|
|Section2= |Section3= }} Squaric acid, also called quadratic acid, because its four carbon atoms approximately form a square, is an organic compound with chemical formula 424. The conjugate base of squaric acid is the hydrogensquarate anion C4HO4−; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion C4O42−. This is one of the oxocarbon anions, which consist only of carbon and oxygen. Squaric acid is a reagent for chemical synthesis, used for instance to make photosensitive squaraine dyes and inhibitors of protein tyrosine phosphatases. ==Chemical properties== Squaric acid is a white crystalline powder with a thermal decomposition point of 245 °C at ambient pressure.〔K.-S. Lee, J. J. Kweon, I.-H. Oh, C. E. Lee (2012) "Polymorphic phase transition and thermal stability in squaric acid ()". J. Phys. Chem. Solids 73 (7): 890–895. doi:10.1016/j.jpcs.2012.02.013〕 The onset of thermal decomposition depends on the different thermodynamic conditions such as heating rates. The structure of squaric acid is not a perfect square, as the carbon–carbon bond lengths are not quite equal. The high acidity with p''K''a = 1.5 for the first proton and p''K''a = 3.4 for the second is attributable to resonance stabilization of the anion.〔 Robert West and David L. Powell (1963), ''New Aromatic Anions. III. Molecular Orbital Calculations on Oxygenated Anions'' J. Am. Chem. Soc. volume 85 issue 17, pages 2577–2579. 〕 Because the negative charges are equally distributed between each oxygen atom, the dianion of squaric acid is completely symmetrical (unlike squaric acid itself) with all C-C and C-O bond lengths identical. Another, quantum mechanical, way of describing the dianion is to assume that the π electrons of the two double-bonded oxygen atoms are shifted to the latter, so that all four oxygens become single-bonded -O− groups and a double positive electric charge is left in the ring of carbon atoms. In this way the ring fits Hückel's rule for aromaticity (2 π-electrons = 4n + 2 with n = 0). The total symmetry of the dianion is a consequence of charge distribution and aromaticity. On the other hand, theoretical calculations indicate that the analogous tetrathiosquarate anion is anti-aromatic.〔Reza Ghiasi and Majid Monajjemi (2007), ''Theoretical study of interaction of alkaline earth metal with and : structure, electronic properties and aromaticity''. Journal of Sulfur Chemistry, Volume 28, Issue 6, pages 537–546 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Squaric acid」の詳細全文を読む スポンサード リンク
|