The solid dihydrate of N,N-di­methyl-n-tetra­decyl­amine oxide

Crystalline N,N-di­methyl-n-tetra­decyl­amine oxide has been prepared by reaction of liquid N,N-di­methyl-n-tetra­decyl­amine with 70% H₂O₂ in the presence of CO₂ as catalyst. The resulting soft low-melting solid was crystallized as the dihydrate, viz. C₁₆H₃₅NO·2H₂O. The extended hydro­carbon chains pack in a parallel fashion, with the N-oxide ends of the mol­ecules forming hydrogen bonds with the water mol­ecules in hydro­philic layers. The N-O distance is 1.411 (3) Å.

S1. Comment

There is a paucity of structural data in the literature on fatty amine oxides. They are commercially important surfactants because of their extensive use in detergents (Maisonneuve, 1991) and surfactant compositions. As a class, these compounds have been both known and commercially available since the mid 1960 s (Devinsky, 1986). End uses for these materials generally vary with respect to the length of the included alkyl chain, with `light' chains (C<sub>8</sub> and C<sub>10</sub>) being especially effective in hard surface degreasers and specialty cleaners (Miller et al., 1995), `medium' chains (C₁₂ and C₁₄) finding broad usage in light-duty detergents and dishwash formulations (Edward, 1963), and `heavy' chains (C₁₆, C₁₈ and higher) having useful properties in fiber treatments, such as fabric softeners, antistatic treatments, and hair-conditioner products (Shapiro, 1970).

Generally, these tertiary amine oxides are manufactured, supplied, and formulated as aqueous solutions. A major departure from this trend was developed by workers at Ethyl Corporation, when they were able to produce a family of stable easy-to-handle liquid and solid compositions with a broad spectrum of tertiary amine oxides dissolved in non-aqueous `solvents' or co-surfactants (Sauer et al., 1991; Hughes et al., 1992). Additional work, at least with several of the medium and heavy alkyl-chain systems, also demonstrated the possibility for the production of essentially solvent-free solid non-hygroscopic powdered forms of these amine oxide surfactants (Smith et al., 1991). Based upon elemental analysis and indirect spectral evidence, it was postulated by these workers that the actual form of these `solid' materials approximated that of an alkyldimethylamine oxide dihydrate. Extensive characterization of these amorphous powders provided much useful data for the formulator, such as melting point, critical micelle concentration (CMC), hydrophile–lipophile balance data (HLB), and solubility data. To date, however, no direct evidence for the exact physical and chemical structure of these solvent-free solid species has been available. We have determined the structure of the title compound, N,N-dimethyl-n-tetradecylamine oxide dihydrate, (II), to better characterize this family of compounds, to provide a more exact understanding of the nature of one of its members, and, at least inferentially, to shed light on the probable structures of other homologues in this family.

The title compound, (II), was prepared by reaction of the starting liquid amine, (I), with 70% H₂O₂ in the presence of CO₂ catalyst (see reaction Scheme below). By using 70% H₂O₂, the 30% available water is apparently sufficient for formation of the solid dihydrate as thin fragile plates. It is fortuitous that the dihydrate is crystalline, as N-alkyl-N,N-dimethylamine oxides are notoriously difficult to crystallize. None are present in the Cambridge Structural Database (CSD; Allen, 2002), except for trimethylamine oxide (Caron et al., 1964). The structure of N,N-dimethylethanolamine N-oxide (Maia et al., 1984) has also been reported. Ammonium salts of N-alkyl-N,N-dimethylamines are common in the CSD, with ten entries present for alkyl = n-tetradecyl.

The asymmetric unit for (II) is shown in Fig. 1. The alkyl group is extended and slightly bowed, with atoms C2, C13 and C14 lying 0.084 (3), 0.071 (3) and 0.064 (3) Å, respectively, to one side of the best plane of C2–C14, while atoms C7 and C8 lie 0.046 (4) and 0.058 (4) Å to the opposite side. The C—C distances of the hydrocarbon chain vary in the range 1.512 (4)–1.526 (4) Å, with a mean value of 1.5180 (11) Å. Most intrachain bond angles of the hydrocarbon chain fall within the narrow range 113.7 (3)–114.8 (3)°, with N1—C1—C3 slightly larger and C1—C2—C3 slightly smaller. The N—O distance compares with a value of 1.404 Å (corrected for libration) in trimethylamine oxide (Caron et al., 1964) and a value of 1.399 Å (mean of three) in N,N-dimethylethanolamine N-oxide (Maia et al., 1984).

The unit cell is shown in Fig. 2, which illustrates the hydrogen-bonded regions near x = 0, where the water molecules associate with the head-to-head hydrophilic ends of the fatty amine N-oxide molecules. In the interior of the cell, the parallel hydrocarbon chains interdigitate along the [010] direction. The hydrophilic region is shown in Fig. 3. Each water molecule donates two hydrogen bonds and accepts one, linking the N-oxide groups into six-oxygen centrosymmetric rings. These rings are further linked by H₂O···H₂O hydrogen bonds to form two-dimensional arrays.

S2. Experimental

The title compound was prepared by reacting liquid tetradecyldimethylamine with 70% hydrogen peroxide in the presence of CO₂ catalyst. A blanket of CO₂ was maintained throughout the addition of the peroxide to the liquid amine. The amine oxidation process is strongly exothermic and the reaction temperature was regulated by the rate of peroxide addition (Elnagar, 2000). The resulting soft solid (m.p. 317 K) was crystallized from methyl ethyl ketone.

S3. Refinement

H atoms on C atoms were placed in calculated positions, with C—H distances in the range 0.98–0.99 Å, and thereafter treated as riding. A torsional parameter was refined for each methyl group. Water H atoms were located in difference maps, and their coordinates were refined (is this correct?), the O—H distance constrained. For H atoms, U_iso = 1.2U_eq of the attached atom (1.5 for methyl and water H atoms).

Computing details

Data collection: COLLECT (Nonius, 2000); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. View of the title compound with displacement ellipsoids at the 50% probability level.
	Fig. 2. The unit cell in the title compound. Only water H atoms are shown.
	Fig. 3. The hydrogen bonding in the title compound. Only the first two C atoms of the C14 chain are illustrated.

N,N-dimethyl-n-tetradecylamine oxide

Crystal data

C16H35NO·2H2O	F(000) = 664
Mr = 293.48	Dx = 1.056 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 3471 reflections
a = 22.782 (8) Å	θ = 2.5–26.0°
b = 8.110 (5) Å	µ = 0.07 mm^−1
c = 9.995 (5) Å	T = 100 K
β = 91.19 (2)°	Plate, colorless
V = 1846.3 (16) Å^3	0.48 × 0.17 × 0.03 mm
Z = 4

Data collection

Nonius KappaCCD (with Oxford Cryostream cooler) diffractometer	1746 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.076
Graphite monochromator	θmax = 26.0°, θmin = 2.6°
ω scans with κ offsets	h = −28→28
15919 measured reflections	k = −10→10
3508 independent reflections	l = −11→11

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)] = 0.086	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.195	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.053P)^2 + 1.8197P] where P = (Fo^2 + 2Fc^2)/3
3508 reflections	(Δ/σ)max = 0.004
196 parameters	Δρmax = 0.32 e Å^−3
4 restraints	Δρmin = −0.20 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
O1	0.91337 (10)	0.6352 (3)	0.16990 (19)	0.0342 (6)
N1	0.89780 (12)	0.6766 (3)	0.3016 (2)	0.0285 (7)
C1	0.83330 (14)	0.6461 (5)	0.3101 (3)	0.0319 (9)
H1A	0.8252	0.5332	0.2769	0.038*
H1B	0.8129	0.7236	0.2484	0.038*
C2	0.80611 (14)	0.6631 (5)	0.4477 (3)	0.0339 (9)
H2A	0.8209	0.5744	0.5075	0.041*
H2B	0.8173	0.7705	0.4878	0.041*
C3	0.73959 (14)	0.6518 (5)	0.4337 (3)	0.0344 (9)
H3A	0.7255	0.7439	0.3765	0.041*
H3B	0.7294	0.5477	0.3871	0.041*
C4	0.70723 (14)	0.6574 (5)	0.5651 (3)	0.0328 (9)
H4A	0.7208	0.5644	0.6220	0.039*
H4B	0.7176	0.7610	0.6124	0.039*
C5	0.64094 (14)	0.6479 (5)	0.5482 (3)	0.0336 (9)
H5A	0.6308	0.5453	0.4992	0.040*
H5B	0.6275	0.7419	0.4922	0.040*
C6	0.60755 (14)	0.6505 (5)	0.6782 (3)	0.0323 (9)
H6A	0.6200	0.5546	0.7331	0.039*
H6B	0.6184	0.7515	0.7284	0.039*
C7	0.54145 (14)	0.6458 (5)	0.6594 (3)	0.0335 (9)
H7A	0.5307	0.5454	0.6082	0.040*
H7B	0.5291	0.7422	0.6049	0.040*
C8	0.50745 (14)	0.6468 (5)	0.7886 (3)	0.0332 (9)
H8A	0.5189	0.5485	0.8417	0.040*
H8B	0.5191	0.7454	0.8411	0.040*
C9	0.44146 (14)	0.6474 (5)	0.7696 (3)	0.0344 (9)
H9A	0.4299	0.5492	0.7166	0.041*
H9B	0.4301	0.7461	0.7169	0.041*
C10	0.40740 (14)	0.6475 (5)	0.8981 (3)	0.0329 (9)
H10A	0.4197	0.7443	0.9521	0.040*
H10B	0.4181	0.5474	0.9498	0.040*
C11	0.34122 (14)	0.6522 (5)	0.8792 (3)	0.0334 (9)
H11A	0.3306	0.7524	0.8275	0.040*
H11B	0.3289	0.5555	0.8251	0.040*
C12	0.30721 (14)	0.6521 (5)	1.0078 (3)	0.0327 (9)
H12A	0.3203	0.7472	1.0629	0.039*
H12B	0.3171	0.5505	1.0584	0.039*
C13	0.24111 (14)	0.6610 (5)	0.9887 (3)	0.0351 (9)
H13A	0.2279	0.5653	0.9345	0.042*
H13B	0.2312	0.7620	0.9374	0.042*
C14	0.20739 (15)	0.6627 (5)	1.1185 (3)	0.0387 (10)
H14A	0.2147	0.5596	1.1673	0.058*
H14B	0.1653	0.6732	1.0982	0.058*
H14C	0.2204	0.7562	1.1736	0.058*
C15	0.93178 (15)	0.5730 (5)	0.3988 (3)	0.0357 (9)
H15A	0.9215	0.4568	0.3854	0.054*
H15B	0.9223	0.6060	0.4902	0.054*
H15C	0.9739	0.5882	0.3849	0.054*
C16	0.91250 (15)	0.8520 (4)	0.3237 (3)	0.0338 (9)
H16A	0.9548	0.8683	0.3129	0.051*
H16B	0.9017	0.8841	0.4145	0.051*
H16C	0.8908	0.9201	0.2585	0.051*
O1W	1.03069 (11)	0.6873 (3)	0.1416 (2)	0.0406 (7)
H1W	0.9922	0.674	0.146	0.061*
H2W	1.0440	0.679	0.0607	0.061*
O2W	0.91913 (12)	0.3044 (4)	0.1151 (2)	0.0466 (8)
H3W	0.9160	0.4099	0.132	0.070*
H4W	0.9203	0.2420	0.187	0.070*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0387 (14)	0.0462 (17)	0.0181 (12)	0.0024 (13)	0.0081 (10)	−0.0031 (11)
N1	0.0320 (17)	0.0320 (18)	0.0217 (15)	0.0022 (14)	0.0031 (12)	−0.0023 (13)
C1	0.031 (2)	0.038 (2)	0.0258 (18)	−0.0057 (18)	−0.0028 (14)	−0.0030 (16)
C2	0.032 (2)	0.050 (3)	0.0203 (17)	−0.0034 (19)	0.0032 (14)	−0.0031 (17)
C3	0.031 (2)	0.049 (3)	0.0227 (18)	−0.0026 (19)	0.0005 (14)	−0.0023 (17)
C4	0.031 (2)	0.042 (2)	0.0244 (18)	0.0005 (19)	0.0008 (14)	−0.0034 (17)
C5	0.029 (2)	0.046 (2)	0.0251 (18)	−0.0029 (19)	−0.0008 (15)	−0.0009 (17)
C6	0.032 (2)	0.043 (2)	0.0223 (17)	0.0004 (19)	0.0011 (14)	0.0014 (17)
C7	0.030 (2)	0.045 (2)	0.0256 (18)	−0.0023 (19)	−0.0003 (14)	−0.0043 (17)
C8	0.031 (2)	0.045 (2)	0.0231 (18)	0.0031 (19)	0.0016 (14)	0.0004 (17)
C9	0.033 (2)	0.045 (3)	0.0249 (18)	−0.0034 (19)	−0.0002 (15)	0.0004 (18)
C10	0.031 (2)	0.045 (2)	0.0224 (18)	−0.0010 (18)	0.0003 (14)	−0.0005 (17)
C11	0.029 (2)	0.044 (2)	0.0272 (18)	−0.0049 (19)	0.0010 (14)	−0.0007 (17)
C12	0.032 (2)	0.043 (2)	0.0231 (18)	−0.0007 (19)	0.0013 (14)	0.0007 (17)
C13	0.031 (2)	0.045 (2)	0.0289 (19)	−0.0029 (19)	−0.0030 (15)	0.0016 (18)
C14	0.036 (2)	0.050 (3)	0.0304 (19)	−0.001 (2)	0.0037 (16)	0.0013 (19)
C15	0.039 (2)	0.041 (2)	0.0274 (19)	0.0105 (19)	−0.0001 (16)	0.0006 (17)
C16	0.036 (2)	0.036 (2)	0.0291 (19)	−0.0029 (18)	0.0082 (15)	−0.0017 (17)
O1W	0.0383 (15)	0.0506 (17)	0.0330 (14)	−0.0010 (14)	0.0064 (12)	−0.0041 (13)
O2W	0.0541 (18)	0.0551 (19)	0.0307 (14)	0.0100 (16)	−0.0024 (12)	−0.0096 (13)

Geometric parameters (Å, º)

O1—N1	1.411 (3)	C9—C10	1.514 (4)
N1—C16	1.477 (4)	C9—H9A	0.9900
N1—C15	1.489 (4)	C9—H9B	0.9900
N1—C1	1.494 (4)	C10—C11	1.516 (4)
C1—C2	1.526 (4)	C10—H10A	0.9900
C1—H1A	0.9900	C10—H10B	0.9900
C1—H1B	0.9900	C11—C12	1.515 (4)
C2—C3	1.522 (4)	C11—H11A	0.9900
C2—H2A	0.9900	C11—H11B	0.9900
C2—H2B	0.9900	C12—C13	1.516 (4)
C3—C4	1.520 (4)	C12—H12A	0.9900
C3—H3A	0.9900	C12—H12B	0.9900
C3—H3B	0.9900	C13—C14	1.522 (4)
C4—C5	1.518 (4)	C13—H13A	0.9900
C4—H4A	0.9900	C13—H13B	0.9900
C4—H4B	0.9900	C14—H14A	0.9800
C5—C6	1.519 (4)	C14—H14B	0.9800
C5—H5A	0.9900	C14—H14C	0.9800
C5—H5B	0.9900	C15—H15A	0.9800
C6—C7	1.514 (4)	C15—H15B	0.9800
C6—H6A	0.9900	C15—H15C	0.9800
C6—H6B	0.9900	C16—H16A	0.9800
C7—C8	1.520 (4)	C16—H16B	0.9800
C7—H7A	0.9900	C16—H16C	0.9800
C7—H7B	0.9900	O1W—H1W	0.88
C8—C9	1.512 (4)	O1W—H2W	0.87
C8—H8A	0.9900	O2W—H3W	0.88
C8—H8B	0.9900	O2W—H4W	0.88
O1—N1—C16	108.0 (2)	C7—C8—H8B	108.6
O1—N1—C15	109.7 (2)	H8A—C8—H8B	107.6
C16—N1—C15	109.4 (3)	C8—C9—C10	114.8 (3)
O1—N1—C1	106.3 (2)	C8—C9—H9A	108.6
C16—N1—C1	111.8 (3)	C10—C9—H9A	108.6
C15—N1—C1	111.6 (3)	C8—C9—H9B	108.6
N1—C1—C2	117.0 (2)	C10—C9—H9B	108.6
N1—C1—H1A	108.0	H9A—C9—H9B	107.5
C2—C1—H1A	108.0	C9—C10—C11	114.8 (3)
N1—C1—H1B	108.0	C9—C10—H10A	108.6
C2—C1—H1B	108.0	C11—C10—H10A	108.6
H1A—C1—H1B	107.3	C9—C10—H10B	108.6
C3—C2—C1	109.5 (2)	C11—C10—H10B	108.6
C3—C2—H2A	109.8	H10A—C10—H10B	107.5
C1—C2—H2A	109.8	C12—C11—C10	114.8 (3)
C3—C2—H2B	109.8	C12—C11—H11A	108.6
C1—C2—H2B	109.8	C10—C11—H11A	108.6
H2A—C2—H2B	108.2	C12—C11—H11B	108.6
C4—C3—C2	114.7 (3)	C10—C11—H11B	108.6
C4—C3—H3A	108.6	H11A—C11—H11B	107.6
C2—C3—H3A	108.6	C11—C12—C13	114.7 (3)
C4—C3—H3B	108.6	C11—C12—H12A	108.6
C2—C3—H3B	108.6	C13—C12—H12A	108.6
H3A—C3—H3B	107.6	C11—C12—H12B	108.6
C5—C4—C3	113.7 (3)	C13—C12—H12B	108.6
C5—C4—H4A	108.8	H12A—C12—H12B	107.6
C3—C4—H4A	108.8	C12—C13—C14	114.2 (3)
C5—C4—H4B	108.8	C12—C13—H13A	108.7
C3—C4—H4B	108.8	C14—C13—H13A	108.7
H4A—C4—H4B	107.7	C12—C13—H13B	108.7
C4—C5—C6	114.7 (3)	C14—C13—H13B	108.7
C4—C5—H5A	108.6	H13A—C13—H13B	107.6
C6—C5—H5A	108.6	C13—C14—H14A	109.5
C4—C5—H5B	108.6	C13—C14—H14B	109.5
C6—C5—H5B	108.6	H14A—C14—H14B	109.5
H5A—C5—H5B	107.6	C13—C14—H14C	109.5
C7—C6—C5	114.1 (3)	H14A—C14—H14C	109.5
C7—C6—H6A	108.7	H14B—C14—H14C	109.5
C5—C6—H6A	108.7	N1—C15—H15A	109.5
C7—C6—H6B	108.7	N1—C15—H15B	109.5
C5—C6—H6B	108.7	H15A—C15—H15B	109.5
H6A—C6—H6B	107.6	N1—C15—H15C	109.5
C6—C7—C8	114.7 (2)	H15A—C15—H15C	109.5
C6—C7—H7A	108.6	H15B—C15—H15C	109.5
C8—C7—H7A	108.6	N1—C16—H16A	109.5
C6—C7—H7B	108.6	N1—C16—H16B	109.5
C8—C7—H7B	108.6	H16A—C16—H16B	109.5
H7A—C7—H7B	107.6	N1—C16—H16C	109.5
C9—C8—C7	114.6 (2)	H16A—C16—H16C	109.5
C9—C8—H8A	108.6	H16B—C16—H16C	109.5
C7—C8—H8A	108.6	H1W—O1W—H2W	113
C9—C8—H8B	108.6	H3W—O2W—H4W	114
O1—N1—C1—C2	173.4 (3)	C5—C6—C7—C8	179.5 (3)
C16—N1—C1—C2	−69.1 (4)	C6—C7—C8—C9	178.2 (3)
C15—N1—C1—C2	53.8 (4)	C7—C8—C9—C10	179.7 (3)
N1—C1—C2—C3	172.1 (3)	C8—C9—C10—C11	178.6 (3)
C1—C2—C3—C4	176.7 (3)	C9—C10—C11—C12	179.9 (3)
C2—C3—C4—C5	179.3 (3)	C10—C11—C12—C13	178.6 (3)
C3—C4—C5—C6	179.1 (3)	C11—C12—C13—C14	−179.4 (3)
C4—C5—C6—C7	178.3 (3)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H1W···O1	0.88	1.85	2.727 (3)	174
O1W—H2W···O2W^i	0.87	1.97	2.832 (4)	170
O2W—H3W···O1	0.88	1.87	2.741 (4)	177
O2W—H4W···O1W^ii	0.88	2.07	2.829 (4)	144

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