organic compounds
4-Hydroxy-2,2,6,6-tetramethylpiperidinium trifluoroacetate
aSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People's Republic of China, bSchool of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, People's Republic of China, and cSchool of Materials Science and Engineering, Shijizhuang Railway Institute, Shijiazhuang 050043, People's Republic of China
*Correspondence e-mail: dengyicoo@yahoo.com.cn
The title compound, C9H20NO+·C2F3O2−, is an important intermediate in the synthesis of hindered light stabilizers. The piperidinium ring adopts a chair conformation with the hydroxyl group in an equatorial position. The crystal packing is stabilized by O—H⋯O and N—H⋯O hydrogen bonds. The CF3 group is disordered over two positions with almost equal site occupancy factors.
Related literature
For general background, see: Borzatta & Carrozza (1991). For related structures, see: Nengfang et al. (2005).
Experimental
Crystal data
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Refinement
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Data collection: CrystalClear (Rigaku/MSC, 2005); cell CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1997); software used to prepare material for publication: CrystalStructure (Rigaku/MSC, 2005).
Supporting information
https://doi.org/10.1107/S1600536807066305/bt2664sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536807066305/bt2664Isup2.hkl
An ethanol solution (10 ml) of 2,2,6,6-tetramethylpiperidin-4-ol (3.2 mmol, 0.5 g) was added dropwise to a stirred aqueous solution (6 ml) of trifluoroacetic acid (3.8 mmol, 0.43 g) at 293 K. Then the reaction mixture was filtered and the filtrate stood for about five days until colourless needle shaped crystals were obtained.
In the absence of anomalous scatterers, Friedel pairs had been merged and the
was arbitrarily assigned. All H atoms were positioned geometrically with C—H ranging from 0.98Å to 1.00Å and refined as riding with Uiso(H)=1.2Ueq(C,N,O) or 1.5eq(Cmethyl). The CF3 group is disordered over two position with a ratio of occupancy factors of 0.459 (1)/0.541 (1). The atoms of the CF3 group were restrained to an isotropic behaviour.4-hydroxyl-2,2,6,6-tetramethylpiperidine is a very important intermediate in the synthesis of hindered light stabilizers (Borzatta & Carrozza, 1991; She et al., 2005). The piperidium ring adopts a chair conformation with the hydroxyl group in an equatorial position. The crystal packing is stabilized by O—H···O and N—H···O hydrogen bonds.
For general background, see: Borzatta & Carrozza (1991). For related structures, see: Nengfang et al. (2005).
Data collection: CrystalClear (Rigaku/MSC, 2005); cell
CrystalClear (Rigaku/MSC, 2005); data reduction: CrystalClear (Rigaku/MSC, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1997); software used to prepare material for publication: CrystalStructure (Rigaku/MSC, 2005).C9H20NO+·C2F3O2− | Dx = 1.320 Mg m−3 |
Mr = 271.28 | Mo Kα radiation, λ = 0.71070 Å |
Orthorhombic, P212121 | Cell parameters from 4344 reflections |
a = 7.6204 (8) Å | θ = 2.1–28.7° |
b = 9.8939 (10) Å | µ = 0.12 mm−1 |
c = 18.099 (2) Å | T = 113 K |
V = 1364.6 (2) Å3 | Needle, colourless |
Z = 4 | 0.22 × 0.20 × 0.16 mm |
F(000) = 576 |
Rigaku Saturn diffractometer | 2039 independent reflections |
Radiation source: rotating anode | 1993 reflections with I > 2σ(I) |
Confocal monochromator | Rint = 0.057 |
Detector resolution: 14.63 pixels mm-1 | θmax = 28.7°, θmin = 2.3° |
ω scans | h = −10→10 |
Absorption correction: multi-scan (CrystalClear; Rigaku/MSC, 2005) | k = −13→13 |
Tmin = 0.974, Tmax = 0.981 | l = −24→24 |
17989 measured reflections |
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)] = 0.057 | H-atom parameters constrained |
wR(F2) = 0.132 | w = 1/[σ2(Fo2) + (0.0506P)2 + 0.4132P] where P = (Fo2 + 2Fc2)/3 |
S = 1.23 | (Δ/σ)max = 0.001 |
2039 reflections | Δρmax = 0.22 e Å−3 |
202 parameters | Δρmin = −0.21 e Å−3 |
48 restraints | Extinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.028 (4) |
C9H20NO+·C2F3O2− | V = 1364.6 (2) Å3 |
Mr = 271.28 | Z = 4 |
Orthorhombic, P212121 | Mo Kα radiation |
a = 7.6204 (8) Å | µ = 0.12 mm−1 |
b = 9.8939 (10) Å | T = 113 K |
c = 18.099 (2) Å | 0.22 × 0.20 × 0.16 mm |
Rigaku Saturn diffractometer | 2039 independent reflections |
Absorption correction: multi-scan (CrystalClear; Rigaku/MSC, 2005) | 1993 reflections with I > 2σ(I) |
Tmin = 0.974, Tmax = 0.981 | Rint = 0.057 |
17989 measured reflections |
R[F2 > 2σ(F2)] = 0.057 | 48 restraints |
wR(F2) = 0.132 | H-atom parameters constrained |
S = 1.23 | Δρmax = 0.22 e Å−3 |
2039 reflections | Δρmin = −0.21 e Å−3 |
202 parameters |
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. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
O1 | 0.9028 (3) | −0.20835 (18) | 0.73102 (11) | 0.0271 (5) | |
H1 | 0.8683 | −0.2769 | 0.7078 | 0.041* | |
O2 | 0.6874 (3) | 0.4099 (2) | 0.58896 (13) | 0.0394 (6) | |
O3 | 0.7885 (3) | 0.5590 (2) | 0.67201 (12) | 0.0398 (6) | |
N1 | 0.8175 (3) | 0.1997 (2) | 0.67551 (12) | 0.0226 (5) | |
H1A | 0.9012 | 0.2378 | 0.7056 | 0.027* | |
H1B | 0.7631 | 0.2695 | 0.6510 | 0.027* | |
C1 | 0.6801 (4) | 0.1339 (3) | 0.72563 (15) | 0.0244 (5) | |
C2 | 0.7571 (4) | 0.0022 (3) | 0.75609 (14) | 0.0241 (5) | |
H2A | 0.8503 | 0.0245 | 0.7922 | 0.029* | |
H2B | 0.6639 | −0.0475 | 0.7827 | 0.029* | |
C3 | 0.8336 (4) | −0.0893 (3) | 0.69685 (14) | 0.0233 (5) | |
H3 | 0.7392 | −0.1152 | 0.6612 | 0.028* | |
C4 | 0.9789 (4) | −0.0143 (3) | 0.65571 (15) | 0.0234 (5) | |
H4A | 1.0300 | −0.0753 | 0.6181 | 0.028* | |
H4B | 1.0729 | 0.0095 | 0.6911 | 0.028* | |
C5 | 0.9142 (4) | 0.1151 (3) | 0.61754 (14) | 0.0241 (6) | |
C6 | 0.6475 (4) | 0.2352 (3) | 0.78794 (18) | 0.0326 (7) | |
H6A | 0.7557 | 0.2477 | 0.8164 | 0.049* | |
H6B | 0.5549 | 0.2009 | 0.8205 | 0.049* | |
H6C | 0.6110 | 0.3220 | 0.7668 | 0.049* | |
C7 | 0.5089 (4) | 0.1126 (3) | 0.68236 (19) | 0.0335 (7) | |
H7A | 0.4141 | 0.0899 | 0.7168 | 0.050* | |
H7B | 0.5244 | 0.0387 | 0.6469 | 0.050* | |
H7C | 0.4789 | 0.1958 | 0.6558 | 0.050* | |
C8 | 1.0678 (4) | 0.2025 (3) | 0.59214 (16) | 0.0302 (6) | |
H8A | 1.0229 | 0.2852 | 0.5691 | 0.045* | |
H8B | 1.1384 | 0.1522 | 0.5562 | 0.045* | |
H8C | 1.1408 | 0.2262 | 0.6348 | 0.045* | |
C9 | 0.7963 (4) | 0.0851 (3) | 0.55106 (16) | 0.0331 (7) | |
H9A | 0.7112 | 0.0149 | 0.5643 | 0.050* | |
H9B | 0.8685 | 0.0536 | 0.5097 | 0.050* | |
H9C | 0.7338 | 0.1675 | 0.5366 | 0.050* | |
C10 | 0.7562 (4) | 0.5187 (3) | 0.60881 (16) | 0.0281 (6) | |
C11 | 0.8356 (12) | 0.6096 (9) | 0.5496 (5) | 0.037 (3) | 0.459 (14) |
F1 | 0.7559 (17) | 0.7299 (6) | 0.5473 (4) | 0.067 (3) | 0.459 (14) |
F2 | 1.0045 (10) | 0.6341 (15) | 0.5611 (4) | 0.086 (4) | 0.459 (14) |
F3 | 0.8194 (18) | 0.5655 (13) | 0.4799 (6) | 0.070 (4) | 0.459 (14) |
C11' | 0.7828 (13) | 0.6165 (8) | 0.5453 (4) | 0.042 (3) | 0.541 (14) |
F1' | 0.6352 (15) | 0.6809 (12) | 0.5291 (5) | 0.126 (4) | 0.541 (14) |
F2' | 0.900 (2) | 0.7116 (8) | 0.5599 (3) | 0.099 (5) | 0.541 (14) |
F3' | 0.8406 (14) | 0.5571 (12) | 0.4839 (5) | 0.070 (4) | 0.541 (14) |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0347 (11) | 0.0164 (8) | 0.0303 (10) | 0.0008 (8) | −0.0069 (9) | 0.0017 (8) |
O2 | 0.0504 (13) | 0.0219 (10) | 0.0458 (12) | −0.0013 (10) | −0.0187 (11) | −0.0011 (9) |
O3 | 0.0615 (15) | 0.0274 (10) | 0.0306 (11) | −0.0091 (11) | −0.0059 (11) | −0.0024 (9) |
N1 | 0.0236 (11) | 0.0177 (10) | 0.0266 (10) | −0.0003 (9) | −0.0010 (9) | −0.0003 (9) |
C1 | 0.0212 (12) | 0.0193 (11) | 0.0327 (14) | −0.0006 (10) | 0.0012 (11) | 0.0002 (10) |
C2 | 0.0220 (12) | 0.0239 (12) | 0.0266 (12) | −0.0010 (11) | 0.0014 (10) | 0.0015 (10) |
C3 | 0.0260 (13) | 0.0189 (11) | 0.0251 (12) | 0.0016 (10) | −0.0050 (10) | 0.0015 (10) |
C4 | 0.0242 (12) | 0.0226 (12) | 0.0235 (12) | 0.0017 (11) | −0.0001 (10) | −0.0021 (10) |
C5 | 0.0278 (13) | 0.0202 (12) | 0.0244 (12) | 0.0020 (11) | −0.0004 (11) | −0.0008 (10) |
C6 | 0.0298 (15) | 0.0269 (14) | 0.0412 (16) | 0.0004 (12) | 0.0098 (13) | −0.0047 (13) |
C7 | 0.0242 (13) | 0.0266 (13) | 0.0498 (18) | 0.0009 (11) | −0.0036 (13) | 0.0032 (14) |
C8 | 0.0356 (16) | 0.0263 (14) | 0.0286 (14) | −0.0029 (13) | 0.0046 (12) | 0.0016 (12) |
C9 | 0.0434 (18) | 0.0272 (14) | 0.0287 (14) | 0.0013 (13) | −0.0109 (13) | 0.0016 (11) |
C10 | 0.0292 (13) | 0.0200 (12) | 0.0351 (14) | 0.0032 (11) | −0.0061 (12) | −0.0015 (11) |
C11 | 0.048 (5) | 0.030 (5) | 0.033 (5) | −0.009 (4) | −0.006 (4) | −0.004 (4) |
F1 | 0.132 (8) | 0.020 (3) | 0.049 (4) | 0.017 (4) | 0.014 (4) | 0.011 (2) |
F2 | 0.064 (5) | 0.129 (9) | 0.065 (4) | −0.058 (5) | 0.007 (3) | 0.005 (5) |
F3 | 0.136 (10) | 0.043 (6) | 0.030 (5) | −0.037 (6) | −0.025 (5) | 0.002 (4) |
C11' | 0.063 (6) | 0.037 (4) | 0.026 (4) | 0.000 (4) | −0.010 (3) | −0.001 (3) |
F1' | 0.123 (7) | 0.136 (7) | 0.120 (6) | 0.051 (6) | −0.004 (5) | 0.080 (5) |
F2' | 0.198 (13) | 0.060 (5) | 0.039 (3) | −0.082 (7) | 0.013 (6) | −0.012 (3) |
F3' | 0.105 (7) | 0.063 (7) | 0.042 (5) | −0.025 (5) | 0.042 (4) | −0.024 (4) |
O1—C3 | 1.431 (3) | C6—H6A | 0.9800 |
O1—H1 | 0.8400 | C6—H6B | 0.9800 |
O2—C10 | 1.250 (3) | C6—H6C | 0.9800 |
O3—C10 | 1.236 (3) | C7—H7A | 0.9800 |
N1—C1 | 1.531 (3) | C7—H7B | 0.9800 |
N1—C5 | 1.531 (3) | C7—H7C | 0.9800 |
N1—H1A | 0.9200 | C8—H8A | 0.9800 |
N1—H1B | 0.9200 | C8—H8B | 0.9800 |
C1—C6 | 1.529 (4) | C8—H8C | 0.9800 |
C1—C2 | 1.531 (4) | C9—H9A | 0.9800 |
C1—C7 | 1.537 (4) | C9—H9B | 0.9800 |
C2—C3 | 1.519 (4) | C9—H9C | 0.9800 |
C2—H2A | 0.9900 | C10—C11' | 1.516 (7) |
C2—H2B | 0.9900 | C10—C11 | 1.525 (8) |
C3—C4 | 1.526 (4) | C11—F2 | 1.326 (9) |
C3—H3 | 1.0000 | C11—F1 | 1.337 (8) |
C4—C5 | 1.536 (4) | C11—F3 | 1.339 (8) |
C4—H4A | 0.9900 | C11'—F2' | 1.324 (8) |
C4—H4B | 0.9900 | C11'—F1' | 1.325 (8) |
C5—C8 | 1.527 (4) | C11'—F3' | 1.333 (8) |
C5—C9 | 1.530 (4) | ||
C3—O1—H1 | 109.5 | H6A—C6—H6B | 109.5 |
C1—N1—C5 | 120.2 (2) | C1—C6—H6C | 109.5 |
C1—N1—H1A | 107.3 | H6A—C6—H6C | 109.5 |
C5—N1—H1A | 107.3 | H6B—C6—H6C | 109.5 |
C1—N1—H1B | 107.3 | C1—C7—H7A | 109.5 |
C5—N1—H1B | 107.3 | C1—C7—H7B | 109.5 |
H1A—N1—H1B | 106.9 | H7A—C7—H7B | 109.5 |
C6—C1—N1 | 105.6 (2) | C1—C7—H7C | 109.5 |
C6—C1—C2 | 110.8 (2) | H7A—C7—H7C | 109.5 |
N1—C1—C2 | 108.2 (2) | H7B—C7—H7C | 109.5 |
C6—C1—C7 | 109.1 (2) | C5—C8—H8A | 109.5 |
N1—C1—C7 | 109.7 (2) | C5—C8—H8B | 109.5 |
C2—C1—C7 | 113.1 (2) | H8A—C8—H8B | 109.5 |
C3—C2—C1 | 113.6 (2) | C5—C8—H8C | 109.5 |
C3—C2—H2A | 108.9 | H8A—C8—H8C | 109.5 |
C1—C2—H2A | 108.9 | H8B—C8—H8C | 109.5 |
C3—C2—H2B | 108.9 | C5—C9—H9A | 109.5 |
C1—C2—H2B | 108.9 | C5—C9—H9B | 109.5 |
H2A—C2—H2B | 107.7 | H9A—C9—H9B | 109.5 |
O1—C3—C2 | 109.1 (2) | C5—C9—H9C | 109.5 |
O1—C3—C4 | 110.1 (2) | H9A—C9—H9C | 109.5 |
C2—C3—C4 | 109.5 (2) | H9B—C9—H9C | 109.5 |
O1—C3—H3 | 109.4 | O3—C10—O2 | 128.9 (3) |
C2—C3—H3 | 109.4 | O3—C10—C11' | 117.9 (4) |
C4—C3—H3 | 109.4 | O2—C10—C11' | 112.8 (4) |
C3—C4—C5 | 113.1 (2) | O3—C10—C11 | 112.4 (4) |
C3—C4—H4A | 109.0 | O2—C10—C11 | 118.2 (4) |
C5—C4—H4A | 109.0 | F2—C11—F1 | 106.4 (8) |
C3—C4—H4B | 109.0 | F2—C11—F3 | 107.3 (8) |
C5—C4—H4B | 109.0 | F1—C11—F3 | 102.6 (8) |
H4A—C4—H4B | 107.8 | F2—C11—C10 | 112.5 (7) |
C8—C5—C9 | 108.9 (2) | F1—C11—C10 | 111.5 (7) |
C8—C5—N1 | 105.4 (2) | F3—C11—C10 | 115.7 (9) |
C9—C5—N1 | 111.2 (2) | F2'—C11'—F1' | 106.0 (8) |
C8—C5—C4 | 111.2 (2) | F2'—C11'—F3' | 104.8 (8) |
C9—C5—C4 | 112.4 (2) | F1'—C11'—F3' | 107.9 (8) |
N1—C5—C4 | 107.6 (2) | F2'—C11'—C10 | 113.2 (6) |
C1—C6—H6A | 109.5 | F1'—C11'—C10 | 111.2 (6) |
C1—C6—H6B | 109.5 | F3'—C11'—C10 | 113.3 (8) |
C5—N1—C1—C6 | 165.7 (2) | O2—C10—C11—F2 | −121.8 (8) |
C5—N1—C1—C2 | 47.0 (3) | C11'—C10—C11—F2 | 164 (3) |
C5—N1—C1—C7 | −76.9 (3) | O3—C10—C11—F1 | −69.1 (8) |
C6—C1—C2—C3 | −166.0 (2) | O2—C10—C11—F1 | 118.7 (8) |
N1—C1—C2—C3 | −50.6 (3) | C11'—C10—C11—F1 | 45 (2) |
C7—C1—C2—C3 | 71.2 (3) | O3—C10—C11—F3 | 174.2 (8) |
C1—C2—C3—O1 | 179.6 (2) | O2—C10—C11—F3 | 2.0 (11) |
C1—C2—C3—C4 | 59.1 (3) | C11'—C10—C11—F3 | −72 (2) |
O1—C3—C4—C5 | −180.0 (2) | O3—C10—C11'—F2' | 25.7 (9) |
C2—C3—C4—C5 | −60.1 (3) | O2—C10—C11'—F2' | −160.6 (8) |
C1—N1—C5—C8 | −166.6 (2) | C11—C10—C11'—F2' | −47 (2) |
C1—N1—C5—C9 | 75.6 (3) | O3—C10—C11'—F1' | −93.5 (8) |
C1—N1—C5—C4 | −47.8 (3) | O2—C10—C11'—F1' | 80.2 (9) |
C3—C4—C5—C8 | 167.3 (2) | C11—C10—C11'—F1' | −167 (3) |
C3—C4—C5—C9 | −70.4 (3) | O3—C10—C11'—F3' | 144.8 (8) |
C3—C4—C5—N1 | 52.4 (3) | O2—C10—C11'—F3' | −41.5 (10) |
O3—C10—C11—F2 | 50.4 (9) | C11—C10—C11'—F3' | 72 (2) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1B···O2 | 0.92 | 1.88 | 2.786 (3) | 169 |
N1—H1A···O1i | 0.92 | 1.96 | 2.869 (3) | 171 |
O1—H1···O3ii | 0.84 | 1.85 | 2.682 (3) | 171 |
Symmetry codes: (i) −x+2, y+1/2, −z+3/2; (ii) x, y−1, z. |
Experimental details
Crystal data | |
Chemical formula | C9H20NO+·C2F3O2− |
Mr | 271.28 |
Crystal system, space group | Orthorhombic, P212121 |
Temperature (K) | 113 |
a, b, c (Å) | 7.6204 (8), 9.8939 (10), 18.099 (2) |
V (Å3) | 1364.6 (2) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.12 |
Crystal size (mm) | 0.22 × 0.20 × 0.16 |
Data collection | |
Diffractometer | Rigaku Saturn |
Absorption correction | Multi-scan (CrystalClear; Rigaku/MSC, 2005) |
Tmin, Tmax | 0.974, 0.981 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 17989, 2039, 1993 |
Rint | 0.057 |
(sin θ/λ)max (Å−1) | 0.676 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.057, 0.132, 1.23 |
No. of reflections | 2039 |
No. of parameters | 202 |
No. of restraints | 48 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.22, −0.21 |
Computer programs: CrystalClear (Rigaku/MSC, 2005), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), SHELXTL (Bruker, 1997), CrystalStructure (Rigaku/MSC, 2005).
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1B···O2 | 0.92 | 1.88 | 2.786 (3) | 169.4 |
N1—H1A···O1i | 0.92 | 1.96 | 2.869 (3) | 171.1 |
O1—H1···O3ii | 0.84 | 1.85 | 2.682 (3) | 170.5 |
Symmetry codes: (i) −x+2, y+1/2, −z+3/2; (ii) x, y−1, z. |
References
Borzatta, V. & Carrozza, P. (1991). European Patent EP 0 462 069. Google Scholar
Bruker (1997). SHELXTL. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Rigaku/MSC (2005). CrystalClear and CrystalStructure. Rigaku/MSC, The Woodlands, Texas, USA. Google Scholar
Nengfang, S., Guo, H. Z., Yin, G. & Wu, A. (2005). Acta Cryst. E61, o2902–o2903. Web of Science CSD CrossRef IUCr Journals Google Scholar
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4-hydroxyl-2,2,6,6-tetramethylpiperidine is a very important intermediate in the synthesis of hindered light stabilizers (Borzatta & Carrozza, 1991; She et al., 2005). The piperidium ring adopts a chair conformation with the hydroxyl group in an equatorial position. The crystal packing is stabilized by O—H···O and N—H···O hydrogen bonds.