3,5-Difluoro­benzoic acid

The title compound, C₇H₄O₂F₂, forms dimers that are stabilized by hydrogen bonds between carboxyl groups. The crystal structure is stabilized by O—H...O and C—H...O hydrogen bonds, as well as weak stacking inter­actions between the mol­ecules. The F atoms form edges of channels that extend along [100].

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.002 Å
• R factor = 0.040
• wR factor = 0.128
• Data-to-parameter ratio = 17.5

checkCIF/PLATON results

No syntax errors found



Alert level C
PLAT153_ALERT_1_C The su's on the Cell Axes are Equal (x 100000) .        100 Ang.
PLAT180_ALERT_3_C Check Cell Rounding: # of Values Ending with 0 =          4

Alert level G
PLAT199_ALERT_1_G Check the Reported _cell_measurement_temperature        293 K
PLAT200_ALERT_1_G Check the Reported _diffrn_ambient_temperature .        293 K

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   2 ALERT level C = Check and explain
   2 ALERT level G = General alerts; check

   3 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   0 ALERT type 2 Indicator that the structure model may be wrong or deficient
   1 ALERT type 3 Indicator that the structure quality may be low
   0 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. Comment

3,5-Difluorobenzoic acid (I) (Fig. 1) belongs to a family of compounds with medical and agricultural applications (Dumas et al., 1999; Pinkus et al., 2003). For example, substituted benzoic acids stimulate skeletal muscle (Moffett and Tang et al., 1968) and thus 3,5-dichlorobenzoic acid has been used for derivative preparation in a cardiac arrhythmia treatment (Lynch and Salata, 1998). 3,5-Difluorobenzoic acid is used as a substrate in the synthesis of 3,5-difluorohydrazide (Qadeer et al., 2007), which is needed for the synthesis of biologically active heterocyclic compounds.

3,5-Difluorobenzoic acid crystallized in the space group P2₁/c with one molecule per asymmetric unit. The packing in the crystal structure of (I) is very similar to that observed for 3,5-dichlorobenzoic acid (Pinkus et al., 2003). In both cases the carboxylic acid groups are involved in dimer formation, by forming stabilizing hydrogen bonds (Table 1, Fig. 2). The packing is also stabilized by intermolecular C4—H4···O2 hydrogen-bond interactions (Table 1). Stacking interactions are weak with distances between centroids and offsets of 3.77Å and 1.37Å respectively. The molecules of (I) are packed in such a way that channels of 2.6Å by 3.3Å wide are formed between halogen substituents (Fig. 2). The volume of the channels in each unit cell, as calculated with PLATON (Spek, 2003), equals 8% of the unit cell volume. The carboxylic acid and benzene groups are almost coplanar with a C2—C1—C7—O2 torsion angle equal to 172°.

Related literature

For related literature, see: Dumas et al. (1999); Lynch & Salata (1998); Moffett & Tang (1968); Pinkus et al. (2003); Qadeer et al. (2007).

S2. Experimental

3,5-Difluorobenzoic acid (97%) was purchased from Aldrich. It was initially dissolved in acetone, and small crystals were obtained by slow evaporation at 293 K. These crystals of I were dissolved in and then recrystallized by evaporation from 1-butanol, which resulted in single crystals suitable for X-ray diffraction study.

S3. Refinement

The aromatic hydrogen atoms were visible in the difference density Fourier map, but during refinement they were replaced in ideal positions with C—H distances equal to 0.93Å and were allowed to refine using the riding model with an isotropic displacement parameter 1.2 times that of the adjacent carbon atom. The hydrogen atom of the carboxylic acid group was localized from the difference map and isotropically refined. The O—H distance was restrained to be 0.82 Å.

Computing details

Data collection: HKL-2000 (Otwinowski & Minor, 1997); cell refinement: HKL-2000; data reduction: HKL-2000; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997) and HKL-3000SM (Minor et al., 2006); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997) and HKL-3000SM; molecular graphics: HKL-3000SM, Mercury (Macrae et al., 2006), ORTEPIII (Burnett & Johnson, 1996) and ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: HKL-3000SM.

Figures

	Fig. 1. A view of the title compound. Displacement ellipsoids are drawn at the 50% probability level and H atoms are drawn as spheres of an arbitrary radius.
	Fig. 2. The molecular packing of compound I shown along [100]. Hydrogen bonds are marked with blue, dashed lines.

3,5-difluorobenzoic acid

Crystal data

C7H4F2O2	F(000) = 320
Mr = 158.10	Dx = 1.484 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71074 Å
Hall symbol: -P 2ybc	Cell parameters from 18569 reflections
a = 3.769 (1) Å	θ = 2.1–28.7°
b = 13.400 (1) Å	µ = 0.14 mm^−1
c = 14.041 (1) Å	T = 293 K
β = 93.78 (1)°	Needle, colorless
V = 707.6 (2) Å^3	0.5 × 0.15 × 0.05 mm
Z = 4

Data collection

Rigaku R-AXIS RAPID diffractometer	1817 independent reflections
Radiation source: fine-focus sealed tube	1277 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.033
Detector resolution: 10 pixels mm^-1	θmax = 28.7°, θmin = 2.1°
ω scans with χ offset	h = −5→5
Absorption correction: multi-scan (Otwinowski et al., 2003)	k = −17→17
Tmin = 0.98, Tmax = 0.99	l = −18→18
18569 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.040	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.128	H atoms treated by a mixture of independent and constrained refinement
S = 1.07	w = 1/[σ^2(Fo^2) + (0.0586P)^2 + 0.0698P] where P = (Fo^2 + 2Fc^2)/3
1817 reflections	(Δ/σ)max < 0.001
104 parameters	Δρmax = 0.16 e Å^−3
1 restraint	Δρmin = −0.16 e Å^−3

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 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 (Ų)

	x	y	z	Uiso*/Ueq
O2	0.1841 (3)	0.48378 (7)	0.39611 (7)	0.0784 (3)
O1	−0.0234 (3)	0.36856 (8)	0.49171 (7)	0.0747 (3)
C7	0.1216 (3)	0.39595 (10)	0.41548 (8)	0.0586 (3)
C1	0.2106 (3)	0.31418 (10)	0.34960 (9)	0.0597 (3)
C6	0.3240 (3)	0.33876 (12)	0.26072 (9)	0.0691 (4)
H6	0.3440	0.4050	0.2421	0.083*
F2	0.5187 (3)	0.28467 (10)	0.11447 (7)	0.1159 (4)
C4	0.3797 (4)	0.16402 (14)	0.22527 (13)	0.0893 (5)
H4	0.4364	0.1135	0.1835	0.107*
C2	0.1801 (4)	0.21588 (11)	0.37749 (11)	0.0724 (4)
H2	0.1043	0.1993	0.4372	0.087*
F1	0.2359 (4)	0.04641 (8)	0.33917 (11)	0.1330 (5)
C5	0.4060 (4)	0.26214 (14)	0.20097 (10)	0.0800 (5)
C3	0.2666 (5)	0.14321 (12)	0.31326 (14)	0.0867 (5)
H1	−0.071 (6)	0.4223 (13)	0.5270 (15)	0.135 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O2	0.1051 (8)	0.0638 (6)	0.0701 (6)	−0.0015 (5)	0.0341 (5)	−0.0054 (4)
O1	0.0966 (7)	0.0682 (6)	0.0624 (6)	0.0010 (5)	0.0283 (5)	−0.0038 (4)
C7	0.0591 (7)	0.0647 (8)	0.0529 (6)	0.0031 (5)	0.0103 (5)	−0.0040 (5)
C1	0.0526 (6)	0.0686 (8)	0.0581 (7)	0.0034 (5)	0.0047 (5)	−0.0124 (5)
C6	0.0631 (7)	0.0854 (9)	0.0596 (7)	0.0001 (6)	0.0096 (6)	−0.0150 (6)
F2	0.1198 (9)	0.1598 (11)	0.0716 (6)	−0.0067 (7)	0.0334 (5)	−0.0382 (6)
C4	0.0775 (10)	0.1029 (13)	0.0870 (11)	0.0128 (8)	0.0021 (8)	−0.0437 (9)
C2	0.0750 (8)	0.0692 (9)	0.0727 (8)	0.0050 (7)	0.0030 (6)	−0.0109 (6)
F1	0.1841 (13)	0.0705 (7)	0.1448 (11)	0.0127 (7)	0.0135 (9)	−0.0238 (6)
C5	0.0657 (8)	0.1118 (13)	0.0634 (8)	0.0016 (8)	0.0100 (6)	−0.0282 (8)
C3	0.0903 (11)	0.0692 (9)	0.0996 (12)	0.0093 (8)	−0.0024 (9)	−0.0239 (8)

Geometric parameters (Å, º)

O2—C7	1.2341 (15)	F2—C5	1.3471 (19)
O1—C7	1.2870 (15)	C4—C3	1.362 (3)
O1—H1	0.898 (15)	C4—C5	1.364 (3)
C7—C1	1.4865 (17)	C4—H4	0.9300
C1—C2	1.381 (2)	C2—C3	1.381 (2)
C1—C6	1.3853 (19)	C2—H2	0.9300
C6—C5	1.374 (2)	F1—C3	1.354 (2)
C6—H6	0.9300
C7—O1—H1	109.9 (15)	C3—C4—H4	121.4
O2—C7—O1	123.58 (11)	C5—C4—H4	121.4
O2—C7—C1	120.76 (11)	C3—C2—C1	117.37 (16)
O1—C7—C1	115.66 (12)	C3—C2—H2	121.3
C2—C1—C6	121.24 (13)	C1—C2—H2	121.3
C2—C1—C7	120.01 (12)	F2—C5—C4	118.30 (14)
C6—C1—C7	118.75 (13)	F2—C5—C6	118.69 (17)
C5—C6—C1	117.88 (15)	C4—C5—C6	123.01 (16)
C5—C6—H6	121.1	F1—C3—C4	118.50 (15)
C1—C6—H6	121.1	F1—C3—C2	118.17 (18)
C3—C4—C5	117.18 (14)	C4—C3—C2	123.33 (17)
O2—C7—C1—C2	171.99 (12)	C3—C4—C5—F2	179.73 (13)
O1—C7—C1—C2	−8.38 (18)	C3—C4—C5—C6	−0.1 (2)
O2—C7—C1—C6	−7.93 (19)	C1—C6—C5—F2	−179.63 (12)
O1—C7—C1—C6	171.70 (12)	C1—C6—C5—C4	0.2 (2)
C2—C1—C6—C5	−0.1 (2)	C5—C4—C3—F1	179.64 (15)
C7—C1—C6—C5	179.81 (12)	C5—C4—C3—C2	−0.1 (3)
C6—C1—C2—C3	−0.1 (2)	C1—C2—C3—F1	−179.56 (14)
C7—C1—C2—C3	−179.99 (13)	C1—C2—C3—C4	0.2 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O2^i	0.90 (2)	1.73 (2)	2.625 (1)	173 (2)
C4—H4···O2^ii	0.93	2.56	3.437 (2)	159

Symmetry codes: (i) −x, −y+1, −z+1; (ii) −x+1, y−1/2, −z+1/2.