著者

野口 大介

出版者

技術・教育研究会

雑誌

技術・教育研究論文誌 = Journal of Technology and Education

巻号頁・発行日

vol.28, no.2, pp.49-58, 2021-12

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The Kolbe-Schmitt reaction is well known as a synthetic process for the industrial production of aromatic hydroxy carboxylic acids, and it is widely learned in chemical education. Several mechanistic studies of the reaction have been conducted over a century; however, it is supposed that it has not been determined yet. Therefore, the author overviews the literature focusing on its proposed reaction mechanisms, including experimental and theoretical research. In 1957, five mechanisms were reviewed: (1) metalation of aromatic nucleus, (2) intermediate formation of metal aryl carbonate, (3) direct nuclear carbonation, (4) tautomeric rearrangement, and (5) chelate formation and electrophilic substitution. After that, more intermediates of some complexes were also proposed, and X-ray crystal analyses revealed the structures of metal phenoxides (reactants) and sodium salicylate (product), both pure and/or solvated; however, it appears that the structural data were not used sufficiently to promote the mechanistic studies of the reaction. Recently, experimental studies by Kosugi et al. (Org. Biomol. Chem., 1(5), 817–821 (2003)) claimed that direct carboxylation occurs, and the complex of metal phenoxide combined with carbon dioxide is not the intermediate of the reaction; on the other hand, theoretical calculations did not support those conclusions. Further effort would be required for technological development and chemical education to integrate the accumulated knowledge of the studies of the Kolbe-Schmitt reaction mechanisms in the future.

著者

野口 大介

出版者

技術・教育研究会

雑誌

技術・教育研究論文誌 = Journal of Technology and Education

巻号頁・発行日

vol.28, no.1, pp.27-36, 2021-06

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Ethylenediaminetetraacetic acid (EDTA) is one of the well-known chelating ligands learned in analytical chemistry and widely used in such as industry and molecular biology to support our daily lives. It is also demonstrated as an acidic reagent in neutralization titration. The chemical structure indicates that EDTA is a tetrabasic acid; however, confusion of its notation exists in some textbooks. Some authors represent it as a neutral molecule, while others say it is a zwitterion. As the appropriate structure of EDTA seemed unclear, I surveyed some published research papers; herein I show in detail the historical controversy concerning the various proposed structures of EDTA, including non-ionized molecule and double zwitterion with ammonium carboxylate groups, both in solid state and aqueous solution. In the 1960s, some structures based on IR spectroscopic analyses were proposed; earlier studies exhibiting non-ionized EDTA molecular structure were almost overturned. Regrettably, without sufficient reference to each other, the idea of EDTA as a neutral molecule in a solid state still remained. After that, X-ray crystallographic studies confirmed the existence of EDTA as a double zwitterion also in a solid state, whereas an XPS investigation deduced that there are two kinds of nitrogen atoms, =N+H– and =N– types, in an EDTA molecule. Recent studies have revisited a double zwitterion form of EDTA in solution; however, some textbooks of analytical chemistry have shown the chemical structure of non-ionized EDTA, subsequently, as if “it returns to the beginning”. The reasons for this difficulty in interpreting the structure of EDTA in aqueous solution are considered to be as follow: (1) the poor solubility of EDTA itself in water made it quite difficult to be measured by IR spectroscopy in solution; (2) unexpected hydrogen bonds could change the frequencies of certain IR-active bonding; (3) there was some unawareness about its states, whether in solid or in solution; (4) the fact that EDTA has two different crystalline modifications with different IR spectra was almost unknown. I hope that the present study will be helpful in making the precise structure of EDTA in solution easily available based on literature for students, with a result that also would be of significance for those who teach and educate chemical analysis and technology.