | 1.3159 (10) | O22—C12 | 1.3213 (10) |
O21—H21O | 0.94 (2) | O22—H22O | 0.912 (19) |
O31—C41 | 1.3411 (10) | O32—C42 | 1.3359 (9) |
O31—C21 | 1.4467 (10) | O32—C22 | 1.4449 (10) |
O41—C41 | 1.2068 (11) | O42—C42 | 1.2093 (10) |
O51—C51 | 1.4212 (11) | O52—C52 | 1.4295 (10) |
O51—H51O | 0.869 (18) | O52—H52O | 0.857 (19) |
C11—C21 | 1.5229 (11) | C12—C22 | 1.5167 (11) |
C21—C31 | 1.5169 (11) | C22—C32 | 1.5136 (12) |
C21—H21 | 1.0000 | C22—H22 | 1.0000 |
C31—H31A | 0.9800 | C32—H32A | 0.9800 |
C31—H31B | 0.9800 | C32—H32B | 0.9800 |
C31—H31C | 0.9800 | C32—H32C | 0.9800 |
C41—C51 | 1.5231 (12) | C42—C52 | 1.5117 (11) |
C51—C61 | 1.5212 (15) | C52—C62 | 1.5216 (14) |
C51—H51 | 1.0000 | C52—H52 | 1.0000 |
C61—H61A | 0.9800 | C62—H62A | 0.9800 |
C61—H61B | 0.9800 | C62—H62B | 0.9800 |
C61—H61C | 0.9800 | C62—H62C | 0.9800 |
| | | |
C11—O21—H21O | 110.5 (12) | C12—O22—H22O | 108.9 (12) |
C41—O31—C21 | 115.52 (6) | C42—O32—C22 | 114.13 (6) |
C51—O51—H51O | 105.1 (12) | C52—O52—H52O | 107.4 (12) |
O11—C11—O21 | 124.19 (8) | O12—C12—O22 | 124.38 (8) |
O11—C11—C21 | 122.75 (7) | O12—C12—C22 | 123.65 (7) |
O21—C11—C21 | 112.96 (7) | O22—C12—C22 | 111.95 (7) |
O31—C21—C31 | 106.52 (6) | O32—C22—C32 | 106.79 (7) |
O31—C21—C11 | 109.47 (6) | O32—C22—C12 | 109.23 (6) |
C31—C21—C11 | 109.69 (6) | C32—C22—C12 | 111.17 (7) |
O31—C21—H21 | 110.4 | O32—C22—H22 | 109.9 |
C31—C21—H21 | 110.4 | C32—C22—H22 | 109.9 |
C11—C21—H21 | 110.4 | C12—C22—H22 | 109.9 |
C21—C31—H31A | 109.5 | C22—C32—H32A | 109.5 |
C21—C31—H31B | 109.5 | C22—C32—H32B | 109.5 |
H31A—C31—H31B | 109.5 | H32A—C32—H32B | 109.5 |
C21—C31—H31C | 109.5 | C22—C32—H32C | 109.5 |
H31A—C31—H31C | 109.5 | H32A—C32—H32C | 109.5 |
H31B—C31—H31C | 109.5 | H32B—C32—H32C | 109.5 |
O41—C41—O31 | 123.44 (8) | O42—C42—O32 | 124.28 (7) |
O41—C41—C51 | 124.36 (8) | O42—C42—C52 | 123.04 (7) |
O31—C41—C51 | 112.20 (7) | O32—C42—C52 | 112.67 (7) |
O51—C51—C61 | 111.72 (7) | O52—C52—C42 | 108.14 (7) |
O51—C51—C41 | 109.16 (7) | O52—C52—C62 | 110.99 (8) |
C61—C51—C41 | 110.04 (8) | C42—C52—C62 | 110.57 (7) |
O51—C51—H51 | 108.6 | O52—C52—H52 | 109.0 |
C61—C51—H51 | 108.6 | C42—C52—H52 | 109.0 |
C41—C51—H51 | 108.6 | C62—C52—H52 | 109.0 |
C51—C61—H61A | 109.5 | C52—C62—H62A | 109.5 |
C51—C61—H61B | 109.5 | C52—C62—H62B | 109.5 |
H61A—C61—H61B | 109.5 | H62A—C62—H62B | 109.5 |
C51—C61—H61C | 109.5 | C52—C62—H62C | 109.5 |
H61A—C61—H61C | 109.5 | H62A—C62—H62C | 109.5 |
H61B—C61—H61C | 109.5 | H62B—C62—H62C | 109.5 |
| | | |
C41—O31—C21—C31 | 171.34 (7) | C42—O32—C22—C32 | 171.42 (7) |
C41—O31—C21—C11 | −70.13 (9) | C42—O32—C22—C12 | −68.27 (8) |
O11—C11—C21—O31 | −41.75 (10) | O12—C12—C22—O32 | −27.49 (10) |
O21—C11—C21—O31 | 141.63 (7) | O22—C12—C22—O32 | 154.31 (7) |
O11—C11—C21—C31 | 74.79 (10) | O12—C12—C22—C32 | 90.10 (10) |
O21—C11—C21—C31 | −101.83 (8) | O22—C12—C22—C32 | −88.10 (9) |
C21—O31—C41—O41 | −1.00 (15) | C22—O32—C42—O42 | −0.05 (12) |
C21—O31—C41—C51 | 178.52 (7) | C22—O32—C42—C52 | 178.90 (6) |
O41—C41—C51—O51 | −44.90 (14) | O42—C42—C52—O52 | −20.96 (12) |
O31—C41—C51—O51 | 135.58 (8) | O32—C42—C52—O52 | 160.08 (7) |
O41—C41—C51—C61 | 78.04 (13) | O42—C42—C52—C62 | 100.74 (11) |
O31—C41—C51—C61 | −101.48 (9) | O32—C42—C52—C62 | −78.22 (9) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O21—H21O···O51i | 0.94 (2) | 1.73 (2) | 2.6617 (10) | 169.4 (19) |
O51—H51O···O12 | 0.869 (18) | 1.902 (18) | 2.7650 (9) | 171.7 (17) |
O22—H22O···O52i | 0.912 (19) | 1.698 (19) | 2.6006 (10) | 169.6 (18) |
O52—H52O···O42 | 0.857 (19) | 2.168 (18) | 2.6730 (9) | 117.4 (15) |
O52—H52O···O11ii | 0.857 (19) | 2.090 (19) | 2.7785 (11) | 136.9 (16) |
Symmetry codes: (i) x+1, y, z; (ii) −x+1/2, −y+1, z+1/2. |
(II) (
S,
S,
S)-3-Hydroxybut-3-en-2-yl
2-[(2-hydroxypropanoyl)oxy]propanoate
top
Crystal data top
C9H14O7 | F(000) = 496 |
Mr = 234.20 | Dx = 1.371 Mg m−3 |
Monoclinic, C2 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: C 2y | Cell parameters from 17211 reflections |
a = 17.3594 (5) Å | θ = 1.8–27.5° |
b = 5.62712 (17) Å | µ = 0.12 mm−1 |
c = 13.9888 (6) Å | T = 150 K |
β = 123.861 (1)° | Needle, colourless |
V = 1134.71 (7) Å3 | 0.48 × 0.12 × 0.09 mm |
Z = 4 | |
Data collection top
Nonius KappaCCD area-detector diffractometer | 1443 independent reflections |
Radiation source: rotating anode | 1358 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.028 |
ϕ and ω scans | θmax = 27.5°, θmin = 1.8° |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | h = −22→22 |
Tmin = 0.707, Tmax = 0.746 | k = −7→7 |
19858 measured reflections | l = −18→18 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.028 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.069 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.07 | w = 1/[σ2(Fo2) + (0.0318P)2 + 0.5155P] where P = (Fo2 + 2Fc2)/3 |
1443 reflections | (Δ/σ)max < 0.001 |
156 parameters | Δρmax = 0.17 e Å−3 |
1 restraint | Δρmin = −0.15 e Å−3 |
Crystal data top
C9H14O7 | V = 1134.71 (7) Å3 |
Mr = 234.20 | Z = 4 |
Monoclinic, C2 | Mo Kα radiation |
a = 17.3594 (5) Å | µ = 0.12 mm−1 |
b = 5.62712 (17) Å | T = 150 K |
c = 13.9888 (6) Å | 0.48 × 0.12 × 0.09 mm |
β = 123.861 (1)° | |
Data collection top
Nonius KappaCCD area-detector diffractometer | 1443 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | 1358 reflections with I > 2σ(I) |
Tmin = 0.707, Tmax = 0.746 | Rint = 0.028 |
19858 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.028 | 1 restraint |
wR(F2) = 0.069 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.07 | Δρmax = 0.17 e Å−3 |
1443 reflections | Δρmin = −0.15 e Å−3 |
156 parameters | |
Special details top
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell e.s.d.'s are taken
into account individually in the estimation of e.s.d.'s in distances, angles
and torsion angles; correlations between e.s.d.'s in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.
planes. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
O1 | 0.17066 (10) | 0.4613 (3) | 0.73316 (11) | 0.0382 (3) | |
O2 | 0.11475 (11) | 0.7716 (3) | 0.61303 (13) | 0.0438 (4) | |
H2O | 0.100 (2) | 0.823 (8) | 0.661 (3) | 0.085 (10)* | |
O3 | 0.25546 (7) | 0.2979 (2) | 0.62751 (10) | 0.0277 (3) | |
O4 | 0.35275 (9) | 0.6008 (3) | 0.72761 (12) | 0.0344 (3) | |
O5 | 0.49699 (7) | 0.2950 (3) | 0.85280 (9) | 0.0276 (3) | |
O6 | 0.50562 (9) | 0.3483 (3) | 0.70013 (11) | 0.0324 (3) | |
O7 | 0.68303 (10) | 0.5210 (3) | 0.84047 (14) | 0.0395 (4) | |
H7O | 0.6698 (19) | 0.645 (6) | 0.803 (2) | 0.055 (8)* | |
C1 | 0.15700 (12) | 0.5649 (3) | 0.64948 (15) | 0.0275 (4) | |
C2 | 0.17984 (11) | 0.4664 (4) | 0.56672 (14) | 0.0258 (4) | |
H2 | 0.1973 | 0.5983 | 0.5344 | 0.031* | |
C3 | 0.09807 (12) | 0.3285 (4) | 0.47044 (15) | 0.0339 (4) | |
H3A | 0.0817 | 0.2000 | 0.5033 | 0.051* | |
H3B | 0.1148 | 0.2609 | 0.4197 | 0.051* | |
H3C | 0.0449 | 0.4354 | 0.4261 | 0.051* | |
C4 | 0.33751 (12) | 0.3925 (3) | 0.70986 (15) | 0.0249 (4) | |
C5 | 0.40548 (11) | 0.1942 (3) | 0.77945 (14) | 0.0247 (4) | |
H5 | 0.4050 | 0.0729 | 0.7269 | 0.030* | |
C6 | 0.38168 (12) | 0.0789 (4) | 0.85764 (15) | 0.0311 (4) | |
H6A | 0.3839 | 0.1985 | 0.9100 | 0.047* | |
H6B | 0.4266 | −0.0474 | 0.9025 | 0.047* | |
H6C | 0.3192 | 0.0108 | 0.8110 | 0.047* | |
C7 | 0.53842 (12) | 0.3725 (3) | 0.80130 (15) | 0.0251 (4) | |
C8 | 0.63027 (12) | 0.4907 (4) | 0.88781 (16) | 0.0334 (4) | |
H8 | 0.6656 | 0.3837 | 0.9562 | 0.040* | |
C9 | 0.61305 (18) | 0.7229 (6) | 0.9273 (3) | 0.0732 (10) | |
H9A | 0.5828 | 0.8350 | 0.8628 | 0.110* | |
H9B | 0.6723 | 0.7893 | 0.9899 | 0.110* | |
H9C | 0.5729 | 0.6947 | 0.9549 | 0.110* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.0537 (8) | 0.0336 (8) | 0.0374 (7) | 0.0070 (7) | 0.0316 (7) | 0.0096 (6) |
O2 | 0.0722 (10) | 0.0341 (8) | 0.0475 (8) | 0.0209 (8) | 0.0472 (8) | 0.0135 (7) |
O3 | 0.0223 (5) | 0.0263 (7) | 0.0298 (6) | 0.0011 (5) | 0.0116 (5) | −0.0027 (5) |
O4 | 0.0326 (7) | 0.0265 (7) | 0.0439 (8) | −0.0043 (6) | 0.0212 (6) | −0.0046 (6) |
O5 | 0.0208 (5) | 0.0392 (8) | 0.0254 (5) | −0.0048 (6) | 0.0145 (5) | 0.0005 (6) |
O6 | 0.0372 (7) | 0.0366 (8) | 0.0339 (6) | 0.0003 (6) | 0.0262 (6) | 0.0004 (6) |
O7 | 0.0396 (7) | 0.0311 (8) | 0.0697 (10) | 0.0041 (7) | 0.0440 (8) | 0.0097 (8) |
C1 | 0.0289 (8) | 0.0272 (9) | 0.0294 (8) | 0.0018 (8) | 0.0181 (7) | 0.0028 (7) |
C2 | 0.0250 (8) | 0.0263 (9) | 0.0277 (8) | 0.0039 (7) | 0.0156 (7) | 0.0032 (7) |
C3 | 0.0287 (9) | 0.0370 (11) | 0.0286 (8) | 0.0039 (9) | 0.0115 (7) | −0.0008 (8) |
C4 | 0.0250 (8) | 0.0282 (9) | 0.0266 (8) | −0.0033 (7) | 0.0176 (7) | −0.0033 (7) |
C5 | 0.0214 (7) | 0.0301 (10) | 0.0250 (7) | −0.0020 (7) | 0.0144 (7) | −0.0019 (7) |
C6 | 0.0266 (8) | 0.0389 (11) | 0.0312 (9) | −0.0060 (8) | 0.0182 (7) | 0.0018 (9) |
C7 | 0.0259 (8) | 0.0243 (8) | 0.0336 (8) | 0.0055 (7) | 0.0217 (7) | 0.0043 (7) |
C8 | 0.0234 (8) | 0.0439 (12) | 0.0401 (9) | −0.0028 (9) | 0.0221 (8) | 0.0023 (10) |
C9 | 0.0515 (13) | 0.087 (2) | 0.105 (2) | −0.0405 (15) | 0.0586 (15) | −0.066 (2) |
Geometric parameters (Å, º) top
O1—C1 | 1.207 (2) | C3—H3B | 0.9800 |
O2—C1 | 1.317 (2) | C3—H3C | 0.9800 |
O2—H2O | 0.89 (3) | C4—C5 | 1.518 (3) |
O3—C4 | 1.346 (2) | C5—C6 | 1.514 (2) |
O3—C2 | 1.450 (2) | C5—H5 | 1.0000 |
O4—C4 | 1.197 (2) | C6—H6A | 0.9800 |
O5—C7 | 1.3430 (19) | C6—H6B | 0.9800 |
O5—C5 | 1.443 (2) | C6—H6C | 0.9800 |
O6—C7 | 1.203 (2) | C7—C8 | 1.516 (3) |
O7—C8 | 1.408 (2) | C8—C9 | 1.512 (4) |
O7—H7O | 0.83 (3) | C8—H8 | 1.0000 |
C1—C2 | 1.522 (2) | C9—H9A | 0.9800 |
C2—C3 | 1.517 (2) | C9—H9B | 0.9800 |
C2—H2 | 1.0000 | C9—H9C | 0.9800 |
C3—H3A | 0.9800 | | |
| | | |
C1—O2—H2O | 109 (3) | O5—C5—H5 | 110.2 |
C4—O3—C2 | 115.09 (15) | C6—C5—H5 | 110.2 |
C7—O5—C5 | 116.91 (13) | C4—C5—H5 | 110.2 |
C8—O7—H7O | 112.4 (19) | C5—C6—H6A | 109.5 |
O1—C1—O2 | 124.60 (17) | C5—C6—H6B | 109.5 |
O1—C1—C2 | 124.65 (17) | H6A—C6—H6B | 109.5 |
O2—C1—C2 | 110.61 (15) | C5—C6—H6C | 109.5 |
O3—C2—C3 | 106.24 (16) | H6A—C6—H6C | 109.5 |
O3—C2—C1 | 109.06 (13) | H6B—C6—H6C | 109.5 |
C3—C2—C1 | 110.50 (14) | O6—C7—O5 | 123.38 (16) |
O3—C2—H2 | 110.3 | O6—C7—C8 | 125.91 (15) |
C3—C2—H2 | 110.3 | O5—C7—C8 | 110.71 (14) |
C1—C2—H2 | 110.3 | O7—C8—C9 | 112.3 (2) |
C2—C3—H3A | 109.5 | O7—C8—C7 | 110.38 (15) |
C2—C3—H3B | 109.5 | C9—C8—C7 | 109.68 (16) |
H3A—C3—H3B | 109.5 | O7—C8—H8 | 108.1 |
C2—C3—H3C | 109.5 | C9—C8—H8 | 108.1 |
H3A—C3—H3C | 109.5 | C7—C8—H8 | 108.1 |
H3B—C3—H3C | 109.5 | C8—C9—H9A | 109.5 |
O4—C4—O3 | 124.84 (18) | C8—C9—H9B | 109.5 |
O4—C4—C5 | 125.72 (17) | H9A—C9—H9B | 109.5 |
O3—C4—C5 | 109.38 (15) | C8—C9—H9C | 109.5 |
O5—C5—C6 | 106.87 (13) | H9A—C9—H9C | 109.5 |
O5—C5—C4 | 108.75 (15) | H9B—C9—H9C | 109.5 |
C6—C5—C4 | 110.54 (14) | | |
| | | |
C4—O3—C2—C3 | 173.83 (14) | O4—C4—C5—O5 | −13.1 (2) |
C4—O3—C2—C1 | −67.06 (18) | O3—C4—C5—O5 | 169.56 (12) |
O1—C1—C2—O3 | −26.7 (2) | O4—C4—C5—C6 | 103.9 (2) |
O2—C1—C2—O3 | 157.42 (15) | O3—C4—C5—C6 | −73.41 (17) |
O1—C1—C2—C3 | 89.7 (2) | C5—O5—C7—O6 | −4.7 (3) |
O2—C1—C2—C3 | −86.2 (2) | C5—O5—C7—C8 | 175.50 (16) |
C2—O3—C4—O4 | −7.0 (2) | O6—C7—C8—O7 | −13.6 (3) |
C2—O3—C4—C5 | 170.40 (13) | O5—C7—C8—O7 | 166.21 (16) |
C7—O5—C5—C6 | 169.37 (16) | O6—C7—C8—C9 | 110.7 (2) |
C7—O5—C5—C4 | −71.28 (18) | O5—C7—C8—C9 | −69.5 (2) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2O···O6i | 0.89 (3) | 2.00 (3) | 2.7986 (19) | 149 (3) |
O2—H2O···O7i | 0.89 (3) | 2.37 (3) | 3.049 (2) | 134 (3) |
O7—H7O···O1ii | 0.83 (3) | 2.03 (3) | 2.843 (2) | 166 (3) |
Symmetry codes: (i) x−1/2, y+1/2, z; (ii) x+1/2, y+1/2, z. |
Experimental details
| (I) | (II) |
Crystal data |
Chemical formula | C6H10O5 | C9H14O7 |
Mr | 162.14 | 234.20 |
Crystal system, space group | Orthorhombic, P212121 | Monoclinic, C2 |
Temperature (K) | 150 | 150 |
a, b, c (Å) | 7.94437 (7), 11.71472 (17), 16.82348 (12) | 17.3594 (5), 5.62712 (17), 13.9888 (6) |
α, β, γ (°) | 90, 90, 90 | 90, 123.861 (1), 90 |
V (Å3) | 1565.70 (3) | 1134.71 (7) |
Z | 8 | 4 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 0.12 | 0.12 |
Crystal size (mm) | 0.38 × 0.36 × 0.24 | 0.48 × 0.12 × 0.09 |
|
Data collection |
Diffractometer | Nonius KappaCCD area-detector diffractometer | Nonius KappaCCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 2008a) | Multi-scan (SADABS; Sheldrick, 2008a) |
Tmin, Tmax | 0.703, 0.747 | 0.707, 0.746 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 69491, 3856, 3674 | 19858, 1443, 1358 |
Rint | 0.026 | 0.028 |
(sin θ/λ)max (Å−1) | 0.807 | 0.649 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.026, 0.076, 1.07 | 0.028, 0.069, 1.07 |
No. of reflections | 3856 | 1443 |
No. of parameters | 219 | 156 |
No. of restraints | 0 | 1 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.31, −0.20 | 0.17, −0.15 |
Selected geometric parameters (Å, º) for (I) topC41—C51 | 1.5231 (12) | C42—C52 | 1.5117 (11) |
| | | |
C41—O31—C21—C31 | 171.34 (7) | C42—O32—C22—C32 | 171.42 (7) |
O11—C11—C21—O31 | −41.75 (10) | O12—C12—C22—O32 | −27.49 (10) |
C21—O31—C41—C51 | 178.52 (7) | C22—O32—C42—C52 | 178.90 (6) |
O31—C41—C51—O51 | 135.58 (8) | O32—C42—C52—O52 | 160.08 (7) |
Hydrogen-bond geometry (Å, º) for (I) top
D—H···A | D—H | H···A | D···A | D—H···A |
O21—H21O···O51i | 0.94 (2) | 1.73 (2) | 2.6617 (10) | 169.4 (19) |
O51—H51O···O12 | 0.869 (18) | 1.902 (18) | 2.7650 (9) | 171.7 (17) |
O22—H22O···O52i | 0.912 (19) | 1.698 (19) | 2.6006 (10) | 169.6 (18) |
O52—H52O···O42 | 0.857 (19) | 2.168 (18) | 2.6730 (9) | 117.4 (15) |
O52—H52O···O11ii | 0.857 (19) | 2.090 (19) | 2.7785 (11) | 136.9 (16) |
Symmetry codes: (i) x+1, y, z; (ii) −x+1/2, −y+1, z+1/2. |
Selected torsion angles (º) for (II) topC4—O3—C2—C3 | 173.83 (14) | O3—C4—C5—O5 | 169.56 (12) |
C2—O3—C4—C5 | 170.40 (13) | C5—O5—C7—C8 | 175.50 (16) |
C7—O5—C5—C4 | −71.28 (18) | | |
Hydrogen-bond geometry (Å, º) for (II) top
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2O···O6i | 0.89 (3) | 2.00 (3) | 2.7986 (19) | 149 (3) |
O2—H2O···O7i | 0.89 (3) | 2.37 (3) | 3.049 (2) | 134 (3) |
O7—H7O···O1ii | 0.83 (3) | 2.03 (3) | 2.843 (2) | 166 (3) |
Symmetry codes: (i) x−1/2, y+1/2, z; (ii) x+1/2, y+1/2, z. |
Lactic acid is the simplest 2-hydroxy acid with a chiral C atom. Hydroxy acids are known to form condensation polymers upon heating and removal of water (Probst et al., 1977). The crystal structures of the dimer, C6H10O5, (I), and the trimer, C9H14O7, (II), are reported here.
Today lactic acid is produced in general by fermentation. Both enantiomeric forms can be produced in this way: (S)-lactic acid, which is the natural form, but also (R)-lactic acid. Pure monomeric (S)-lactic acid has a melting point of 326 K (Borsook et al., 1933). However, due to the hygroscopicity of crystalline (S)-lactic acid, its high solubility in water and the phenomenon of condensation polymerization, the commercial product is not a solid, but a concentrated solution in water. A commercial solution of 90wt% lactic acid at equilibrium and 293 K contains about 65wt% of monomeric lactic acid, 15–20wt% of dimer and about 5wt% of higher oligomers. The crystal structure of lactic acid (Schouten et al., 1994) shows an almost planar molecule, with the hydroxy group coplanar with the carboxylic acid group.
Lactic acid and its derivatives are used in a wide range of applications (Datta, 2005). Lactic acid and its alkalimetal salts are used in food applications for reasons of pH control, taste and antimicrobial activity (Bogaert & Naidu, 2000; Shelef, 1994). Apart from food applications, lactic acid and its derivatives are also used in technical applications (e.g. lactic acid in household cleaning, ethyl lactate as a solvent in electronics), medical applications (e.g. sodium lactate in infusion liquor) and cosmetic applications. In the last decade, the application of lactic acid as monomer in the synthesis of poly(lactic acid) (PLA) has gained importance. PLA is used as a biobased polymer in packaging, but also in other polymer applications (Jem et al., 2010).
The condensation polymerization of lactic acid occurs at room temperature. The process starts with the formation of a dimer (lactoyllactic acid), followed by a trimer etc. (Holten, 1971). At high temperatures (473 K) and reduced pressure, a degree of polymerization of 10–20 can be achieved. As far as we know, no data on the pure solid dimer or trimer of (S)-lactic acid have been published previously.
Compound (I) crystallizes with two independent molecules in the asymmetric unit (Z' = 2). A plot of these molecules is shown in Fig. 1. The backbone of both molecules is essentially planar, with the torsion angles about C4x—O3x and O3x—C2x indicating trans conformations (Table 1; x = 1 or 2). The main differences between the two molecules are the conformations of the end groups, which are both involved in intermolecular hydrogen bonding (see below), and the differences can thus be ascribed to crystal packing effects. The torsion angles about C1x—C2x differ by about 15° between the two molecules; the torsion angles about C4x—C5x differ by about 25°. These differences can also be seen in an overlay plot of the two molecules (Fig. 2). Thermal motion analysis using the THMA11 program (Schomaker & Trueblood, 1998) reveals that the second molecule is more rigid than the first, with Rthma = 0.167 versus 0.280 (Rthma = {[Σ(wΔU)2]/[Σ(wUobs)2]}1/2). A possible explanation is the presence of an intramolecular O—H···O interaction in the second molecule, which makes the molecule more rigid. A comparison of the bond lengths in both molecules using a half-normal probability plot (Abrahams & Keve, 1971) is shown in Fig. 3 and reveals that the largest difference is 7.00σ. This difference is between the bonds C41—C51 in the first molecule and C42—C52 in the second molecule. The latter bond is part of the five-membered ring formed by the intramolecular hydrogen bond.
Compound (II) crystallizes with one independent molecule in the asymmetric unit (Z' = 1), which is shown in Fig. 4. The molecule consists of two essentially planar parts, with the torsion angles about C2—O3, O3—C4, C4—C5 and O5—C7 corresponding to trans conformations. The central C5—O5 bond adopts a gauche conformation (Table 3). If a segmented model is applied in the rigid-body analysis in THMA11 and the molecule is allowed to move about this C5—O5 bond, Rthma drops from 0.263 to 0.148. Segementation about other bonds of the molecule does not lead to such a large improvement. We therefore consider this as a strong indication that the two planar parts of the molecule are independent rigid groups.
The two independent molecules in (I) form hydrogen-bonded chains in the direction of the crystallographic a axis. Carboxylic acid atom O21 acts as donor and ester atom O51 of the translated molecule is the acceptor (Table 2). An identical situation is found for the second residue and the graph-set descriptors (Bernstein et al., 1995) for the two chains are consequently both C(8). The two chains are linked, with hydroxy groups O51 and O52 of one chain as donors and carboxylic acid atoms O12 and O11 of the other chain as acceptors, best described with the graph-set descriptor D(2). The overall pattern formed by intermolecular hydrogen bonding is thus a two-dimensional network in the ac plane (Fig. 5). The hydroxy group at O52 is actually bifurcated, with an angle sum of 357 (2)° at H52O. This additionally forms an intramolecular hydrogen bond with atom O42 as acceptor, thereby imparting molecular rigidity (see above); here the graph-set notation is S(5). The corresponding interaction at O51 is considered much weaker, due to an O—H···O angle of only 94.9 (12)° and a very long H···O distance of 2.625 (16) Å.
In (II) we also find a two-dimensional hydrogen-bonding pattern (Fig. 6). Chains with graph-set descriptor C(10) run along the [110] diagonal. Carboxylic acid atom O2 is the hydrogen-bond donor and ester atom O6 the acceptor (Table 4). Atom H2O is bifurcated, with an angle sum of 361 (5)°, and also involved in a hydrogen-bonded chain in the b direction. In this chain, atom O2 is the donor and hydroxy group O7 the acceptor. In a cooperative fashion, atom O7 is then also a donor, with carboxylic atom O1 as acceptor. Consequently, the graph-set descriptor here is C22(6). The combination of the [110] and the [010] chains results in a two-dimensional network in the ab plane.
Both molecules (I) and (II) only consist of the very weak anomalous scatterers C, H and O. For Mo Kα radiation the a priori estimation of the Bijvoet differences (Flack & Shmueli, 2007) results in the very low value of Friedif = 7 for both structures. The standard uncertainties in the Flack parameters (Flack, 1983), x = 0.0 (3) for (I) and x = -0.1 (8) for (II), were so large that the absolute structure could not be determined reliably by this method (Flack & Bernardinelli, 2000). For the least-squares refinement of both structures the Friedel pairs have therefore been averaged. Interestingly, using the method of Hooft et al. (2008) on the unmerged data allowed the absolute structure of (I) to be confirmed. Here, the absolute structure is not determined during the least-squares refinement as with the Flack parameter, but likelihood calculations are applied on the Bijvoet differences. For (I), the Hooft parameter yields a value of y = -0.02 (9) based on all 3024 Bijvoet pairs and assuming a Gaussian distribution of errors. The probability that the absolute configuration is correct is 1.000. In the case of (II), the standard uncertainty in the Hooft parameter y = 0.2 (2) is too high to draw reliable conclusions. Here, a probability of 0.987 for the correct absolute structure is only attained by making the assumption that the crystal is enantiomerically pure. Allowing the possibility of an inversion twin reduces the probability to 0.536.