Crystallographic and spectroscopic characterization of 2-bromo-p-tolu­aldehyde

Lanham, E.E.; Tanski, J.M.

doi:10.1107/S2056989025006127

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Volume 81| Part 8| August 2025| Pages 744-747

https://doi.org/10.1107/S2056989025006127

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Crystallographic and spectroscopic characterization of 2-bromo-p-tolualdehyde

Eden E. Lanham ^a and Joseph M. Tanski ^a ^*

^aDepartment of Chemistry, Vassar College, Poughkeepsie, NY 12604, USA
^*Correspondence e-mail: [email protected]

Edited by J. Reibenspies, Texas A & M University, USA (Received 14 June 2025; accepted 9 July 2025; online 29 July 2025)

The title compound (systematic name: 2-bromo-4-methylbenzaldehyde), C₈H₇BrO, is a halogenated benzaldehyde derivative. The molecule contains an aldehyde moiety, ortho bromine, and para methyl group. Packing via van der Waals forces, the molecules are arranged with both offset face-to-face and an edge-to-face π-stacking interaction revealed by Hirshfeld surface characterization.

Keywords: crystal structure; π-stacking; benzaldehyde derivative.

CCDC reference: 2471403

1. Chemical context

Benzaldehydes are a class of molecules commonly used as key ingredients in many natural fruit flavorings (Jabba et al., 2020 ; Kosmider et al., 2016 ). Benzaldehydes exhibit a wide range of properties; some of their derivatives have been investigated as potential carcinogens, particularly as flavor ingredients commonly found in e-cigarettes (Jabba et al., 2020). Conversely, others have been researched for their anticancer properties (Saitoh & Saya, 2016 ; Takeuchi et al., 1978 ). The title compound, 2-bromo-4-methylbenzaldehyde (I), more widely known as 2-bromo-p-tolualdehyde, is a benzaldehyde derivative with a bromine and methyl group. This compound may be synthesized by converting 2-bromo-4-methylaniline to the aldehyde via the benzenediazonium chloride (Jolad & Rajagopalan, 1966 ). The title compound has seen recent applications in the synthesis of non-linear optical materials (Rahman et al., 2025 ), as the starting material in the preparation of a protein kinase inhibitor (Defois et al., 2024 ), and in ruthenium-catalyzed aldehyde annulation to prepare indenes (Ma et al., 2025 ).

2. Structural commentary

The molecular structure of the title compound (Fig. 1) shows the aldehyde group located at the para position relative to the methyl group on the aromatic ring. The aldehyde is oriented with the oxygen atom rotated opposite an ortho bromine atom to avoid electronic repulsion, resulting in an intramolecular Br⋯H1A interaction at 2.7895 (14) Å and an angle between the aldehyde group and the plane of the aromatic ring of 10.60 (13)°.

Figure 1
A view of 2-bromo-p-tolualdehyde (I)

with the atom-numbering scheme. Displacement ellipsoids are shown at the 50% probability level.

3. Supramolecular features

The title compound packs together in the solid state via van der Waals forces (Fig. 2). The shortest distance between bromine atoms in adjacent molecules is 3.9641 (3) Å, which exceeds the sum of the van der Waals radii of bromine (3.70 Å; (Bondi, 1964 ). In an offset head-to-tail stacking motif running parallel to the crystallographic b-axis, the molecules of 2-bromo-p-tolualdehyde are arranged with the polar aldehyde and bromine groups in proximity to each other with a Br⋯H_aldehyde interaction, Br⋯H1A, with H–acceptor distance of 3.0651 (14) Å, longer than the observed intramolecular Br⋯H interaction. The stacks further pack in an offset fashion to maximize hydrophobic-like interactions of the non-polar tolyl groups. Both face-to-face and edge-to-face π-stacking geometrical arrangement of the aromatic rings are apparent, although both are highly offset (Fig. 3). The face-to-face π-stacking arrangement is characterized by a ring centroid-to-centroid distance of 3.9641 (3) Å, centroid-to-plane distance of 3.410 (1) Å, and ring-offset slippage parameter of 2.021 (2) Å. The edge-to-face π-stacking arrangement is revealed by the Hirshfeld surface calculated with CrystalExplorer21 (Spackman et al., 2021 ), mapped over d_norm in the range −0.0622 to 1.0811 a.u. (Fig. 4). The brighter red spot on the right of the surface indicates the offset edge-to-face C—H⋯π interaction of the C8—H8A proton directed towards the C7–C8 edge of the molecule within the surface, with an H8A⋯C7–C8 bond centroid distance of 2.770 Å. The red spot at the bottom of the surface represents the equivalent interaction originating from the molecule within the surface. The directionality of the C—H⋯π interaction is confirmed when the surface is mapped over d_e in the range 1.0656 to 2.3698 a.u. and d_i in the range 1.0646 to 2.3113 a.u. (Fig. 5), with a bright-red spot on the right of the surface when mapped over d_e indicating a short contact from the surface to the atom outside, and conversely a bright-red spot on the bottom of the surface when mapped over d_i indicating a short contact from the surface to the atom inside. The less intense red spot on the left of the d_norm surface indicates a weak C—H⋯O interaction, C6—H6B⋯O, with an H–acceptor distance of 2.655 (10) Å. The two-dimensional fingerprint plots (Fig. 6) show that no single interaction dominates, the most important interatomic contacts, summing to 91.8%, being (b) H⋯H (34.6%), (c) Br⋯H/H⋯Br (20.4%), (d) O⋯H/H⋯O (17.1%), (e) C⋯H/H⋯C (13.0%) and (f) C⋯C (6.7%) contacts. Additionally, Br contacts with C, Br and O together contribute 5.7%, and with the contribution of 2.4% O⋯C contacts, the total reaches 99.9%.

Figure 2
A view of the molecular packing in 2-bromo-p-tolualdehyde (I)

. Displacement ellipsoids are shown at the 50% probability level.

Figure 3
A view of the face-to-face and edge-to-face (C8—H8A⋯πⁱ shown via dashed lines) π-stacking geometrical arrangement in 2-bromo-p-tolualdehyde (I)

. Displacement ellipsoids are shown at the 50% probability level. Symmetry code: (i) $[{1\over 2}]$ − x, − $[{1\over 2}]$ + y, $[{3\over 2}]$ − z.

Figure 4
Hirshfeld surface of 2-bromo-p-tolualdehyde (I)

mapped over d_norm showing via dashed lines the C8—H8A⋯πⁱ and C6—H6B⋯Oⁱⁱ interactions. Symmetry codes: (i) $[{1\over 2}]$ − x, − $[{1\over 2}]$ + y, $[{3\over 2}]$ − z; (ii) − $[{1\over 2}]$ + x, $[{1\over 2}]$ − y, − $[{1\over 2}]$ + z.

Figure 5
Hirshfeld surface of 2-bromo-p-tolualdehyde (I)

mapped over d_i (left) and d_e (right) showing the C8—H8A⋯πⁱ interaction.

Figure 6
The (a) full two-dimensional fingerprint plot for 2-bromo-p-tolualdehyde (I)

and individual fingerprint plots for (b) H⋯H (34.6%), (c) Br⋯H/H⋯Br (20.4%), (d) O⋯H/H⋯O (17.1%), (e) C⋯H/H⋯C (13.0%) and (f) C⋯C (6.7%) contacts.

4. Database survey

The Cambridge Structural Database (version 6.00, April 2025; Groom et al., 2016 ) contains related aromatic bromobenzaldehydes. 4-Bromobenzaldehyde (CSD Refcode YICFEV01; Ndima et al., 2021 ) crystallizes with two molecules in the asymmetric unit and features C—Br bond lengths of 1.891 and 1.895 Å, similar to the title compound at 1.9040 (13) Å, whereas the smaller angle between the aldehyde group and the plane of the aromatic rings of 0.85 and 2.07° in the related structure emphasizes in the impact of the ortho bromine regiochemistry in the title compound, which twists the aldehyde 10.60 (13)° out of the plane of the aromatic ring. Similarly to the title compound, related structure 1-bromo-2-naphthaldehyde (Refcode FADWIQ; Koppenhoefer & Bats, 1986 ) also has the aldehyde oriented with the oxygen atom rotated opposite the ortho bromine atom resulting in an intramolecular Br⋯H interaction at 2.653 Å, although the aldehyde carbonyl appears to be fully conjugated to the naphthyl, displaying an angle between the aldehyde group and the plane of the aromatic ring of 0°.

5. Synthesis and crystallization

2-Bromo-p-tolualdehyde (technical grade) was purchased from Aldrich Chemical Company, USA, and was used as received.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon were included in calculated positions and refined using a riding model with: C—H = 0.95, 0.95 and 0.98 Å and U_iso(H) = 1.2, 1.2 and 1.5 × U_eq(C) of the aryl, aldehyde and methyl C-atoms, respectively.

Table 1
Experimental details

Crystal data
Chemical formula	C₈H₇BrO
M_r	199.05
Crystal system, space group	Monoclinic, P2₁/n
Temperature (K)	125
a, b, c (Å)	11.4432 (8), 3.9641 (3), 16.8225 (11)
β (°)	102.838 (1)
V (Å³)	744.03 (9)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	5.45
Crystal size (mm)	0.38 × 0.33 × 0.25

Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.26, 0.34
No. of measured, independent and observed [I > 2σ(I)] reflections	17211, 2269, 2159
R_int	0.023
(sin θ/λ)_max (Å⁻¹)	0.714

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.017, 0.045, 1.08
No. of reflections	2269
No. of parameters	93
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.43, −0.34
Computer programs: Bruker Instrument Service, APEX3 and SAINT (Bruker, 2013 ), SHELXT2018/2 (Sheldrick, 2015a ), SHELXL2017/1 (Sheldrick, 2015b ), SHELXTL2014 (Sheldrick, 2008 ), OLEX2 (Dolomanov et al., 2009 ), and Mercury (Macrae et al., 2020 ).

7. Analytical Data

¹H NMR (Bruker Avance III HD 400 MHz, CDCl₃): δ 2.4 (s, 3H, CH₃), δ 7.23 (dd, 1H, C_arylH, J_ortho = 7.8 Hz, J_meta = 0.8 Hz), δ 7.46 (s, 1H, C_arylH), δ 7.82 (d, 1H, C_arylH, J_ortho = 7.8 Hz), δ 10.32 (s, 1H, CHO). ¹³C NMR (¹³C{¹H}, 100.6 MHz, CDCl₃): δ 20.2 (CH₃), δ 127.0 (C_aryl), δ 128.9 (C_arylH), δ 129.9 (C_arylH), δ 131.2 (C_aryl), δ 134.1 (C_arylH), δ 146.7 (C_aryl), δ 190.2 (CHO). IR (Thermo Nicolet iS50, ATR, cm⁻¹): 3029 (w, C_arylH str), 2953 (w, C_alkylH str), 2923 (w, C_alkylH str), 2858 (m) and 2752 (w) (CHO Fermi doublet), 1683 (s, CO str), 1648 (m), 1598 (s), 1560 (m), 1499 (w), 1482 (m), 1445 (m), 1380 (s), 1292 (w), 1267 (s), 1207 (s), 1141 (m), 1038 (s), 998 (m), 873 (m), 864 (s), 818 (s), 780 (s), 693 (s), 672 (m), 611 (s), 532 (m), 440 (s), 434 (s). GC/MS (Hewlett-Packard MS 5975/GC 7890): M+ = 199 amu.

Supporting information

CCDC reference: 2471403

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S2056989025006127/jy2061sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989025006127/jy2061Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2056989025006127/jy2061Isup3.cml

checkCIF report

Computing details top

2-Bromo-4-methylbenzaldehyde top

Crystal data top

C₈H₇BrO	F(000) = 392
M_r = 199.05	D_x = 1.777 Mg m⁻³
Monoclinic, P2₁/n	Mo Kα radiation, λ = 0.71073 Å
a = 11.4432 (8) Å	Cell parameters from 9969 reflections
b = 3.9641 (3) Å	θ = 2.4–30.5°
c = 16.8225 (11) Å	µ = 5.45 mm⁻¹
β = 102.838 (1)°	T = 125 K
V = 744.03 (9) Å³	Plate, colourless
Z = 4	0.38 × 0.33 × 0.25 mm

Data collection top

Bruker APEXII CCD diffractometer	2269 independent reflections
Radiation source: sealed X-ray tube, Bruker APEX-II CCD	2159 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.023
Detector resolution: 8.3333 pixels mm^-1	θ_max = 30.5°, θ_min = 2.0°
φ and ω scans	h = −16→16
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	k = −5→5
T_min = 0.26, T_max = 0.34	l = −23→24
17211 measured reflections

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.017	H-atom parameters constrained
wR(F²) = 0.045	w = 1/[σ²(F_o²) + (0.0209P)² + 0.4808P] where P = (F_o² + 2F_c²)/3
S = 1.08	(Δ/σ)_max = 0.003
2269 reflections	Δρ_max = 0.43 e Å⁻³
93 parameters	Δρ_min = −0.34 e Å⁻³
0 restraints	Extinction correction: SHELXL2017/1 (Sheldrick 2015b)
Primary atom site location: dual	Extinction coefficient: 0.0085 (7)

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Br	0.61962 (2)	0.86694 (3)	0.61166 (2)	0.01803 (5)
O	0.28890 (10)	0.2568 (3)	0.56866 (7)	0.0305 (2)
C1	0.37283 (12)	0.4513 (4)	0.58656 (8)	0.0204 (3)
H1A	0.40198	0.554074	0.543759	0.024*
C2	0.43181 (11)	0.5358 (3)	0.67170 (7)	0.0147 (2)
C3	0.53957 (11)	0.7144 (3)	0.69284 (8)	0.0138 (2)
C4	0.59307 (11)	0.7850 (3)	0.77369 (8)	0.0159 (2)
H4A	0.666864	0.904404	0.78654	0.019*
C5	0.53869 (12)	0.6811 (3)	0.83594 (8)	0.0173 (2)
C6	0.59545 (14)	0.7624 (4)	0.92352 (8)	0.0246 (3)
H6A	0.652269	0.948864	0.925516	0.037*
H6B	0.637915	0.563344	0.949925	0.037*
H6C	0.533058	0.827998	0.952079	0.037*
C7	0.43039 (12)	0.5029 (3)	0.81548 (8)	0.0188 (2)
H7A	0.392109	0.431425	0.857244	0.023*
C8	0.37879 (11)	0.4303 (3)	0.73479 (8)	0.0180 (2)
H8A	0.305944	0.306442	0.722046	0.022*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br	0.01877 (7)	0.01856 (7)	0.01827 (7)	−0.00280 (4)	0.00737 (5)	0.00088 (4)
O	0.0269 (5)	0.0362 (6)	0.0245 (5)	−0.0129 (5)	−0.0029 (4)	−0.0015 (5)
C1	0.0202 (6)	0.0224 (6)	0.0171 (6)	−0.0019 (5)	0.0008 (5)	−0.0002 (5)
C2	0.0132 (5)	0.0148 (5)	0.0153 (5)	0.0013 (4)	0.0018 (4)	−0.0001 (4)
C3	0.0139 (5)	0.0127 (5)	0.0155 (5)	0.0013 (4)	0.0047 (4)	0.0000 (4)
C4	0.0144 (5)	0.0147 (5)	0.0176 (6)	0.0014 (4)	0.0016 (4)	−0.0014 (4)
C5	0.0201 (6)	0.0161 (5)	0.0153 (5)	0.0061 (4)	0.0028 (4)	−0.0007 (4)
C6	0.0315 (7)	0.0266 (7)	0.0145 (6)	0.0045 (6)	0.0024 (5)	−0.0029 (5)
C7	0.0192 (6)	0.0201 (6)	0.0189 (6)	0.0044 (5)	0.0082 (5)	0.0028 (5)
C8	0.0141 (5)	0.0191 (6)	0.0216 (6)	0.0004 (4)	0.0056 (4)	0.0013 (5)

Geometric parameters (Å, º) top

Br—C3	1.9040 (13)	C5—C7	1.4013 (19)
O—C1	1.2162 (18)	C5—C6	1.5071 (19)
C1—C2	1.4796 (18)	C6—H6A	0.98
C1—H1A	0.95	C6—H6B	0.98
C2—C3	1.3977 (17)	C6—H6C	0.98
C2—C8	1.3984 (18)	C7—C8	1.3849 (19)
C3—C4	1.3904 (17)	C7—H7A	0.95
C4—C5	1.3941 (19)	C8—H8A	0.95
C4—H4A	0.95

O—C1—C2	123.27 (13)	C7—C5—C6	120.89 (13)
O—C1—H1A	118.4	C5—C6—H6A	109.5
C2—C1—H1A	118.4	C5—C6—H6B	109.5
C3—C2—C8	117.74 (11)	H6A—C6—H6B	109.5
C3—C2—C1	123.22 (12)	C5—C6—H6C	109.5
C8—C2—C1	119.04 (12)	H6A—C6—H6C	109.5
C4—C3—C2	121.51 (12)	H6B—C6—H6C	109.5
C4—C3—Br	117.40 (9)	C8—C7—C5	120.44 (12)
C2—C3—Br	121.08 (9)	C8—C7—H7A	119.8
C3—C4—C5	120.15 (12)	C5—C7—H7A	119.8
C3—C4—H4A	119.9	C7—C8—C2	121.29 (12)
C5—C4—H4A	119.9	C7—C8—H8A	119.4
C4—C5—C7	118.85 (12)	C2—C8—H8A	119.4
C4—C5—C6	120.25 (13)

O—C1—C2—C3	169.24 (14)	C3—C4—C5—C7	0.54 (19)
O—C1—C2—C8	−10.0 (2)	C3—C4—C5—C6	−178.96 (12)
C8—C2—C3—C4	0.20 (18)	C4—C5—C7—C8	0.30 (19)
C1—C2—C3—C4	−179.10 (12)	C6—C5—C7—C8	179.80 (13)
C8—C2—C3—Br	179.59 (9)	C5—C7—C8—C2	−0.9 (2)
C1—C2—C3—Br	0.29 (17)	C3—C2—C8—C7	0.65 (19)
C2—C3—C4—C5	−0.80 (19)	C1—C2—C8—C7	179.99 (12)
Br—C3—C4—C5	179.79 (9)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1A···Brⁱ	0.95	3.07	3.5842 (14)	116

Symmetry code: (i) −x+1, −y+1, −z+1.

Acknowledgements

This work was supported by Vassar College.

Funding information

X-ray facilities were provided by the US National Science Foundation (grant Nos. 0521237 and 0911324 to JMT).

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