X-ray Studies on 2:6-Dihydroxy Benzoic Acid and Its Salts

Cant, Elizabeth M (1959) X-ray Studies on 2:6-Dihydroxy Benzoic Acid and Its Salts. PhD thesis, University of Glasgow.

Abstract

This thesis describes an investigation of 2:6-dihydroxy benzoic acid and its salts by X-ray analysis. The unit cell dimensions of 2:6-dihydroxy benzoic acid monohydrate have been obtained and the possible space group is discussed. The main part of the work is concerned with the crystal and molecular structure of the potassium and rubidium salts of 2:6-dihydroxy benzoic acid and a complete structure analysis of the potassium salt has been carried out. The crystals of 2:6-dihydroxy benzoic acid monohydrate are orthorhombic, space group Pna2 1 (C9 20) or Pnma (D 16 2h) with 4 molecules per unit cell. The unit cell dimensions are a = 6.77. b = 9.67 and c = 11.92 A. Potassium 2:6-dihydroxy benzoate crystallises in the monoclinic space group P2 1/n (C5 2h) with four molecules per unit cell. The unit cell dimensions are a = 3.94, b = 15.10 and c = 13.37 A. beta = 99 11' The structure of the compound has been elucidated by the Isomorphous Replacement method, in which use was made of the isomorphous rubidium salt. Refinement proceeded from a study of Fourier syntheses in all three zones and Fourier difference syntheses projecting on (100). Electron density projections on the (100) plane gave good resolution of all the atoms in the asymmetric unit. The shortness of the a axis prevented good resolution in the h01 and hk0 zones and the x co-ordinates were obtained largely by trial and error methods after an approximate x co-ordinate for the potassium ion had been obtained from a study of the far-out reflections in the hk0 and h01 zones. However, from the electron density projections, x co-ordinates could be assigned to the potassium ion and most of the oxygen atoms. The co-ordinate standard deviations are less than .02 A. The molecule is planar within experimental error and bond lengths vary between 2.75 and 3.03 A for potassiura-oxygen bonds, between 1.25 and 1.40 A for carbon-oxygen bonds and between 1.39 and 1.53 A for carbon-carbon bonds. There are two intramolecular hydrogen bonds joining the oxygen atoms of the carboxyl group to the phenolic oxygens; these are 2.50 and 2.56 A in length. There are no discrete molecules and the packing is determined by the co-ordination of the organic residues to a central metal ion by potassium-oxygen links in all directions, involving both phenolic and carboxylic oxygens. The potassium is surrounded by seven bonded oxygens, six of which are arranged to form a trigonal prism. The ion shows a preferential attraction to one of the two oxygen atoms of the carboxyl group and has an attraction of similar magnitude to both of the phenolic oxygens. The bond lengths within the organic residue are reasonably consistent with Cochran's values for salicylic acid. It is expected that the work on the potassium salt will shortly be published.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Organic chemistry, Analytical chemistry
• Date of Award: 1959
• Depositing User: Enlighten Team
• Unique ID: glathesis:1959-79329
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 05 Mar 2020 10:43
• Last Modified: 05 Mar 2020 10:43
• URI: https://theses.gla.ac.uk/id/eprint/79329