3-(2-Formylphenoxy)propanoic acid

In the structure of the title compound, C10H10O4, the carboxyl group forms a catemer motif in the [100] direction instead of the expected dimeric structures. The carboxylic acid group is found in the syn conformation and the three-dimensional organization in the crystal is based on C—H⋯O and O—H⋯O interactions.

In the structure of the title compound, C 10 H 10 O 4 , the carboxyl group forms a catemer motif in the [100] direction instead of the expected dimeric structures. The carboxylic acid group is found in the syn conformation and the three-dimensional organization in the crystal is based on C-HÁ Á ÁO and O-HÁ Á ÁO interactions.

Comment
The title compound was synthesized as a precursor for asymmetric PPV-type [poly(p-phenylene vinylene)] oligomers. These compounds are promising candidates as the active materials in organic memories (Bandyopadhyay & Pal, 2003) and as non-linear optical (NLO) materials with a high first-order hyperpolarizability (Chemla, 1987). Besides the condition that these oligomers should bear donor and acceptor substituents connected by a π-system, it is of critical importance for their usefulness as an NLO material and as a bistable organic memory material that the crystal packing is non-centrosymmetric.
In particular, the compound should crystallize in a polar space group. To influence the crystal packing by means of the crystal engineering methodology in order to meet this criterion, it is necessary to introduce certain statistically well chosen synthons (Desiraju, 1997). Therefore, we opted for carboxylic acid functional groups (powerful hydrogen bond donors) in the basic structure of the organic semiconductor. To combine the electronic (A-π-D) and the structural (non-centrosymmetric space group) requirements, we used the Williamson ether synthesis to prepare the title compound as a building block for a PPV-based semiconductor bearing a carboxylic acid moiety.
The geometry of the aldehyde contains no surprises. The methylene fragments are in synclinal conformation and the torsion angle is 65.9 (3)°. As a result and due to the repulsion between the lone pairs of oxygen atoms O2 and O3, the carboxylic acid group, found in the syn conformation, is twisted out of the plane of the phenyl ring. The O2···O3 distance Two intramolecular CH···O hydrogen bonds involving the aldehyde group can be identified: the relevant parameters of C6-H6···O1 and C11-H1···O2 are given in Table 1, entries 1 and 2. The incorporation of a carboxylic acid group was expected to yield the corresponding dimer synthon, but the crystal structure of the title compound reveals a catemer motif in the [1 0 0] direction (Fig. 2), consisting of an infinite chain of hydrogen bonds. The O3···O4 distance is 2.618 (3) Å (Table   1, entry 3) which is not surprising for a hydrogen bond of moderate strength (Steiner, 2002). The chains are intertwined through O1···Cg contacts in which the aldehyde oxygen atom contacts the center of the aromatic ring [O1···Cg i 3.508 (3) Å, C11-O1···Cg i 78.47 (17)°, symm. code i = 1-x, -y, -z]. O1 is also involved in weak CH···O hydrogen bonds with H21a (  . 3). Two additional hydrogen bonds can be observed: H21a contacts O3 and H3 contacts O4 (Table 1, entries 6 and 7, respectively).

Experimental
Salicylic aldehyde and 3-chloropropanoic acid were obtained from ACROS and used as received. The general procedure of Gresham was followed (Gresham et al., 1949). 8 g of NaOH in 20 ml of distilled water were added to a stirred solution of supplementary materials sup-2 12.2 g (0.1 mol) of salicylic aldehyde and 10.9 g (0.1 mol) of 3-chloropropanoic acid in 80 ml of distilled water. After heating under reflux for 4 h, the mixture was acidified with 19.5 ml of conc. HCl. Unreacted salicylic aldehyde was removed by steam distillation and the resulting mixture was placed in the refrigerator. The resulting tan-coloured needles were collected by filtration in a 20% yield. M.p. 381 K (uncorrected). Crystals suitable for the diffraction experiment were grown by slow

Refinement
The positions of the hydrogen atoms were derived from the electron density difference map and X-H bonding distances are situated between 0.89 (3) Å and 1.03 (3) Å. Fig. 1. Molecular structure of the title compound with the numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are represented as spheres with an arbitrary radius.  Table 1 for details.  Table 1 for details.  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 Rfactors(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.