Azelaic acid

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The crystal structure of heptane-1,7-dicarboxylic acid (azelaic acid), C 9 H 16 O 4 , has been redetermined at 180 K. The molecular units have twofold symmetry and are linked via the ubiquitous syn±syn carboxylic acid dimer to form in®nite chains running approximately along the [401] vector.

Comment
Two polymorphs of azelaic acid, (I), have been reported previously: the form crystallizes in P2 1 /c (Caspari, 1928;Housty & Hospital, 1967) and the form crystallizes in C2/c (Housty & Hospital, 1967). For both polymorphs, the structures present in the CSD (AZELAC10 and AZELAC01; Allen & Kennard, 1993) are derived from room-temperature data with R factors ca 10% and ambiguities in the treatment of H atoms. We have, therefore, re-examined azelaic acid and report here the structure of the form measured at 180 K to signi®cantly greater precision.
The H atom of the carboxylic acid group was located in a difference Fourier map and re®ned without restraint. All other H atoms were placed geometrically and allowed to ride during subsequent re®nement with an isotropic displacement parameter ®xed at 1.2 times that for the C atom to which they are attached.
We thank the EPSRC for ®nancial assistance with purchase of the CCD diffractometer.  Two polymorphs of azelaic acid, (I), have been reported previously: the α form crystallizes in P2 1 /c (Caspari, 1928;Housty & Hospital, 1967) and the β form crystallizes in C2/c (Housty & Hospital, 1967). For both polymorphs, the structures present in the CSD (AZELAC10 and AZELAC01; Allen & Kennard, 1993) are derived from room-temperature data with R factors ca 10% and ambiguities in the treatment of H atoms. We have, therefore, re-examined azelaic acid and report here the structure of the β form measured at 180 K to significantly greater precision.

S2. Experimental
Azelaic acid was obtained from Aldrich and recrystallized from ethanol.

S3. Refinement
The H atom of the carboxylic acid group was located in a difference Fourier map and refined without restraint. All other H atoms were placed geometrically and allowed to ride during subsequent refinement with an isotropic displacement parameter fixed at 1.2 times that for the C atom to which they are attached.

Figure 1
The molecular unit of the title compound showing displacement ellipsoids at the 50% probability level.   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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.