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

Article abstract of DOI:10.1107/S0108270108005799

Molecules of 2,4-dimethyl-benzoic 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 b

Full text of DOI:10.1107/S0108270108005799

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
this is illustrated by the different values of the C2—C3—C8
and C4—C3—C8 angles, which are 124.23 (14) and
118.57 (14)ꢀ, respectively. In contrast, the C4—C5—C9 and
C6—C5—C9 angles are nearly equal and the methyl group in
the para position is located symmetrically.
ISSN 0108-2701
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 H1Á Á ÁO2i and O1Á Á ÁO2i distances
Conformation of hydrogen-bonded
dimeric o-methyl-substituted benzoic
acids
˚
are 1.71 (3) and 2.646 (2) A, respectively, and the O1—
H1Á Á ÁO2i angle is 176 (3)ꢀ [symmetry code: (i) Àx, Ày, 1 À z].
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 noncoplanarity of
the carboxyl group and the benzene ring 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, Á. This parameter was calculated by adding the
values of the C2—C3—C8 and C4—C3—C8 angles and
subtracting 240ꢀ from the sum. The second possibility for
relieving steric strain is a twist of the carboxyl group around
the Car—Ccarboxyl bond. To examine these parameters for
benzoic acid derivatives, we performed a search of the
Cambridge Structural Database (CSD, Version 5.28 of August
2007; Allen, 2002; Bruno et al., 2002). The following restric-
tions 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.
´
Marek Glinski, Ewa Wilczkowska, Izabela D. Madura and
Janusz Zachara*
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664
Warsaw, Poland
Correspondence e-mail: janzac@ch.pw.edu.pl
Received 11 January 2008
Accepted 29 February 2008
Online 8 March 2008
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.
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, along 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) A. The relevant C—
C—O angles are also different, at 114.34 (14) and 124.02 (14)ꢀ.
The differences between these bond lengths (Ád) and angles
(Á’) obey the relation given by Borthwick (1980), i.e. Á’ =
˚
À100 Â Ád (’ in degrees and d in A). The carboxyl group is
Figure 1
twisted with respect to the benzene ring. The dihedral angle
between the plane of the benzene ring and the plane formed
by atoms O1/O2/C1 is 14.06 (3)ꢀ. The methyl group in the
ortho position is splayed away from the carboxyl group and
The hydrogen-bonded dimeric motif of (I), showing 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. [Symmetry code: (i) Àx, Ày, 1 À z.]
o208 # 2008 International Union of Crystallography
doi:10.1107/S0108270108005799
Acta Cryst. (2008). C64, o208–o210

organic compounds
factors, such as crystal packing forces and weak intermolecular
interactions.
Experimental
The synthesis of the title compound 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 sublima-
tion.
Crystal data
3
˚
C9H10O2
Mr = 150.17
Monoclinic, P21=c
V = 786.8 (3) A
Z = 4
Figure 2
Mo Kꢁ radiation
ꢂ = 0.09 mmÀ1
T = 293 (2) K
0.25 Â 0.20 Â 0.15 mm
A scatter plot showing the relation between the in-plane splaying, Á, 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 penta-
substituted benzoic acid derivatives.
˚
a = 5.4656 (12) A
˚
b = 13.660 (2) A
˚
c = 10.6755 (19) A
ꢀ = 99.182 (16)ꢀ
Data collection
Siemens P3 diffractometer
1551 measured reflections
1399 independent reflections
953 reflections with I > 2ꢃ(I)
Rint = 0.014
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 Á ranges from 5.6 to 7.1ꢀ,
whereas that of the twist angle ranges from 1.0 to 14.1ꢀ. 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 Á and the
twist fall in the ranges 2.4–4.0 and 32.6–51.6ꢀ, respectively. It
should be noted that for pure 2,4,6-trimethylbenzoic acid
measured at 100 K (Wilson & Goeta, 2004), the values of Á
and the twist angle are 2.9 and 42.7ꢀ, respectively. When the
same compound forms a cocrystal with 1,4-bis(pentafluoro-
phenyl)butadiyne (Gdaniec et al., 2003), these values change
to 4.0 and 32.6ꢀ, 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 90ꢀ is observed.
2 standard reflections
every 70 reflections
intensity decay: 4.1%
Refinement
R[F2 > 2ꢃ(F2)] = 0.038
wR(F2) = 0.106
S = 1.02
1399 reflections
108 parameters
H atoms treated by a mixture of
independent and constrained
refinement
À3
˚
Áꢄmax = 0.14 e A
Áꢄmin = À0.13 e A
À3
˚
H atoms were positioned geometrically and constrained to ride on
˚
their parent atoms, with C—H = 0.93–0.96 A and Uiso(H) = 1.2Ueq(C)
or 1.5Ueq(methyl 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 carboxyl H
atom was located in a difference Fourier map and its positional and
isotropic displacement parameters were freely refined.
Data collection: P3/P4-PC Diffractometer Program (Siemens,
1991); cell refinement: P3/P4-PC Diffractometer Program; data
reduction: XDISK (Siemens, 1991); program(s) used to solve struc-
ture: SHELXS97 (Sheldrick, 2008); program(s) used to refine struc-
ture: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPIII
(Burnett & Johnson, 1996); software used to prepare material for
publication: SHELXL97 and PLATON (Spek, 2003).
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 90ꢀ with a
loss of resonance stabilization. The structures of 2,4,6-
trimethylbenzoic acid molecules in various molecular envir-
onments are good examples of the fact that the molecular
conformation is also to some extent dependent on other
Table 1
Selected geometric parameters (A, ).
ꢀ
˚
O1—C1
1.3085 (19)
O2—C1
1.2251 (18)
O1—C1—O2
O1—C1—C2
O2—C1—C2
C4—C3—C8
121.63 (14)
114.34 (14)
124.02 (14)
118.57 (14)
C2—C3—C8
C4—C5—C9
C6—C5—C9
124.23 (14)
121.52 (16)
120.86 (16)
O1—C1—C2—C3
165.38 (15)
O2—C1—C2—C3
À14.6 (3)
ꢁ
´
Acta Cryst. (2008). C64, o208–o210
Glinski et al. C9H10O2 o209

organic compounds
Burnett, M. N. & Johnson, C. K. (1996). ORTEPIII. Report ORNL-6895. Oak
Ridge National Laboratory, Tennessee, USA.
Byrn, M. P., Curtis, C. J., Hsiou, Y., Khan, S. I., Sawin, P. A., Tendick, S. K.,
Terzis, A. & Strouse, C. E. (1993). J. Am. Chem. Soc. 115, 9480–9491.
Etter, M. C. (1990). Acc. Chem. Res. 23, 120–126.
´
Gdaniec, M., Jankowski, W., Milewska, M. J. & Połonski, T. (2003). Angew.
Chem. Int. Ed. 42, 3903–3906.
Kosolapoff, G. M. (1947). J. Am. Chem. Soc. 69, 1651–1652.
Leiserowitz, L. (1976). Acta Cryst. B32, 775–802.
This work was supported by Warsaw University of Tech-
nology.
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: FA3135). Services for accessing these data are
described at the back of the journal.
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.
Siemens (1991). P3/P4-PC Diffractometer Program (Version 4.27) and XDISK
(Version 4.20.2 PC). Siemens Analytical X-ray Instruments Inc., Madison,
Wisconsin, USA.
References
Allen, F. H. (2002). Acta Cryst. B58, 380–388.
´
Barcon, A., Cote, M. L., Brunskill, A. P. J., Thompson, H. W. & Lalancette,
R. A. (1997). Acta Cryst. C53, 1842–1845.
Spek, A. L. (2003). J. Appl. Cryst. 36, 7–13.
Westheimer, F. H. (1956). Steric Effects in Organic Chemistry, edited by M. S.
Newman, pp. 523–555. New York: John Wiley & Sons.
Wilson, C. C. & Goeta, A. E. (2004). Angew. Chem. Int. Ed. 43, 2095–2099.
Borthwick, P. W. (1980). Acta Cryst. B36, 628–632.
Bruno, I. J., Cole, J. C., Edgington, P. R., Kessler, M., Macrae, C. F., McCabe, P.,
Pearson, J. & Taylor, R. (2002). Acta Cryst. B58, 389–397.
ꢁ
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o210 Glinski et al. C9H10O2
Acta Cryst. (2008). C64, o208–o210