Acetic anhydride at 100 K: the first crystal structure determination

Acetic anhydride (ethanoic anhydride), (CH₃CO)₂O, is a widely used acetyl­ation reagent in organic synthesis. The crystal and mol­ecular structure, as determined by single-crystal X-ray analysis at 100 K, is reported for the first time. A crystal of the title compound (m.p. 200 K) suitable for X-ray diffraction was grown from the melt at low temperature. The title compound crystallizes in the ortho­rhom­bic space group Pbcn, with Z = 4. In the crystal, the mol­ecule adopts an exact C₂-symmetric conformation about a crystallographic twofold axis. The mol­ecules are densely packed. Two of the methyl H atoms form short inter­molecular contacts to a neighbouring carbonyl O atom, which can be viewed as weak hydrogen bonds.

Computing details

Data collection: APEX2 (Bruker, 2013); cell refinement: SAINT (Bruker, 2013); data reduction: SAINT (Bruker, 2013); program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2014); software used to prepare material for publication: enCIFer (Allen et al., 2004).

Ethanoic anhydride

Crystal data

C4H6O3	Dx = 1.346 Mg m^−3
Mr = 102.09	Cu Kα radiation, λ = 1.54178 Å
Orthorhombic, Pbcn	Cell parameters from 1147 reflections
a = 8.0707 (14) Å	θ = 5.5–41.1°
b = 7.8733 (13) Å	µ = 1.01 mm^−1
c = 7.9305 (13) Å	T = 100 K
V = 503.93 (15) Å^3	Cylinder, colourless
Z = 4	1.32 × 0.50 × 0.50 mm
F(000) = 216

Data collection

Bruker AXS X8 Proteum diffractometer	452 independent reflections
Radiation source: 0.2 x 2 mm focus rotating anode	445 reflections with I > 2σ(I)
MONTEL graded multilayer optic monochromator	Rint = 0.051
Detector resolution: 16.67 pixels mm^-1	θmax = 67.4°, θmin = 7.9°
φ– and ω–scans	h = −9→9
Absorption correction: gaussian (SADABS; Bruker, 2012)	k = −9→9
Tmin = 0.472, Tmax = 0.669	l = −7→8
19642 measured reflections

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)] = 0.025	H-atom parameters constrained
wR(F^2) = 0.064	w = 1/[σ^2(Fo^2) + (0.0276P)^2 + 0.1605P] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max < 0.001
452 reflections	Δρmax = 0.16 e Å^−3
35 parameters	Δρmin = −0.14 e Å^−3
0 restraints	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.012 (2)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
O1	0.14484 (9)	0.04403 (9)	0.34929 (9)	0.0280 (3)
O2	0.0000	0.26741 (12)	0.2500	0.0253 (3)
C1	0.08205 (11)	0.17820 (12)	0.37607 (12)	0.0204 (3)
C2	0.08774 (13)	0.27697 (12)	0.53580 (13)	0.0245 (3)
H2A	0.1315	0.2051	0.6263	0.037*
H2B	−0.0243	0.3148	0.5652	0.037*
H2C	0.1597	0.3761	0.5211	0.037*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0304 (5)	0.0292 (5)	0.0246 (5)	0.0064 (3)	−0.0029 (3)	−0.0002 (3)
O2	0.0345 (6)	0.0211 (5)	0.0203 (6)	0.000	−0.0097 (4)	0.000
C1	0.0187 (5)	0.0230 (5)	0.0195 (6)	−0.0035 (4)	−0.0005 (4)	0.0041 (4)
C2	0.0303 (6)	0.0247 (6)	0.0186 (6)	−0.0046 (4)	−0.0021 (4)	0.0011 (4)

Geometric parameters (Å, º)

O1—C1	1.1907 (12)	C2—H2A	0.9800
O2—C1^i	1.3897 (11)	C2—H2B	0.9800
O2—C1	1.3897 (11)	C2—H2C	0.9800
C1—C2	1.4871 (14)
C1^i—O2—C1	119.29 (10)	C1—C2—H2B	109.5
O1—C1—O2	121.52 (9)	H2A—C2—H2B	109.5
O1—C1—C2	127.06 (9)	C1—C2—H2C	109.5
O2—C1—C2	111.30 (8)	H2A—C2—H2C	109.5
C1—C2—H2A	109.5	H2B—C2—H2C	109.5
C1^i—O2—C1—O1	32.13 (7)	C1^i—O2—C1—C2	−151.49 (8)

Symmetry code: (i) −x, y, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2A···O1^ii	0.98	2.64	3.5750 (13)	159
C2—H2C···O1^iii	0.98	2.47	3.3567 (13)	151

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