Conformation of hydrogen-bonded dimeric o-methyl-substituted benzoic acids

Molecules of 2,4-dimethyl­benzoic acid, C₉H₁₀O₂, form typical centrosymmetric hydrogen-bonded dimers. The carboxyl group is twisted with respect to the benzene ring and the methyl group in the ortho position shows evasive in-plane splaying. The relation between the in-plane splaying and the twist angle of the carboxyl group for various ortho-substituted dimeric derivatives of benzoic acid is presented. It shows how the steric strains are released depending on the numbers and positions of the substituents.

S1. Comment

2,4-Dimethylbenzoic acid, (I), was obtained by the acetylation of m-xylene by acetic acid in the presence of P₂O₅ as catalyst. According to Kosolapoff (1947), in the course of this reaction mono- and diacetyl derivatives of m-xylene form, and the latter is found in the high-boiling fraction. Our studies reveal that the compound is actually 2,4-dimethylbenzoic acid, (I).

Molecules of (I) crystallize in the form of centrosymmetric hydrogen-bonded dimers. One such dimer is presented in Fig. 1, with the atom-numbering scheme. The carboxyl group in (I) shows C—O bond-length alteration, with distances of 1.3085 (19) and 1.2251 (18) Å. The relevant C—C—O angles are also different, at 114.34 (14) and 124.02 (14)°. The differences between these bond lengths (Δd) and angles (Δϕ) obey the relation given by Borthwick (1980), i.e. Δϕ = -100 × Δd (ϕ in degrees, d in Å). The carboxyl group is twisted with respect to the phenyl ring. The dihedral angle between the plane of the phenyl ring and the plane of atoms O1, O2, C1 is 14.06 (3)°. The methyl group in the ortho-position is splayed away from the carboxyl group and this is illustrated by the different values of the angles C2—C3—C8 and C4—C3—C8, which are 124.23 (14) and 118.57 (14)°, respectively. In contrast, the angles C4—C5—C9 and C6—C5—C9 are nearly equal and the methyl group in the para position is located symmetrically.

The centrosymetric hydrogen-bonded dimer of (I) is described using the common motif which occurs in most other monocarboxylic acids (Leiserowitz, 1976), namely R²₂(8) (Etter, 1990) (Fig. 1). The H1··· O2ⁱ and O1···O2ⁱ distances are 1.71 (3) and 2.646 (2) Å, respectively [symmetry code: (i) -x, -y, 1 - z]. The O1—H1···O2ⁱ angle is 176 (3)°. The molecular packing of (I) is best described as comprising puckered sheets of carboxylic acid hydrogen-bonded dimers but there are no significant interactions between dimers.

The observed in-plane splaying and the non-coplanarity of the carboxyl group and the phenyl ring of (I) indicate that the molecular conformation is influenced by the presence of a methyl group in the ortho-position. In other methylbenzoic acid derivatives, it was found that there are two ways in which the steric strain caused by the closeness of a carboxyl and a methyl group can be relieved (Barcon et al., 1997). First of all, the carboxyl and the o-methyl groups exhibit evasive in-plane splaying, Δ. This parameter was calculated by adding the values of the C2—C3—C8 and C4—C3—C8 angles and subtracting 240° from the sum. The second possibility for relieving steric strain is a twist of the carboxyl group around the C_benz—C_carboxyl bond. To examine these parameters for benzoic acid derivatives, we performed a search of the Cambridge Structural Database (CSD, Version 5.28, August 2007; Allen, 2002; Bruno et al., 2002). The following restrictions were made: R < 10%, at least one methyl group present in an ortho-position, and the remaining substituents restricted to be only H, methyl, Cl or Br. Moreover, only acids forming hydrogen-bonded dimeric structures were taken into account. In the case of two acids, namely 2-methylbenzoic acid (Byrn et al., 1993) and 2,4,6-trimethylbenzoic acid (Gdaniec et al., 2003), structural data for the acid molecules acting as guest molecules or found in cocrystals were also included. This provided us with a set of 14 molecules. The angle of twist and the angle for combined in-plane splay have been calculated for (I) and all the structures obtained from the CSD search. In the case of acids with methyl groups in both ortho-positions, the in-plane splay was taken as the mean of the two values.

The scatter graph (Fig. 2) shows the relation between the twist of the carboxyl group and the in-plane splay. The data are separated into three clusters. The points at the bottom right-hand corner correspond to the structures of mono-ortho-substituted acids. The value of Δ ranges from 5.6 to 7.1°, whereas that of the twist angle ranges from 1.0 to 14.1°. The parameters of the benzoic acid derivatives with a methyl group in both ortho-positions and with both meta-positions vacant lie in the middle cluster. In this cluster, the values of Δ and the twist fall in the ranges 2.4–4.0° and 32.6–51.6°, respectively. It should be noted that for pure 2,4,6-trimethylbenzoic acid measured at 100 K (Wilson & Goeta, 2004), the values of Δ and the twist angle are 2.9 and 42.7°, respectively. When the same compound forms a cocrystal with 1,4-bis(pentafluorophenyl)butadiyne (Gdaniec et al., 2003), these values change to 4.0 and 32.6°, respectively. Moreover, the data provided by Wilson & Goeta (2004) for pure 2,4,6-trimethylbenzoic acid measured at different temperatures in the range 100–290 K show small changes of the parameters in question. At the top left-hand corner of Fig. 2 there is a cluster of points, which corresponds to the pentasubstituted acids. The buttressing effect (Westheimer, 1956) of the substituents in the meta-position hinders the in-plane splaying but a twist angle of approximately 90° is observed.

Summarizing, the observed relationship shows that the in-plane splaying and the twist angle of the carboxyl group are interdependent parameters with a correlation factor of 0.98 (4). Steric strains introduced by substituents in the ortho-position are primarily released by in-plane splaying. However, if the buttressing effect of groups in the meta-positions occurs, in-plane splaying is unfavourable. Thus, the twist of the carboxyl group becomes significant and approaches 90° with a loss of resonance stabilization. The structures of 2,4,6-trimethylbenzoic acid molecules in various molecular environments are good examples of the fact that the molecular conformation is also to some extent dependent on other factors, such as crystal packing forces and weak intermolecular interactions.

Related literature

For related literature, see: Allen (2002); Barcon et al. (1997); Borthwick (1980); Bruno et al. (2002); Byrn et al. (1993); Etter (1990); Gdaniec et al. (2003); Kosolapoff (1947); Leiserowitz (1976); Westheimer (1956); Wilson & Goeta (2004).

S2. Experimental

The synthesis was carried out according to the procedure described by Kosolapoff (1947). m-Xylene was treated with acetic acid in the presence of P₂O₅ as catalyst and the high-boiling fraction was purified by distillation under reduced pressure. Crystals of (I) suitable for X-ray studies were obtained by sublimation.

S3. Refinement

H atoms were positioned geometrically and constrained to ride on their parent atoms, with C—H = 0.93–0.96 Å and U_iso(H) = 1.2 (1.5 for methyl groups)U_eq(C). The methyl group in the para-position (C9) was modelled as an idealized disordered rotating group with a refined occupancy factor of 0.62 (2) for the major conformer. The H atom attached to the O atom was located in a difference Fourier map and its positional and isotropic displacement parameters were freely refined.

Computing details

Data collection: P3/P4-PC Software (Siemens, 1991); cell refinement: P3/P4-PC Software (Siemens, 1991); data reduction: XDISK (Siemens, 1991); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL (Sheldrick, 2008) and PLATON (Spek, 2003).

Figures

	Fig. 1. The hydrogen-bonded dimeric motif of (I), with the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii. Hydrogen bonds are shown as dashed lines.
	Fig. 2. A scatter plot, showing the relation between the in-plane splaying, Δ, and the twist angle of the carboxyl group. Data corresponding to mono-ortho-substituted acids are represented by circles, triangles show the data for di-ortho-substituted molecules and squares denote the group of pentasubstituted benzoic acid derivatives.

2,4-dimethylbenzoic acid

Crystal data

C9H10O2	F(000) = 320
Mr = 150.17	Dx = 1.268 Mg m^−3
Monoclinic, P21/c	Melting point: 399 K
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 5.4656 (12) Å	Cell parameters from 30 reflections
b = 13.660 (2) Å	θ = 12–35°
c = 10.6755 (19) Å	µ = 0.09 mm^−1
β = 99.182 (16)°	T = 293 K
V = 786.8 (3) Å^3	Prism, colourless
Z = 4	0.25 × 0.20 × 0.15 mm

Data collection

Siemens P3 diffractometer	Rint = 0.014
Radiation source: fine-focus sealed tube	θmax = 25.1°, θmin = 2.4°
Graphite monochromator	h = 0→6
profile data from ω/2θ scans	k = 0→16
1551 measured reflections	l = −12→12
1399 independent reflections	2 standard reflections every 70 reflections
953 reflections with I > 2σ(I)	intensity decay: 4.1%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.038	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.106	w = 1/[σ^2(Fo^2) + (0.0589P)^2 + 0.0045P] where P = (Fo^2 + 2Fc^2)/3
S = 1.02	(Δ/σ)max < 0.001
1399 reflections	Δρmax = 0.14 e Å^−3
108 parameters	Δρmin = −0.13 e Å^−3
0 restraints	Extinction correction: SHELXL (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.027 (5)

Special details

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 I > 2σ(I) is used only for calculating R-factors and is not relevant to the choice of reflections for refinement.

The final INS file has the following FVAR for the disorder modeling of the methyl group in the para position:.. FVAR 0.95579 0.62447.. C9 1 0.799433 0.161461 0.003738 11.00000 0.06554 0.08787 = 0.05795 - 0.00187 0.02813 - 0.01426 AFIX 127 H9A 2 0.727850 0.131595 - 0.075029 21.00000 - 1.50000 H9B 2 0.966920 0.139066 0.027405 21.00000 - 1.50000 H9C 2 0.798567 0.231333 - 0.006003 21.00000 - 1.50000 H9D 2 0.934375 0.203067 0.039278 - 21.00000 - 1.50000 H9E 2 0.695304 0.195597 - 0.063157 - 21.00000 - 1.50000 H9F 2 0.863658 0.103329 - 0.029749 - 21.00000 - 1.50000

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
O1	0.2092 (3)	−0.04671 (8)	0.40080 (13)	0.0705 (4)
O2	0.1068 (3)	0.10353 (8)	0.45001 (11)	0.0664 (4)
H1	0.100 (5)	−0.0646 (19)	0.456 (2)	0.124 (9)*
C1	0.2168 (3)	0.04842 (12)	0.38754 (15)	0.0482 (4)
C2	0.3670 (3)	0.08183 (11)	0.29241 (14)	0.0444 (4)
C3	0.3528 (3)	0.17777 (11)	0.24470 (14)	0.0427 (4)
C4	0.4962 (3)	0.19996 (12)	0.15155 (14)	0.0485 (4)
H4	0.4875	0.2631	0.1185	0.058*
C5	0.6499 (3)	0.13376 (13)	0.10575 (14)	0.0515 (4)
C6	0.6626 (3)	0.04037 (13)	0.15618 (16)	0.0594 (5)
H6	0.7669	−0.0057	0.1282	0.071*
C7	0.5231 (3)	0.01497 (12)	0.24693 (16)	0.0566 (5)
H7	0.5331	−0.0485	0.2788	0.068*
C8	0.1907 (3)	0.25625 (11)	0.28568 (17)	0.0560 (5)
H8A	0.2539	0.2755	0.3712	0.084*
H8B	0.0250	0.2317	0.2815	0.084*
H8C	0.1893	0.3118	0.2305	0.084*
C9	0.7994 (3)	0.16146 (16)	0.00374 (17)	0.0685 (6)
H9A	0.7278	0.1316	−0.0750	0.103*	0.62 (2)
H9B	0.9669	0.1391	0.0274	0.103*	0.62 (2)
H9C	0.7986	0.2313	−0.0060	0.103*	0.62 (2)
H9D	0.9344	0.2031	0.0393	0.103*	0.38 (2)
H9E	0.6953	0.1956	−0.0632	0.103*	0.38 (2)
H9F	0.8637	0.1033	−0.0297	0.103*	0.38 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0968 (10)	0.0422 (7)	0.0851 (10)	−0.0020 (7)	0.0529 (9)	0.0069 (6)
O2	0.0905 (9)	0.0467 (7)	0.0739 (8)	−0.0050 (6)	0.0492 (7)	−0.0021 (6)
C1	0.0563 (10)	0.0429 (9)	0.0485 (9)	−0.0034 (8)	0.0180 (8)	0.0005 (8)
C2	0.0494 (9)	0.0424 (8)	0.0434 (9)	−0.0031 (7)	0.0139 (7)	−0.0005 (7)
C3	0.0433 (9)	0.0430 (8)	0.0426 (9)	−0.0072 (7)	0.0088 (7)	−0.0018 (7)
C4	0.0536 (10)	0.0466 (9)	0.0467 (9)	−0.0096 (8)	0.0121 (8)	0.0035 (7)
C5	0.0480 (9)	0.0634 (11)	0.0452 (9)	−0.0108 (9)	0.0138 (8)	−0.0017 (8)
C6	0.0613 (11)	0.0601 (11)	0.0626 (11)	0.0054 (9)	0.0278 (10)	−0.0034 (9)
C7	0.0662 (12)	0.0480 (9)	0.0611 (11)	0.0052 (8)	0.0275 (10)	0.0063 (8)
C8	0.0627 (11)	0.0432 (9)	0.0657 (11)	−0.0015 (8)	0.0217 (9)	0.0012 (8)
C9	0.0655 (12)	0.0879 (14)	0.0579 (11)	−0.0143 (11)	0.0281 (9)	−0.0019 (10)

Geometric parameters (Å, º)

O1—C1	1.3085 (19)	C6—C7	1.370 (2)
O1—H1	0.94 (3)	C6—H6	0.9300
O2—C1	1.2251 (18)	C7—H7	0.9300
C1—C2	1.476 (2)	C8—H8A	0.9600
C2—C7	1.389 (2)	C8—H8B	0.9600
C2—C3	1.404 (2)	C8—H8C	0.9600
C3—C4	1.394 (2)	C9—H9A	0.9600
C3—C8	1.500 (2)	C9—H9B	0.9600
C4—C5	1.376 (2)	C9—H9C	0.9600
C4—H4	0.9300	C9—H9D	0.9600
C5—C6	1.382 (2)	C9—H9E	0.9600
C5—C9	1.510 (2)	C9—H9F	0.9600
C1—O1—H1	111.2 (16)	C3—C8—H8C	109.5
O1—C1—O2	121.63 (14)	H8A—C8—H8C	109.5
O1—C1—C2	114.34 (14)	H8B—C8—H8C	109.5
O2—C1—C2	124.02 (14)	C5—C9—H9A	109.5
C7—C2—C3	119.24 (14)	C5—C9—H9B	109.5
C7—C2—C1	118.50 (14)	H9A—C9—H9B	109.5
C3—C2—C1	122.25 (14)	C5—C9—H9C	109.5
C4—C3—C2	117.19 (14)	H9A—C9—H9C	109.5
C4—C3—C8	118.57 (14)	H9B—C9—H9C	109.5
C2—C3—C8	124.23 (14)	C5—C9—H9D	109.5
C5—C4—C3	123.72 (15)	H9A—C9—H9D	141.1
C5—C4—H4	118.1	H9B—C9—H9D	56.3
C3—C4—H4	118.1	H9C—C9—H9D	56.3
C4—C5—C6	117.62 (14)	C5—C9—H9E	109.5
C4—C5—C9	121.52 (16)	H9A—C9—H9E	56.3
C6—C5—C9	120.86 (16)	H9B—C9—H9E	141.1
C7—C6—C5	120.69 (16)	H9C—C9—H9E	56.3
C7—C6—H6	119.7	H9D—C9—H9E	109.5
C5—C6—H6	119.7	C5—C9—H9F	109.5
C6—C7—C2	121.53 (16)	H9A—C9—H9F	56.3
C6—C7—H7	119.2	H9B—C9—H9F	56.3
C2—C7—H7	119.2	H9C—C9—H9F	141.1
C3—C8—H8A	109.5	H9D—C9—H9F	109.5
C3—C8—H8B	109.5	H9E—C9—H9F	109.5
H8A—C8—H8B	109.5
O1—C1—C2—C3	165.38 (15)	C8—C3—C4—C5	−179.40 (15)
O2—C1—C2—C3	−14.6 (3)	C3—C4—C5—C6	−0.4 (3)
O1—C1—C2—C7	−13.1 (2)	C3—C4—C5—C9	179.40 (15)
O2—C1—C2—C7	166.88 (17)	C4—C5—C6—C7	1.1 (3)
C7—C2—C3—C4	0.9 (2)	C9—C5—C6—C7	−178.74 (17)
C1—C2—C3—C4	−177.55 (15)	C5—C6—C7—C2	−0.7 (3)
C7—C2—C3—C8	179.67 (16)	C3—C2—C7—C6	−0.3 (3)
C1—C2—C3—C8	1.2 (2)	C1—C2—C7—C6	178.24 (16)
C2—C3—C4—C5	−0.6 (2)