Crystallographic and spectroscopic characterization of (R)-O-acetylmandelic acid

The title compound is a resolved chiral ester derivative of mandelic acid containing an acetate group and a carboxylic acid group, which engage in intermolecular hydrogen bonding, forming chains extending parallel to [001].


Chemical context
Chiral, resolved carboxylic acids have played an important role as chiral NMR shift reagents (Lovely & Wenzel, 2008;Parker, 1991). The title compound, (R)-(À)-2-acetoxy-2phenylacetic acid (I), commonly known as (R)-O-acetylmandelic acid, is a chiral, resolved derivative of mandelic acid. The compound may be synthesized by acetylation of the parent -hydroxy acid with acetic anhydride in pyridine (Ornelas et al., 2013). When racemic, resolution of the compound with free amino acids has been demonstrated (Szeleczky et al., 2015). The title compound has been employed as a chiral NMR shift reagent (Parker, 1991).

Structural commentary
The molecular structure of the title compound ( Fig. 1) shows the R confguration about carbon atom C1, and that the molecule does not engage in intramolecular or pairwise hydrogen bonding. The absolute structure parameters confirm the R assignment, with Flack x = À0.01 (4) and Hooft y = À0.02 (4), calculated with PLATON (Spek, 2009).

Supramolecular features
The molecules pack together in the solid state via van der Waals forces and hydrogen bonding between the carboxylic acid OH group and the carbonyl oxygen atom of the ester on a ISSN 2056-9890 neighboring molecule, O1-H1Á Á ÁO4 i [symmetry code (i) Àx + 1 2 , Ày + 1, z À 1 2 ] with a donor-acceptor distance of 2.676 (2) Å (Table 1). These interactions create zigzag hydrogen-bonded chains that extend parallel to the c axis of the unit cell (Fig. 2). Notably, there is no face-to-face or edgeto-face -stacking.

Database survey
The Cambridge Structural Database (Groom et al., 2016) contains several related mandelic acid ester structures.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon were included in calculated positions and refined using a riding model with C-H = 0.95, 0.98 and 1.00 Å and U iso (H) = 1.2, 1.5 and 1.2 Â U eq (C) of the aryl, methyl and methine C atoms, respectively. The position of the carboxylic acid hydrogen atom was found in the difference map and the atom refined semi-freely using a distance restraint d(O-H) = 0.84 Å , and U iso (H) = 1.2U eq (O).  (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008), OLEX2 (Dolomanov et al., 2009), Mercury (Macrae et al., 2008) and PLATON (Spek, 2009).

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.