Abstract
As part of an educational exercise designed to introduce school students to the technique of single-crystal X-ray diffraction and enhance their understanding of primary and secondary bonding, a group of nine secondary school students was given the opportunity to prepare new compounds and to solve and refine data collected on the crystalline materials they had prepared. Their investigation of the alkali metal salts of 4-hydroxybenzoic acid (H2hba) yielded nine new compounds and their structures are described in this article. Whilst the salts might be expected to have similar atomic arrangements, there are significant differences in their structures. Although H2hba is a relatively simple organic molecule, it displays remarkable coordinative flexibility, forming ionic solids containing the uncharged molecule, the monoanion Hhba- or the dianion hba2-. A common feature of the structures is their layered arrangement: alternating hydrophilic layers made up of closely packed metal-oxygen polyhedra separated by the hydrophobic component of the hydroxybenzoate linking units. Close packing of these units seems to be a dominant influence in determining the overall structure. The hydroxybenzoate units are usually both parallel and antiparallel with their immediate neighbours, with packing that can be edge-to-face, face-to-face or a mixture of the two. Hydrogen bonding plays a key role in the structure of most compounds and a short strong hydrogen bond (SSHB) is observed in two of the networks. The compounds of 4-hydroxybenzoic acid, C7H6O3, described here are: poly[di-μ-aqua-μ-4-oxidobenzoato-dilithium], [Li2(C7H4O3)(H2O)2]n, 1, poly[triaqua-μ-4-oxidobenzoato-dilithium], [Li2(C7H4O3)(H2O)3]n, 2, poly[μ-4-hydroxybenzoato-lithium], [Li(C7H5O3)]n, 3, catena-poly[4-hydroxybenzoate [[diaquasodium]-di-μ-aqua]], {[Na(H2O)4](C7H5O3)}n, 4, poly[di-μ-aqua-aqua-μ-4-hydroxybenzoato-potassium], [K(C7H5O3)(H2O)3]n, 5, poly[μ-aqua-μ-4-hydroxybenzoato-potassium], [K(C7H5O3)(H2O)]n, 6, poly[aqua-μ-4-hydroxybenzoato-rubidium], [Rb(C7H5O3)(H2O)]n, 7, poly[aqua-μ-4-hydroxybenzoato-caesium], [Cs(C7H5O3)(H2O)]n, 8, poly[[μ-aqua-aqua(μ-4-hydroxybenzoato)(4-hydroxybenzoic acid)sodium] monohydrate], {[Na(C7H5O3)(C7H6O3)(H2O)2]·H2O}n, 9, poly[[(μ-4-hydroxybenzoato)(μ-4-hydroxybenzoic acid)rubidium] monohydrate], {[K(C7H5O3)(C7H6O3)]·H2O}n, 10, and poly[[(μ-4-hydroxybenzoato)(μ-4-hydroxybenzoic acid)rubidium] monohydrate], {[Rb(C7H5O3)(C7H6O3)]·H2O}n, 11.
Highlights
In the study of chemistry, an understanding of the various types of chemical bonding is essential and it is not surprising that introductory chemistry courses at the secondary school level tend to have a strong emphasis on primary and secondary bonding
Each of the compounds formed by the reaction of the alkali metal hydroxides and H2hba in aqueous solution can be classified within one of three categories according to the metal–Hnhba ratio in the crystal structure (n = 0, 1 or 2)
Given that H2hba is a relatively simple organic molecule, the salts that crystallize from aqueous solution when H2hba reacts with alkali metal hydroxides might be expected to form only a few different structures
Summary
In the study of chemistry, an understanding of the various types of chemical bonding is essential and it is not surprising that introductory chemistry courses at the secondary school level tend to have a strong emphasis on primary and secondary bonding. It is somewhat surprising that the role of X-ray crystallography in providing detailed representations of molecules is poorly recognized in many secondary school chemistry courses worldwide. It would be fair to say that for a large part of the 20th century, crystal structures were determined by expert crystallographers who had extensive training in the technique and possessed a detailed understanding of the theory that underpinned the collection of data, the structural solution and the refinement process. For many chemists it was an unavailable technique unless one was able to collaborate with a crystallographer
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