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The title compound, C8H12O3, crystallizes as acid-to-ketone hydrogen-bonding catemers, in which hydrogen bonds progress from the carboxyl group of each mol­ecule to the ketone group of a translationally related neighbor [O...O = 2.738 (3) Å and O—H...O = 153 (4)°]. Four separate hy­drogen-bonding chains proceed through the cell in centrosymmetrically related pairs along axes lying in the ab plane. Three intermolecular C—H...O close contacts exist involving both carboxyl O atoms. Factors contributing to the choice of hydrogen-bonding mode are discussed.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270103024545/fr1436sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270103024545/fr1436Isup2.hkl
Contains datablock I

CCDC reference: 229099

Comment top

Our study of the crystallography of ketocarboxylic acids explores the molecular characteristics that control their five known hydrogen-bonding modes. Beyond the dimeric and catemer (chain) modes seen in simple acids, the presence of an additional receptor in the system leads to another three observed hydrogen-bonding modes involving the ketone. The title compound, (I), crystallizes with acid-to-ketone catemeric hydrogen bonding. We have previously shown that this pattern, overall the second most commonly encountered, becomes predominant wherever centrosymmetry is precluded (Thompson & Lalancette, 2003) or rendered difficult. Among the latter cases are those like (I), where molecular flexibility is severely restricted (Barcon et al., 2002). When this occurs, the acid is often unable to find a dimeric crystallization mode of suitably low potential energy vis-a-vis alternative packing modes, and catemeric hydrogen-bonding results. We have studied several examples of simple cyclohexane and cyclopentane keto acids in which such low degrees of conformational flexibility are found to lead to acid-to-ketone catemers.

Fig. 1 shows the asymmetric unit for compound (I). In either chair conformation for (I) one of the two cis-related substituents must be axial. The steric requirements of the sp3 methyl group are normally significantly greater than for the sp2 carboxyl group (Hirsch, 1967). However, in the present case, an axial methyl encounters only a single 1,3-diaxial interaction (at C6) because of the presence of the C4 ketone, while an axial carboxyl has interactions at both C3 and C5. The result is that, in (I), the methyl group is the less sterically demanding, and therefore the preferred, axial substituent. Once those demands (and the need to stagger the methyl groups) have been met, the resulting conformer is quite rigid, in contrast to the dynamic situation in solution. The only remaining conformational option is rotation of the carboxyl unit about C1—C7. In (I), this group is turned so that the C1—C6 bond coincides closely with the carboxyl plane [torsion angle O2—C7—C1—C6 = −0.5 (4)°]. The dihedral angle for the ketone (O1/C3/C4/C5) versus the carboxyl (O2/O3/C7/C1) is 46.7 (2)°.

The partial averaging of C—O bond lengths and C—C—O angles by disorder often seen in acids is unique to the carboxyl-pairing hydrogen-bonding mode, whose geometry permits transposition of the two carboxyl O atoms. With catemers and other non-dimeric acid modes, no significant averaging is observed. For (I), the C—O bond lengths are 1.194 (3) and 1.332 (4) Å, with C—C—O angles of 126.1 (3) and 111.9 (3)°. Our own survey of 56 keto acid structures which are not acid dimers gives average values of 1.200 (10) and 1.32 (2) Å, and 124.5 (14) and 112.7 (17)° for these lengths and angles, in accord with typical values of 1.21 and 1.31 Å, and 123 and 112° cited for highly ordered dimeric carboxyls (Borthwick, 1980).

Fig. 2 illustrates the packing of the cell and the hydrogen-bonding arrangement. There are no hydrogen-bonding connections among any of the four molecules within the chosen cell. Rather, each is part of one of four separate translational hydrogen-bonding chains proceeding through the cell along axes lying in the ab plane [O···O = 2.738 (3) Å and O—H···O = 153 (4)°]. Because of the centrosymmetry of the cell, the chains are counterdirectionally paired. Those at the ends of the chosen cell advance by one cell each, either positively or negatively, in both a and b. The remaining molecules are glide-related to those at the cell ends, and their chains proceed either by one cell positively in a and one negatively in b or vice versa.

We characterize the geometry of H bonding to carbonyls using a combination of the H···OC angle and the H···OC—C torsion angle. These describe the approach of the H atom to the O in terms of its deviation from, respectively, CO axiality (ideal = 120°) and planarity with the carbonyl (ideal = 0°). In (I), these angles are H···OC = 138.0 (11)° and H···OC—C = −52.2 (2)°. The latter angle represents hydrogen bonding markedly out of the plane of the ketone carbonyl and would appear to be far from `ideal'.

Three intermolecular C—H···O close contacts were found for the carboxylic acid O atoms [O2···H5A (2.58 Å) to a molecule translationally related in a, and O3···H6B (2.65 Å) and O3···H8B (2.6 Å), both to the same neighbor translationally related in b]. These distances all lie within the 2.7 Å range we standardly employ for non-bonded C—H···O packing interactions (Steiner, 1997). Using compiled data for a large number of such contacts, Steiner & Desiraju (1998) find significant statistical directionality even as far out as 3.0 Å, and conclude that these are legitimately viewed as `weak hydrogen bonds', with a greater contribution to packing forces than simple van der Waals attractions.

The solid-state (KBr) IR spectrum of (I) has CO absorptions at 1732 (COOH) and 1693 cm−1 (ketone). This peak separation conforms to the shifts seen typically in catemers, due, respectively, to removal of hydrogen bonding from the acid CO and addition of hydrogen bonding to the ketone. In CHCl3 solution, these absorptions are seen, probably reversed, at 1710 and 1705 cm−1, consistent with dimerically hydrogen-bonded carboxyl and a normal ketone.

Experimental top

A commercially available technical (90% pure) grade of Hagemann's ester (4-carbethoxy-3-methyl-2-cyclohexen-1-one) was hydrogenated in 95% ethanol with a 5% Pd/C catalyst, and the concentrated liquid product was directly saponified without purification. After passage through a short column of Al2O3 and distillation in a short-path coldfinger apparatus, the product crystallized upon refrigeration. Crystals suitable for X-ray, m.p. 333 K, were obtained from hexane/diethyl ether. An attempt to produce the trans-epimer by base-catalyzed equilibration of the ketal ester led to an approximately 4:1 epimer mixture, which showed no inclination to crystallize.

Refinement top

All H atoms for (I) were found in electron-density difference maps but were placed in calculated positions for the C-bound H atoms (0.97 Å for methylene, 0.98 Å for methine, and 0.96 Å for methyl H atoms) and allowed to refine as riding models on their respective C atoms; their displacement parameters were fixed at 120% of those of their respective C atoms for the methane and methylene, and 150% for the methyl H atoms. The hydroxyl H was allowed to vary positionally, and its displacement parameter was fixed at 0.08 Å2.

Computing details top

Data collection: XSCANS (Siemens, 1996); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXS97 in SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 in SHELXTL; molecular graphics: SHELXP97 in SHELXTL; software used to prepare material for publication: SHELXL97 in SHELXTL.

Figures top
[Figure 1] Fig. 1. The asymmetric unit for (I). Displacement ellipsoids are set at the 20% probability level.
[Figure 2] Fig. 2. A packing diagram, with extracellular molecules included to illustrate the four hydrogen-bonding catemers. For clarity, all carbon-bound H atoms except for the methyl H atoms, have been removed. Displacement ellipsoids are set at the 20% probability level.
(±)-cis-2-Methyl-4-oxocyclohexanecarboxylic acid top
Crystal data top
C8H12O3F(000) = 336
Mr = 156.18Dx = 1.284 Mg m3
Monoclinic, P21/nMelting point: 333 K
Hall symbol: -P 2ynMo Kα radiation, λ = 0.71073 Å
a = 6.754 (1) ÅCell parameters from 37 reflections
b = 5.725 (1) Åθ = 3.3–11.4°
c = 21.078 (4) ŵ = 0.10 mm1
β = 97.46 (2)°T = 296 K
V = 808.1 (2) Å3Parallelepiped, colourless
Z = 40.50 × 0.20 × 0.04 mm
Data collection top
Siemens P4
diffractometer
910 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.051
Graphite monochromatorθmax = 25.0°, θmin = 2.0°
2θ/θ scansh = 18
Absorption correction: numerical
(SHELXTL; Sheldrick, 1997)
k = 16
Tmin = 0.976, Tmax = 0.996l = 2525
2174 measured reflections3 standard reflections every 97 reflections
1415 independent reflections intensity decay: variation <1.5%
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.064Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.172H atoms treated by a mixture of independent and constrained refinement
S = 1.05 w = 1/[σ2(Fo2) + (0.076P)2 + 0.2612P]
where P = (Fo2 + 2Fc2)/3
1415 reflections(Δ/σ)max < 0.001
104 parametersΔρmax = 0.24 e Å3
0 restraintsΔρmin = 0.17 e Å3
Crystal data top
C8H12O3V = 808.1 (2) Å3
Mr = 156.18Z = 4
Monoclinic, P21/nMo Kα radiation
a = 6.754 (1) ŵ = 0.10 mm1
b = 5.725 (1) ÅT = 296 K
c = 21.078 (4) Å0.50 × 0.20 × 0.04 mm
β = 97.46 (2)°
Data collection top
Siemens P4
diffractometer
910 reflections with I > 2σ(I)
Absorption correction: numerical
(SHELXTL; Sheldrick, 1997)
Rint = 0.051
Tmin = 0.976, Tmax = 0.9963 standard reflections every 97 reflections
2174 measured reflections intensity decay: variation <1.5%
1415 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0640 restraints
wR(F2) = 0.172H atoms treated by a mixture of independent and constrained refinement
S = 1.05Δρmax = 0.24 e Å3
1415 reflectionsΔρmin = 0.17 e Å3
104 parameters
Special details top

Experimental. crystal mounted on glass fiber using cyanoacrylate cement

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.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
O10.0883 (4)0.0724 (5)0.14777 (13)0.0797 (9)
O20.8062 (4)0.4162 (5)0.06269 (12)0.0702 (8)
O30.7282 (4)0.7032 (4)0.12498 (11)0.0579 (7)
H30.846 (6)0.744 (7)0.1219 (17)0.080*
C10.4817 (4)0.4173 (5)0.09837 (12)0.0364 (7)
H1A0.38740.53760.08050.044*
C20.4491 (4)0.3801 (5)0.16876 (13)0.0419 (8)
H2A0.46600.53150.19050.050*
C30.2342 (5)0.3000 (6)0.17092 (15)0.0541 (9)
H3A0.21860.25830.21460.065*
H3B0.14480.42940.15850.065*
C40.1741 (5)0.0959 (6)0.12828 (16)0.0529 (9)
C50.2245 (5)0.1144 (6)0.06164 (15)0.0534 (9)
H5A0.13300.22290.03780.064*
H5B0.20630.03710.04110.064*
C60.4364 (4)0.1960 (5)0.05910 (13)0.0424 (8)
H6A0.45540.22590.01500.051*
H6B0.52870.07380.07550.051*
C70.6886 (4)0.5046 (6)0.09293 (13)0.0419 (7)
C80.5983 (5)0.2106 (6)0.20406 (13)0.0569 (10)
H8A0.73160.26600.20210.085*
H8B0.58220.05940.18440.085*
H8C0.57540.19970.24800.085*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0748 (18)0.0694 (18)0.0942 (19)0.0358 (15)0.0083 (15)0.0070 (15)
O20.0501 (14)0.0806 (18)0.0843 (18)0.0041 (13)0.0255 (13)0.0184 (15)
O30.0524 (15)0.0501 (14)0.0724 (15)0.0155 (12)0.0125 (12)0.0105 (12)
C10.0378 (15)0.0331 (15)0.0379 (15)0.0035 (13)0.0033 (12)0.0012 (12)
C20.0495 (18)0.0398 (16)0.0376 (15)0.0050 (15)0.0098 (13)0.0101 (13)
C30.055 (2)0.059 (2)0.0520 (18)0.0085 (17)0.0207 (15)0.0058 (17)
C40.0407 (18)0.051 (2)0.066 (2)0.0074 (17)0.0040 (16)0.0018 (18)
C50.057 (2)0.0464 (19)0.0534 (19)0.0075 (17)0.0046 (15)0.0131 (16)
C60.0483 (18)0.0424 (17)0.0358 (14)0.0005 (15)0.0025 (12)0.0056 (13)
C70.0415 (17)0.0446 (18)0.0398 (15)0.0041 (15)0.0060 (13)0.0056 (14)
C80.069 (2)0.064 (2)0.0366 (15)0.0044 (19)0.0001 (15)0.0065 (16)
Geometric parameters (Å, º) top
O1—C41.222 (4)C3—H3A0.9700
O2—C71.194 (3)C3—H3B0.9700
O3—C71.332 (4)C4—C51.492 (4)
O3—H30.84 (4)C5—C61.513 (4)
C1—C71.502 (4)C5—H5A0.9700
C1—C61.523 (4)C5—H5B0.9700
C1—C21.543 (4)C6—H6A0.9700
C1—H1A0.9800C6—H6B0.9700
C2—C81.522 (4)C8—H8A0.9600
C2—C31.528 (4)C8—H8B0.9600
C2—H2A0.9800C8—H8C0.9600
C3—C41.498 (4)
C7—O3—H3109 (3)C4—C5—C6112.8 (3)
C7—C1—C6111.0 (2)C4—C5—H5A109.0
C7—C1—C2111.7 (2)C6—C5—H5A109.0
C6—C1—C2111.4 (2)C4—C5—H5B109.0
C7—C1—H1A107.5C6—C5—H5B109.0
C6—C1—H1A107.5H5A—C5—H5B107.8
C2—C1—H1A107.5C5—C6—C1111.2 (2)
C8—C2—C3111.4 (3)C5—C6—H6A109.4
C8—C2—C1112.9 (2)C1—C6—H6A109.4
C3—C2—C1109.0 (2)C5—C6—H6B109.4
C8—C2—H2A107.8C1—C6—H6B109.4
C3—C2—H2A107.8H6A—C6—H6B108.0
C1—C2—H2A107.8O2—C7—O3121.9 (3)
C4—C3—C2113.6 (3)O2—C7—C1126.1 (3)
C4—C3—H3A108.8O3—C7—C1111.9 (3)
C2—C3—H3A108.8C2—C8—H8A109.5
C4—C3—H3B108.8C2—C8—H8B109.5
C2—C3—H3B108.8H8A—C8—H8B109.5
H3A—C3—H3B107.7C2—C8—H8C109.5
O1—C4—C5123.3 (3)H8A—C8—H8C109.5
O1—C4—C3121.1 (3)H8B—C8—H8C109.5
C5—C4—C3115.6 (3)
C7—C1—C2—C857.6 (3)C3—C4—C5—C646.5 (4)
C6—C1—C2—C867.2 (3)C4—C5—C6—C151.1 (4)
C7—C1—C2—C3178.0 (3)C7—C1—C6—C5177.0 (2)
C6—C1—C2—C357.2 (3)C2—C1—C6—C557.8 (3)
C8—C2—C3—C473.7 (3)C6—C1—C7—O20.5 (4)
C1—C2—C3—C451.6 (4)C2—C1—C7—O2124.5 (3)
C2—C3—C4—O1132.8 (3)C6—C1—C7—O3177.6 (2)
C2—C3—C4—C547.6 (4)C2—C1—C7—O357.4 (3)
O1—C4—C5—C6133.9 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O3—H3···O1i0.84 (4)1.96 (4)2.738 (3)153 (4)
Symmetry code: (i) x+1, y+1, z.

Experimental details

Crystal data
Chemical formulaC8H12O3
Mr156.18
Crystal system, space groupMonoclinic, P21/n
Temperature (K)296
a, b, c (Å)6.754 (1), 5.725 (1), 21.078 (4)
β (°) 97.46 (2)
V3)808.1 (2)
Z4
Radiation typeMo Kα
µ (mm1)0.10
Crystal size (mm)0.50 × 0.20 × 0.04
Data collection
DiffractometerSiemens P4
diffractometer
Absorption correctionNumerical
(SHELXTL; Sheldrick, 1997)
Tmin, Tmax0.976, 0.996
No. of measured, independent and
observed [I > 2σ(I)] reflections
2174, 1415, 910
Rint0.051
(sin θ/λ)max1)0.595
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.064, 0.172, 1.05
No. of reflections1415
No. of parameters104
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)0.24, 0.17

Computer programs: XSCANS (Siemens, 1996), XSCANS, SHELXS97 in SHELXTL (Sheldrick, 1997), SHELXL97 in SHELXTL, SHELXP97 in SHELXTL.

Selected geometric parameters (Å, º) top
O2—C71.194 (3)O3—C71.332 (4)
O2—C7—C1126.1 (3)O3—C7—C1111.9 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O3—H3···O1i0.84 (4)1.96 (4)2.738 (3)153 (4)
Symmetry code: (i) x+1, y+1, z.
 

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