Sodium phenoxy­acetate hemihydrate

The structure of the hemihydrate of sodium phenoxy­acetate, Na⁺·C₈H₇O₃^-·0.5H₂O, has been redetermined at low temperature (160 K). The structure consists of ribbons containing octahedral NaO₆ units, and half of the Na₂O₂ squares within the ribbon are bridged by water mol­ecules which lie across twofold rotation axes in C2/c. The phenyl substituents lie on the outside of the ribbon.

S1. Comment

The structure of the title compound, (I), was first solved by X-ray photography (R = 0.096; Prout et al., 1971) and has now been redetermined at 160 K (R = 0.052) during an investigation of the impact of impurities on the crystallization of phenoxyacetic acid. The structure consists of a ribbon-like Na–O core, running parallel to the [001] direction, which is bridged above and below by water molecules. As in the acid salt (Evans et al., 2001), the phenyl groups form hydrocarbon layers on the outside of the Na–O ribbon. The ribbon consists of sodium ions surrounded by six O atoms in a distorted octahedral arrangement. The shortest Na···Na bridging distances [range 3.179 (2)–4.0384 (16) Å] are similar to those reported for other bridging systems (Albertsson, 1973).

Hydrates of other metal salts of phenoxyacetate have been determined. Some, like the sodium salt, are oligomeric, namely the lead (Archer et al., 1996; Mak et al., 1985) magnesium and cobalt salts (Smith et al., 1980), whereas the cadmium (Mak et al., 1985) and copper (Prout et al., 1968) derivatives contain discrete units.

Two structural features were identified in the CO₂^- group in the title compound. First, the distortion of the exo-C1 angles C15–C10–O1 and C11–C10–O1 [123.3 (2) and 116.4 (2)°, respectively], which deviate from the trigonal angle (120°). The distortion is caused by the steric requirements of the side chain and is found throughout the phenoxyalkanoic acid series (Kennard et al., 1982; Evans et al., 2001). Secondly, the C—O distances for both C2—O3 and C2—O2 are identical [1.251 (3) Å], and the former of the O3–C2–C3 and O2–C2–C3 angles [113.6 (2) and 119.0 (2) Å] deviates from the trigonal value. In disordered carboxylic acid groups (Leiserowitz, 1976), both bond lengths are also the same, but the O—C—C angles are both close to 120°. In the title compound, the similarity in C—O distances arises because the CO₂^- group is delocalized. However, the O2 and O3 atoms do not have similar environments, as O3 is involved in a hydrogen bond with water (see Table 2) and O2 is not. In contrast, O2 has many more contacts with sodium ions (see Table 1) as it is orientated into the NaO core.

Related literature

For related literature, see: Albertsson et al. (1973); Archer et al. (1996); Evans et al. (2001); Kennard et al. (1982); Leiserowitz (1976); Mak et al. (1985); Prout et al. (1968, 1971); Smith et al. (1980).

S2. Experimental

A sample of powdered sodium hydrogen bis(phenoxyacetate) hydrate (Sigma–Aldrich) was dissolved in Edinburgh tap water and single crystals of sodium phenoxyacetate hemihydrate were obtained by slow evaporation of the aqueous solution.

S3. Refinement

The water H atoms were located in the difference map and their coordinates were freely refined, but their displacement parameters were treated as riding on oxygen (1.2U_eq of the bound atom). H atoms bonded to C atoms were treated as riding, with C—H distances in the range 0.95–0.99 Å.

Computing details

Data collection: XSCANS (Siemens, 1996); cell refinement: XSCANS; data reduction: SHELXTL (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

Fig. 1. The molecular structure of the title compound viewed down the b axis. The unit cell is superimposed, one-fifth of the original size. Displacement ellipsoids are shown at the 60% probability level. [Symmetry codes: (A) 1 - x, -y, -z; (B) 1 - x, y, 0.5 - z; (C) x, -y, z - 0.5; (D) x, y, z - 1.]

Sodium phenoxyacetate hemihydrate

Crystal data

Na^+·C8H7O3^−·0.5H2O	F(000) = 760
Mr = 183.13	Dx = 1.530 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 34.149 (11) Å	Cell parameters from 32 reflections
b = 7.654 (3) Å	θ = 5.3–12.3°
c = 6.144 (2) Å	µ = 0.16 mm^−1
β = 98.02 (3)°	T = 160 K
V = 1590.2 (10) Å^3	Needle, colourless
Z = 8	0.74 × 0.26 × 0.05 mm

Data collection

Bruker P4 diffractometer	1127 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.069
Graphite monochromator	θmax = 25.0°, θmin = 2.4°
ω scans	h = −40→1
Absorption correction: ψ scan (North et al., 1968)	k = −9→1
Tmin = 0.719, Tmax = 0.792	l = −7→7
1719 measured reflections	3 standard reflections every 97 reflections
1400 independent reflections	intensity decay: none

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)] = 0.052	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.139	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0816P)^2 + 1.4634P] where P = (Fo^2 + 2Fc^2)/3
1400 reflections	(Δ/σ)max < 0.001
117 parameters	Δρmax = 0.36 e Å^−3
0 restraints	Δρmin = −0.49 e Å^−3

Special details

Experimental. Collimator size 1.5 mm

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
Na1	0.46847 (3)	0.17124 (13)	0.03367 (15)	0.0158 (3)
O1	0.40157 (5)	0.2474 (2)	0.1427 (3)	0.0191 (5)
O2	0.46095 (5)	0.1196 (2)	0.4207 (3)	0.0176 (4)
O3	0.44708 (5)	0.2918 (3)	0.6954 (3)	0.0193 (5)
O4	0.5000	0.4255 (4)	0.2500	0.0225 (6)
H4	0.5200 (9)	0.492 (5)	0.247 (5)	0.027*
C2	0.44134 (7)	0.2340 (3)	0.5031 (4)	0.0142 (5)
C3	0.40562 (7)	0.3153 (3)	0.3616 (4)	0.0165 (6)
H3A	0.3814	0.2896	0.4273	0.020*
H3B	0.4089	0.4437	0.3578	0.020*
C10	0.36439 (7)	0.2537 (3)	0.0232 (4)	0.0149 (5)
C11	0.35837 (8)	0.1527 (3)	−0.1669 (4)	0.0192 (6)
H11A	0.3796	0.0877	−0.2116	0.023*
C12	0.32123 (8)	0.1475 (4)	−0.2904 (4)	0.0239 (6)
H12A	0.3171	0.0784	−0.4199	0.029*
C13	0.29003 (8)	0.2417 (4)	−0.2277 (4)	0.0244 (6)
H13A	0.2645	0.2352	−0.3115	0.029*
C14	0.29651 (8)	0.3456 (4)	−0.0413 (4)	0.0225 (6)
H14A	0.2753	0.4118	0.0014	0.027*
C15	0.33357 (8)	0.3539 (4)	0.0838 (4)	0.0194 (6)
H15A	0.3379	0.4273	0.2095	0.023*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na1	0.0206 (6)	0.0161 (6)	0.0109 (5)	0.0006 (4)	0.0027 (4)	−0.0010 (4)
O1	0.0202 (9)	0.0267 (10)	0.0107 (9)	0.0048 (8)	0.0037 (7)	−0.0033 (8)
O2	0.0210 (9)	0.0157 (9)	0.0170 (9)	0.0031 (7)	0.0062 (7)	0.0026 (7)
O3	0.0238 (10)	0.0225 (10)	0.0114 (9)	−0.0012 (8)	0.0016 (7)	0.0004 (7)
O4	0.0324 (15)	0.0138 (14)	0.0232 (14)	0.000	0.0107 (12)	0.000
C2	0.0162 (12)	0.0127 (12)	0.0147 (12)	−0.0044 (10)	0.0060 (10)	0.0021 (10)
C3	0.0211 (13)	0.0178 (13)	0.0114 (12)	0.0019 (10)	0.0047 (10)	−0.0009 (10)
C10	0.0186 (12)	0.0150 (12)	0.0115 (12)	−0.0019 (10)	0.0035 (9)	0.0036 (10)
C11	0.0261 (14)	0.0184 (13)	0.0142 (12)	0.0009 (11)	0.0067 (10)	0.0002 (11)
C12	0.0325 (15)	0.0264 (15)	0.0128 (13)	−0.0080 (12)	0.0029 (11)	−0.0013 (11)
C13	0.0206 (13)	0.0323 (16)	0.0194 (14)	−0.0057 (12)	−0.0003 (11)	0.0037 (12)
C14	0.0214 (13)	0.0254 (15)	0.0218 (14)	0.0026 (11)	0.0067 (11)	0.0036 (12)
C15	0.0228 (13)	0.0215 (14)	0.0139 (12)	0.0012 (11)	0.0033 (10)	0.0002 (11)

Geometric parameters (Å, º)

Na1—O3^i	2.298 (2)	C2—C3	1.528 (3)
Na1—O2^ii	2.335 (2)	C3—H3A	0.9900
Na1—O2^iii	2.420 (2)	C3—H3B	0.9900
Na1—O2	2.459 (2)	C10—C11	1.392 (4)
Na1—O4	2.513 (2)	C10—C15	1.394 (4)
Na1—O1	2.537 (2)	C11—C12	1.385 (4)
Na1—Na1^iii	3.179 (2)	C11—H11A	0.9500
Na1—Na1^iv	3.454 (2)	C12—C13	1.385 (4)
Na1—Na1^ii	4.0384 (16)	C12—H12A	0.9500
O1—C10	1.376 (3)	C13—C14	1.387 (4)
O1—C3	1.431 (3)	C13—H13A	0.9500
O2—C2	1.251 (3)	C14—C15	1.388 (4)
O3—C2	1.251 (3)	C14—H14A	0.9500
O4—H4	0.85 (3)	C15—H15A	0.9500
O3^i—Na1—O2^ii	96.08 (7)	O3—C2—O2	127.4 (2)
O3^i—Na1—O2^iii	110.87 (7)	O3—C2—C3	113.6 (2)
O2^ii—Na1—O2^iii	86.86 (7)	O2—C2—C3	119.0 (2)
O3^i—Na1—O2	151.19 (7)	O1—C3—C2	110.6 (2)
O2^ii—Na1—O2	96.45 (6)	O1—C3—H3A	109.5
O2^iii—Na1—O2	95.65 (7)	C2—C3—H3A	109.5
O3^i—Na1—O4	103.07 (7)	O1—C3—H3B	109.5
O2^ii—Na1—O4	156.46 (7)	C2—C3—H3B	109.5
O2^iii—Na1—O4	73.67 (6)	H3A—C3—H3B	108.1
O2—Na1—O4	73.01 (6)	O1—C10—C11	116.4 (2)
O3^i—Na1—O1	87.83 (7)	O1—C10—C15	123.5 (2)
O2^ii—Na1—O1	103.40 (7)	C11—C10—C15	120.1 (2)
O2^iii—Na1—O1	157.84 (7)	C12—C11—C10	119.5 (2)
O2—Na1—O1	64.04 (6)	C12—C11—H11A	120.3
O4—Na1—O1	91.01 (6)	C10—C11—H11A	120.3
C10—O1—C3	116.72 (18)	C11—C12—C13	121.0 (3)
C10—O1—Na1	131.72 (14)	C11—C12—H12A	119.5
C3—O1—Na1	111.06 (14)	C13—C12—H12A	119.5
C2—O2—Na1^v	126.13 (15)	C12—C13—C14	119.2 (3)
C2—O2—Na1^iii	114.81 (16)	C12—C13—H13A	120.4
Na1^v—O2—Na1^iii	93.14 (7)	C14—C13—H13A	120.4
C2—O2—Na1	114.37 (15)	C13—C14—C15	120.8 (3)
Na1^v—O2—Na1	114.75 (8)	C13—C14—H14A	119.6
Na1^iii—O2—Na1	81.32 (7)	C15—C14—H14A	119.6
C2—O3—Na1^vi	134.45 (17)	C14—C15—C10	119.4 (2)
Na1—O4—Na1^iii	78.48 (10)	C14—C15—H15A	120.3
Na1—O4—H4	137 (2)	C10—C15—H15A	120.3
Na1^iii—O4—H4	101 (2)
O3^i—Na1—O1—C10	33.2 (2)	O4—Na1—O2—Na1^iii	−52.29 (5)
O2^ii—Na1—O1—C10	−62.6 (2)	O1—Na1—O2—Na1^iii	−152.04 (7)
O2^iii—Na1—O1—C10	−178.4 (2)	Na1^iv—Na1—O2—Na1^iii	61.42 (7)
O2—Na1—O1—C10	−153.3 (2)	Na1^ii—Na1—O2—Na1^iii	104.16 (5)
O4—Na1—O1—C10	136.2 (2)	Na1^v—Na1—O2—Na1^iii	89.47 (8)
Na1^iii—Na1—O1—C10	−175.07 (19)	O3^i—Na1—O4—Na1^iii	−159.36 (7)
Na1^iv—Na1—O1—C10	−86.3 (2)	O2^ii—Na1—O4—Na1^iii	−15.73 (13)
Na1^ii—Na1—O1—C10	−29.5 (2)	O2^iii—Na1—O4—Na1^iii	−51.12 (5)
Na1^v—Na1—O1—C10	−125.5 (2)	O2—Na1—O4—Na1^iii	50.23 (5)
O3^i—Na1—O1—C3	−138.19 (16)	O1—Na1—O4—Na1^iii	112.64 (6)
O2^ii—Na1—O1—C3	126.07 (16)	Na1^iv—Na1—O4—Na1^iii	−41.18 (4)
O2^iii—Na1—O1—C3	10.2 (3)	Na1^ii—Na1—O4—Na1^iii	−92.96 (10)
O2—Na1—O1—C3	35.36 (15)	Na1^v—Na1—O4—Na1^iii	31.05 (3)
O4—Na1—O1—C3	−35.15 (16)	Na1^vi—O3—C2—O2	−35.7 (4)
Na1^iii—Na1—O1—C3	13.59 (16)	Na1^vi—O3—C2—C3	143.41 (18)
Na1^iv—Na1—O1—C3	102.40 (18)	Na1^v—O2—C2—O3	62.3 (3)
Na1^ii—Na1—O1—C3	159.13 (14)	Na1^iii—O2—C2—O3	−52.0 (3)
Na1^v—Na1—O1—C3	63.18 (15)	Na1—O2—C2—O3	−143.5 (2)
O3^i—Na1—O2—C2	−25.2 (3)	Na1^v—O2—C2—C3	−116.7 (2)
O2^ii—Na1—O2—C2	−140.46 (14)	Na1^iii—O2—C2—C3	128.97 (19)
O2^iii—Na1—O2—C2	132.08 (16)	Na1—O2—C2—C3	37.4 (3)
O4—Na1—O2—C2	61.08 (16)	C10—O1—C3—C2	156.0 (2)
O1—Na1—O2—C2	−38.66 (16)	Na1—O1—C3—C2	−31.3 (2)
Na1^iii—Na1—O2—C2	113.38 (18)	O3—C2—C3—O1	178.2 (2)
Na1^iv—Na1—O2—C2	174.80 (16)	O2—C2—C3—O1	−2.6 (3)
Na1^ii—Na1—O2—C2	−142.46 (15)	C3—O1—C10—C11	−165.4 (2)
Na1^v—Na1—O2—C2	−157.2 (2)	Na1—O1—C10—C11	23.6 (3)
O3^i—Na1—O2—Na1^v	131.95 (14)	C3—O1—C10—C15	14.5 (3)
O2^ii—Na1—O2—Na1^v	16.70 (13)	Na1—O1—C10—C15	−156.49 (19)
O2^iii—Na1—O2—Na1^v	−70.77 (9)	O1—C10—C11—C12	177.5 (2)
O4—Na1—O2—Na1^v	−141.76 (9)	C15—C10—C11—C12	−2.4 (4)
O1—Na1—O2—Na1^v	118.50 (10)	C10—C11—C12—C13	0.1 (4)
Na1^iii—Na1—O2—Na1^v	−89.47 (8)	C11—C12—C13—C14	1.5 (4)
Na1^iv—Na1—O2—Na1^v	−28.04 (10)	C12—C13—C14—C15	−0.9 (4)
Na1^ii—Na1—O2—Na1^v	14.70 (11)	C13—C14—C15—C10	−1.3 (4)
O3^i—Na1—O2—Na1^iii	−138.58 (13)	O1—C10—C15—C14	−176.9 (2)
O2^ii—Na1—O2—Na1^iii	106.17 (7)	C11—C10—C15—C14	3.0 (4)
O2^iii—Na1—O2—Na1^iii	18.70 (9)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4···O3^vii	0.85 (3)	2.00 (3)	2.809 (3)	157 (3)

Symmetry code: (vii) −x+1, −y+1, −z+1.