2-(4-Hy­droxy­phen­yl)acetamide

Kester, A.; King, T.; Bond, M.R.

doi:10.1107/S2414314625010077

organic compounds

IUCrDATA

ISSN: 2414-3146

Volume 10| Part 11| November 2025| x251007

https://doi.org/10.1107/S2414314625010077

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2-(4-Hydroxyphenyl)acetamide

Allison Kester,^a Taylor King ^a and Marcus R. Bond ^a ^*

^aDepartment of Chemistry and Physics, Southeast Missouri State University, Cape Girardeau, MO 63701, USA
^*Correspondence e-mail: [email protected]

Edited by W. T. A. Harrison, University of Aberdeen, United Kingdom (Received 29 October 2025; accepted 11 November 2025; online 14 November 2025)

In the title molecule, C₈H₉NO₂, which is an isomer of acetaminophen [N-(4-hydroxyphenyl)acetamide], the acetamide group plane subtends a dihedral angle of 89.95 (5)° with respect to the phenyl ring plane with the –NH₂ group directed outward, in contrast to an in vacuo DFT geometry optimization in which the –NH₂ group is directed inward. In the extended structure, N—H⋯O hydrogen bonds organize molecules into stacks propagating along [100], with additional hydrogen bonding linking neighboring parallel stacks. A survey of known structures indicates that the structures of 2-phenylacetamide molecules with any substitution at the 4-position on the phenyl ring can demonstrate different orientations for the acetamide group ranging from the –NH₂ group directed almost completely outward to the –NH₂ group directed almost completely inward.

Keywords: crystal structure; atenolol; acetaminophen; DFT geometry optimization.

CCDC reference: 2502168

Structure description

The title molecule, C₈H₉NO₂ (I), is an isomer of N-(4-hydroxyphenyl)acetamide [Cambridge Structural Database (CSD) refcodes: HXACAN01–67], also known as acetaminophen or paracetamol in different countries. The mean C_a—N (a = amide) bond length in structures of acetaminophen calculated from values in the CSD [1.346 (25) Å] is 0.026 Å longer than in (I) (Fig. 1). The short C—N bond length in (I) is consistent with the well known ‘amide resonance' effect (Kemnitz & Loewen, 2007 ). Bond lengths and angles within the amide group of (I) agree with mean values found for 2-substituted acetamide groups in the CSD [C—N/=O: 1.32 (5)/1.23 (5) Å; N—C=O/C—C=O/C—C—N: 122 (5)/121 (5)/116 (5)°; 1620 hits, CSD Version 5.00, August 2025 updates, Groom et al., 2016 ). The –NH₂ group in (I) is almost planar with slight pyramidalization [the N atom lies 0.017 Å above the C12/N11/(H1a, H1b) mean plane] so that sp² hybridization can be assigned to N11. The C_ar—OH (ar = aromatic) bond length [1.381 (3) Å] found in the structure of the parent acid, 4-hydroxyphenylacetic acid (QAPBAL; Gracin & Fischer, 2005 ) is some 0.019 Å longer than in (I). The C_ar—O—H angle agrees within 1 s.u. of 109.5° so that sp³ hybridization can be assume for O41.

Figure 1
Displacement ellipsoid plot of (I) at the 50% level with labels for all atoms.

The core atoms of (I) (C1–C6/C11/O41) are effectively planar [root-mean-square deviation (RMSD) = 0.003 Å] as are those of the acetamide group (C11, C12, O11, and N11; RMSD = 0.005 Å) with mean plane normals perpendicular [89.98 (5)°]. The –NH₂ group is directed outward from the phenyl ring and the carboxyl O-atom directed inward but not fully, as shown by the C1—C11—C12—N11 torsion angle of −135.91 (12)°. In contrast, since the N atom is bound to the phenyl ring in acetaminophen, the acetamide plane and the core atom plane are more closely aligned, e.g. with angles of 20.56 (5)° (monoclinic form I, HXACAN64) and 16.97 (5)° (orthorhombic form II, HXACAN65) between mean plane normals in recent structure determinations (Weatherston, 2024 ). In QAPBAL, the mean plane of the acid group is almost perpendicular to the phenyl ring mean plane [93.22 (14)°]. Here the carboxyl O-atom is directed inward, but with a larger torsion angle magnitude [159.8 (3)°]. A DFT geometry optimization [B3LYP, 6311+G(d,p); GAMESS (Schmidt et al., 1993 )] of (I) in vacuo results in the acetamide plane almost perpendicular (90.33°) to the plane of the core atoms, but with the carboxyl O atom directed outward, the –NH₂ group directed inward, and a torsion angle of −11.68° (Fig. 2). A semi-empirical, partially relaxed scan of this torsion angle (MOPAC2016, Version 19.255L, PM7 Hamiltonian; Stewart, 2016 ) shows a steady rise in energy from its optimized value to a maximum value as the torsion angle approaches 180° (Fig. 3). A MOL file of the optimized geometry has been placed in the supporting information.

Figure 2
Capped stick plots of the DFT-optimized geometry (color scheme: C, gray; H, white; N, blue; O, red) superimposed on the experimental geometry (light gray) of (I).

Figure 3
Plot of a semiempirical, partially relaxed scan in 1° increments of the C1—C11—C12—N11 torsion angle in (I) starting at the DFT-optimized value to a value of 180.3°. The acetamide group is constrained to be approximately planar and to be approximately perpendicular to the phenyl ring during the scan. Energy (kJ mol⁻¹) is plotted on the vertical axis with the torsion angle (°) plotted on the horizontal axis.

A search of the CSD for 2-phenylacetamide molecules with any substitution at the 4-position on the phenyl ring yielded 21 hits that are dominated by pharmaceutically related compounds or natural products. Ten of these are structures of atenolol (CEZVIN and CIDHAZ; de Castro et al., 2007 ), a β blocker medication for treatment of high blood pressure (Heel et al., 1979 ), or its salts or cocrystal: [succinate (DETHIU; Cai et al., 2006 ), nicotinate and isonicotinate (GUJBOG and GUJCAT; Botes et al., 2024 ), fumarate and adipate (IGUWUG and UHOGUX; Shajan et al., 2024 ), 4-aminobenzoate (JIRWIR; Lou et al., 2007 ), chloride (WEWLOC; Rama Kumar et al., 2018 ), and binaphthylphosphate (QAJYIL; Wang & Chen, 2011 )]. The atenolol molecule possesses a substituted propoxy group at the 4-position and, as a result, (I) is a common reagent in its synthetic preparation (Procopio et al., 2024 ).

The crystal structures of atenolol show an orientation for the acetamide group similar to that in (I), i.e., approximately perpendicular interplanar angles [86.01 (9)° for CEZVIN, 90.43 (11) and 86.74 (11)° for CIDHAZ] and similar torsion angle magnitudes [141.24 (19)° for CEZVIN, 135.0 (3) and 142.1 (2)° for CIDHAZ]. The acetamide groups in the salts and cocrystal show a range of orientations. For GUJBOG, JIRWIR, UHOGUX, and WEWLOC, the –NH₂ group is directed more outward, for DETHIEU, GUJCAT, and IGUWUG neither the –NH₂ nor –C=O groups are directed outward significantly, while for QAJYIL the –NH₂ group is directed more inward. In the structures of two other compounds, the natural product millingtojanine A (BAKWUJ; Jumai et al., 2021 ) and 2-carboxamidomethyl-4,5-dimethoxy-phenyl-N,N-diethylsulfonamide (CXMESX; Hamodrakas et al., 1977 ), the amide group is directed almost completely inward and similar to the orientation found in the DFT geometry optimization. At the opposite extreme is the structure of 2-(4-chlorophenyl)acetamide (OCETAT; Ma et al., 2011 ) in which the –NH₂ group is directed almost completely outward [torsion angle = 178.6 (2)°]. A histogram of torsion angle magnitudes for these compounds (Fig. 4) shows the full range of –NH₂ group orientations with a mean value of 115° (standard deviation = 42°) and a median of 121.7°. The orientation of the acetamide group appears to depend on competition between minimizing the molecular energy and optimizing the intermolecular hydrogen-bonding interactions, e.g., an outward-directed –NH₂ group may be more available as a hydrogen-bond donor if a suitable acceptor atom is present.

Figure 4
Histogram of the C—C—C—N torsion angle magnitude frequency for 2-phenylacetamide molecules with any substitution at the 4-position on the phenyl ring.

The –NH₂ group in (I) is a hydrogen-bond donor to carboxyl and to hydroxyl O atoms while the hydroxyl group is a hydrogen-bond donor to a carboxyl O atom, each to different molecules (Fig. 5). The N—H⋯O=C interaction links molecules into stacks along a with the other interactions linking neighboring parallel stacks (Fig. 6). By comparison, OCETAT is found in the same space group as (I) and with a slightly larger molecular volume [197.9 (2) Å³ versus 186.12 (2) Å³ in (I)], but with the chloro substituent not involved in N—H hydrogen bonding. In this case, the extended structure consists of herringbone bilayers with the acetamide groups linked by N—H⋯O hydrogen bonding on the outside of the bilayer while the 4-chloro substituents abut each other in the middle. Hydrogen-bond geometrical data for (I) are presented in Table 1.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O41—H41⋯O11ⁱ	0.914 (18)	1.92 (2)	2.7703 (13)	154.1 (16)
N11—H11a⋯O11ⁱⁱ	0.947 (16)	2.041 (16)	2.9365 (16)	157.1 (13)
N11—H11b⋯O41ⁱⁱⁱ	1.000 (15)	1.965 (16)	2.9646 (19)	178.0 (13)
Symmetry codes: (i) $[-x, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (ii) ; (iii) $[-x+{\script{1\over 2}}, -y+1, z+{\script{1\over 2}}]$ .

Figure 5
Donor hydrogen-bond interactions by a given molecule in (I) to three neighboring molecules. Atoms are drawn as circles of arbitrary radii and hydrogen bonds are indicated by dashed lines.

Figure 6
Unit-cell packing diagram for (I) viewed down a with b vertical and c horizontal. Atoms are drawn as circles of arbitrary radii and hydrogen bonds are indicated by dashed lines.

Synthesis and crystallization

2-(4-Hydroxyphenyl)acetamide (Aldrich, 99%) was dissolved in methanol and diffraction-quality crystals grown by slow evaporation at room temperature.

Refinement

Crystal data, data collection, and structure refinement details are listed in Table 2. Structure solution and initial refinement using an independent atom model occurred within the Bruker APEX3 software package (Version 2019/11–0; Bruker 2019 ) followed by Hirshfeld atom refinement within the OLEX2–1.5 system using NoSpherA2 (Kleemiss et al., 2021 ; Midgley et al., 2021 ). Non-spherical atomic form factors were derived from electron density determined by DFT calculations using ORCA 5.0 (B3LYP functional, def2-SVP basis set; Neese, 2022 ). All atoms were refined anisotropically. Two low angle reflections with F_o << F_c were presumed to be blocked by the beam catcher and omitted from the refinement. A secondary extinction correction coefficient was refined to a value of 0.017 (2).

Table 2
Experimental details

Crystal data
Chemical formula	C₈H₉NO₂
M_r	151.17
Crystal system, space group	Orthorhombic, P2₁2₁2₁
Temperature (K)	295
a, b, c (Å)	5.0935 (2), 9.5089 (4), 15.3708 (7)
V (Å³)	744.46 (6)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	0.10
Crystal size (mm)	0.49 × 0.21 × 0.17

Data collection
Diffractometer	Bruker D8 Quest Eco CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.689, 0.746
No. of measured, independent and observed [I ≥ 2u(I)] reflections	21517, 1908, 1483
R_int	0.057
(sin θ/λ)_max (Å⁻¹)	0.675

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.038, 0.048, 1.13
No. of reflections	1908
No. of parameters	182
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.20, −0.21
Absolute structure	Hooft et al. (2010 )
Absolute structure parameter	−0.1 (5)
Computer programs: SAINT (Bruker, 2019), SHELXT2018/2 (Sheldrick, 2015a ), OLEX2.refine (Bourhis et al., 2015 ), SHELXL2018/3 (Sheldrick, 2015b ), ORTEP-3 for Windows (Farrugia, 2012 ), Mercury (Macrae et al., 2020 ) and publCIF (Westrip, 2010 ).

Structural data

CCDC reference: 2502168

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2414314625010077/hb4542sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314625010077/hb4542Isup2.hkl

MOL file for DFT geometry optimized molecule. DOI: https://doi.org/10.1107/S2414314625010077/hb4542sup3.mol

Supporting information file. DOI: https://doi.org/10.1107/S2414314625010077/hb4542Isup4.cml

checkCIF report

Computing details top

2-(4-Hydroxyphenyl)acetamide top

Crystal data top

C₈H₉NO₂	D_x = 1.349 Mg m⁻³
M_r = 151.17	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P2₁2₁2₁	Cell parameters from 6683 reflections
a = 5.0935 (2) Å	θ = 3.4–26.3°
b = 9.5089 (4) Å	µ = 0.10 mm⁻¹
c = 15.3708 (7) Å	T = 295 K
V = 744.46 (6) Å³	Rod, colourless
Z = 4	0.49 × 0.21 × 0.17 mm
F(000) = 320.229

Data collection top

Bruker D8 Quest Eco CCD diffractometer	1483 reflections with I ≥ 2u(I)
φ and ω scans	R_int = 0.057
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θ_max = 28.7°, θ_min = 3.4°
T_min = 0.689, T_max = 0.746	h = −6→6
21517 measured reflections	k = −12→12
1908 independent reflections	l = −20→20

Refinement top

Refinement on F²	All H-atom parameters refined
Least-squares matrix: full	w = 1/[σ²(F_o²) + (0.0163P)² + 0.0113P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.038	(Δ/σ)_max = 0.001
wR(F²) = 0.048	Δρ_max = 0.20 e Å⁻³
S = 1.13	Δρ_min = −0.20 e Å⁻³
1908 reflections	Extinction correction: Zachariasen, Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
182 parameters	Extinction coefficient: 0.017 (2)
0 restraints	Absolute structure: Hooft et al. (2010)
0 constraints	Absolute structure parameter: −0.1 (5)
Primary atom site location: dual

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

	x	y	z	U_iso*/U_eq
O11	0.22820 (14)	0.47323 (8)	0.44521 (5)	0.0404 (2)
O41	−0.0450 (2)	0.24609 (10)	0.07225 (7)	0.0437 (3)
H41	−0.152 (3)	0.169 (2)	0.0717 (11)	0.069 (6)
N11	0.6580 (3)	0.49778 (15)	0.46884 (8)	0.0407 (3)
H11a	0.829 (3)	0.4650 (16)	0.4559 (9)	0.057 (4)
H11b	0.620 (3)	0.5855 (16)	0.5025 (10)	0.054 (5)
C1	0.3607 (2)	0.27802 (11)	0.30625 (6)	0.0329 (3)
C2	0.1609 (2)	0.18008 (13)	0.29703 (8)	0.0360 (3)
H2	0.111 (3)	0.1155 (13)	0.3516 (8)	0.065 (4)
C3	0.0224 (3)	0.16737 (13)	0.21944 (8)	0.0374 (3)
H3	−0.129 (3)	0.0902 (14)	0.2123 (7)	0.068 (4)
C4	0.0839 (2)	0.25385 (11)	0.14971 (7)	0.0326 (3)
C5	0.2843 (3)	0.35193 (14)	0.15763 (9)	0.0391 (3)
H5	0.332 (3)	0.4167 (14)	0.1032 (7)	0.064 (4)
C6	0.4203 (3)	0.36305 (14)	0.23543 (8)	0.0396 (3)
H6	0.573 (3)	0.4385 (14)	0.2415 (9)	0.076 (5)
C11	0.5106 (3)	0.29262 (15)	0.39025 (9)	0.0405 (3)
H11c	0.459 (3)	0.2104 (13)	0.4351 (9)	0.085 (6)
H11d	0.716 (3)	0.2869 (16)	0.3787 (9)	0.084 (5)
C12	0.4544 (2)	0.42959 (11)	0.43655 (7)	0.0283 (3)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O11	0.0233 (4)	0.0482 (5)	0.0498 (5)	0.0059 (4)	−0.0030 (4)	−0.0130 (4)
O41	0.0497 (6)	0.0443 (6)	0.0370 (5)	−0.0048 (5)	−0.0088 (5)	0.0048 (5)
H41	0.054 (12)	0.097 (15)	0.055 (11)	−0.021 (11)	−0.004 (10)	0.011 (12)
N11	0.0260 (6)	0.0446 (8)	0.0517 (7)	0.0003 (6)	−0.0017 (6)	−0.0046 (6)
H11a	0.040 (9)	0.071 (10)	0.062 (10)	0.011 (9)	−0.003 (9)	0.008 (9)
H11b	0.043 (10)	0.042 (9)	0.077 (12)	−0.001 (8)	−0.013 (8)	−0.026 (8)
C1	0.0345 (6)	0.0314 (6)	0.0328 (6)	0.0065 (6)	0.0001 (5)	−0.0032 (5)
C2	0.0413 (7)	0.0353 (7)	0.0313 (7)	−0.0011 (6)	0.0057 (6)	0.0027 (6)
H2	0.088 (11)	0.057 (9)	0.050 (8)	−0.018 (9)	−0.001 (8)	0.020 (7)
C3	0.0400 (7)	0.0362 (8)	0.0360 (7)	−0.0103 (6)	0.0014 (6)	0.0010 (6)
H3	0.094 (11)	0.062 (10)	0.047 (8)	−0.035 (10)	−0.002 (8)	0.012 (7)
C4	0.0343 (7)	0.0295 (6)	0.0339 (6)	−0.0007 (6)	0.0023 (5)	−0.0003 (5)
C5	0.0460 (8)	0.0381 (7)	0.0332 (7)	−0.0083 (6)	0.0013 (6)	0.0050 (6)
H5	0.082 (10)	0.075 (10)	0.034 (7)	−0.025 (9)	−0.027 (8)	0.020 (7)
C6	0.0402 (8)	0.0379 (7)	0.0407 (8)	−0.0100 (6)	−0.0024 (6)	0.0005 (5)
H6	0.097 (12)	0.066 (9)	0.065 (9)	−0.057 (10)	−0.013 (9)	0.008 (8)
C11	0.0396 (9)	0.0395 (8)	0.0424 (8)	0.0136 (7)	−0.0090 (7)	−0.0042 (6)
H11c	0.137 (16)	0.037 (8)	0.080 (11)	−0.002 (9)	−0.042 (11)	0.015 (9)
H11d	0.071 (11)	0.091 (12)	0.090 (12)	0.038 (11)	−0.006 (9)	−0.059 (9)
C12	0.0237 (6)	0.0342 (6)	0.0270 (6)	0.0037 (5)	0.0007 (5)	0.0022 (5)

Geometric parameters (Å, º) top

O11—C12	1.2319 (12)	C2—C3	1.3908 (16)
O41—H41	0.914 (18)	C3—H3	1.072 (13)
O41—C4	1.3618 (15)	C3—C4	1.3868 (15)
N11—H11a	0.947 (16)	C4—C5	1.3878 (16)
N11—H11b	1.000 (15)	C5—H5	1.067 (11)
N11—C12	1.3197 (16)	C5—C6	1.3862 (18)
C1—C2	1.3869 (16)	C6—H6	1.062 (13)
C1—C6	1.3895 (16)	C11—H11c	1.075 (14)
C1—C11	1.5063 (17)	C11—H11d	1.065 (15)
C2—H2	1.069 (11)	C11—C12	1.5115 (17)

C4—O41—H41	109.9 (11)	H5—C5—C4	119.1 (7)
H11b—N11—H11a	124.2 (14)	C6—C5—C4	119.63 (12)
C12—N11—H11a	118.8 (9)	C6—C5—H5	121.3 (7)
C12—N11—H11b	116.8 (9)	C5—C6—C1	121.48 (12)
C6—C1—C2	118.11 (11)	H6—C6—C1	118.9 (8)
C11—C1—C2	121.42 (11)	H6—C6—C5	119.6 (8)
C11—C1—C6	120.47 (12)	H11c—C11—C1	110.9 (7)
H2—C2—C1	118.6 (7)	H11d—C11—C1	110.6 (8)
C3—C2—C1	121.20 (12)	H11d—C11—H11c	108.2 (11)
C3—C2—H2	120.2 (7)	C12—C11—C1	112.78 (10)
H3—C3—C2	120.9 (6)	C12—C11—H11c	106.1 (7)
C4—C3—C2	119.78 (12)	C12—C11—H11d	108.0 (8)
C4—C3—H3	119.3 (6)	N11—C12—O11	121.91 (12)
C3—C4—O41	122.32 (11)	C11—C12—O11	121.20 (11)
C5—C4—O41	117.89 (11)	C11—C12—N11	116.86 (12)
C5—C4—C3	119.79 (11)

O11—C12—C11—C1	45.69 (11)	C2—C1—C11—C12	−111.31 (12)
O41—C4—C3—C2	−179.97 (11)	C2—C3—C4—C5	0.39 (14)
O41—C4—C5—C6	−179.99 (11)	C3—C2—C1—C6	−0.34 (13)
N11—C12—C11—C1	−135.91 (12)	C3—C2—C1—C11	179.75 (11)
C1—C2—C3—C4	−0.05 (13)	C3—C4—C5—C6	−0.33 (13)
C1—C6—C5—C4	−0.07 (14)	C5—C6—C1—C11	−179.69 (11)
C2—C1—C6—C5	0.40 (13)	C6—C1—C11—C12	68.78 (12)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O41—H41···O11ⁱ	0.914 (18)	1.92 (2)	2.7703 (13)	154.1 (16)
N11—H11a···O11ⁱⁱ	0.947 (16)	2.041 (16)	2.9365 (16)	157.1 (13)
N11—H11b···O41ⁱⁱⁱ	1.000 (15)	1.965 (16)	2.9646 (19)	178.0 (13)

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

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