rac-Hydroxyisovaleric acid

The title compound is the constitutional isomer of α-hydroxybutanoic acid. In the crystal, hydrogen bonds involving the alcoholic hydroxyl group give rise to centrosymmetric dimers that are extended to sheets perpendicular to the crystallographic c axis.

The title compound (systematic name: rac-2-hydroxy-3-methylbutanoic acid), C 5 H 10 O 3 , is the constitutional isomer of �-hydroxybutanoic acid.In the crystal, hydrogen bonds involving the alcoholic hydroxyl group give rise to centrosymmetric dimers that are extended to sheets perpendicular to the crystallographic c axis.

Structure description
The Krebs Cycle -also known as Citric Acid Cycle -is at the centre of metabolic processes in aerobic organisms.It involves a number of hydroxycarboxylic acids that constitute intriguing chelating ligands for a variety of transition metals of pharmaceutical interest (McMurry, 2008).These potential ligands classify as chelate ligands, which have found widespread use in coordination chemistry due to the increased stability of coordination compounds they can form in comparison to monodentate ligands (Gade, 1998).Hydroxycarboxylic acids are a particularly interesting class of ligands as they offer two functional groups that, depending on the experimental conditions, can either act as fully neutral, fully anionic or mixed neutral-anionic donors.Upon varying the substitution pattern on the hydrocarbon backbone, the acidity of the respective hydroxyl groups can be fine-tuned over a wide range and they may, thus, serve as probes for establishing the rules in which pK a range coordination to various central atoms can be observed.Furthermore, the steric pretence of potential substituents may give rise to unique coordination and bonding patterns.Given the multidentate nature of hydroxycarboxylic acids encountered in the Krebs Cycle it appears prudent to investigate simpler 'cut outs' with a more limited number of donor sites to avoid complexer mixtures of reaction products in envisioned synthesis procedures, thus prompting the diffraction study of the title compound to allow for comparisons of metrical parameters of the free ligand and the ligand in envisioned coordination compounds.The present study falls into the ambit of our continued interest into structural aspects of alpha-hydroxycarboxylic acids such as 1-hydroxycyclopropanecarboxylic acid (Betz & Klu ¨fers, 2007a), 1-hydroxycyclobutanecarboxylic acid (Betz & Klu ¨fers, 2007b), 1-hydroxycyclopentanecarboxylic acid (Betz & Klu ¨fers, 2007c) or tert-butylglycolic acid (Betz et al., 2007).Furthermore, geometrical data for glycolic acid (Ellison et al., 1971;Pijper, 1971) and l-lactic acid (Schouten et al., 1994;Yang et al., 2021) is apparent in the literature while, to the best of our knowledge, none of the various hydroxy-n-butanoic acids have been subjected to diffraction studies.Only one report provides the crystal and molecular structure for gamma-hydroxybutanoic acid as a solvent molecule in a barium-supported tetraphenylimidodiphosphinato compound (Morales-Juarez et al., 2005).
The asymmetric unit of the title compound is shown in Fig. 1 and contains one complete molecule.C-O bond lengths are found to be 1.4175 (11) A ˚for the alcoholic hydroxyl group and 1.2064 ( 12) and 1.3143 (11) A ˚for the carboxylic acid group and, thus, lie in the normal range reported for other hydroxycarboxylic acids whose metrical parameters have been deposited with the Cambridge Structural Database (Groom et al., 2016).The alcoholic hydroxy group adopts a staggered conformation relative to the two terminal methyl groups with the relevant C-C-C-O torsional angles measuring À 58.32 (11) and 66.60 (12) � .
In the crystal, classical hydrogen bonds of the O-H� � �O type (Table 1) are apparent that involve all hydroxyl groups as donors and the oxygen atom of the alcoholic hydroxyl group and the carbonyl oxygen atom as acceptors.The hydrogen bonds supported by the alcoholic hydroxyl group as donor and the carbonyl oxygen atom as acceptor connect the individual molecules into centrosymmetric dimers, which are further extended to sheets perpendicular to the crystallographic c axis by means of the carboxylic acid's hydroxyl group as donor and the alcoholic hydroxyl group's oxygen atom as acceptor.In terms of graph-set analysis (Etter et al., 1990;Bernstein et al., 1995), the descriptor for these hydrogen bonds is C 1 1 (5)R 2 2 (10) on the unary level.A depiction of the hydrogen-bonding pattern is shown in Fig. 2.

Synthesis and crystallization
The compound was obtained commercially (Fluka).Crystals suitable for the diffraction studies were taken directly from the provided material.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.The molecular structure of the title compound, with atom labels and anisotropic displacement ellipsoids (drawn at 50% probability level).(Sheldrick, 2008), PLATON (Spek, 2020) and Mercury (Macrae et al., 2020).

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.

data reports data-2
IUCrData (2024).9, x231093 Refinement.The carbon-bound H atom of the methine group was placed in a calculated position1 (C-H 1.00 Å) and was included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C).The H atoms of the methyl groups were allowed to rotate with a fixed angle around the C-C bond to best fit the experimental electron density (HFIX 137 in the SHELX program suite (Sheldrick, 2008)), with U(H) set to 1.5U eq (C).The H atoms of the hydroxyl groups were allowed to rotate with a fixed angle around the C-C bond to best fit the experimental electron density (HFIX 147 in the SHELX program suite (Sheldrick, 2008)), with U(H) set to 1.5U eq (O).
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å Figure 1

Table 2
Experimental details.