Uric Acid
From WikidChem
[edit]Slide 12
All of the acids that Scheele discovered or co discovered, with the exception of uric acid, have a carboxylic acid group. Although a carboxylic acid group has an O and an OH attached to a C, it is not the same as a combined carbonyl and alcohol. A combined carbonyl and alcohol would have a double bond between the C and the O and a single bond between the C and the OH. If the group acted as an acid and lost the H from the OH, the pKa of this substance would be the same as the pKa for an alcohol, approximately 16. The carboxylic acid group, however, does not have a strict double bond between the C and the O and a strict single bond between the C and the OH. Its actual shape is a resonance structure between the two structures on this slide. The resonance between the two structures is shown by the double-headed arrow connecting them. When a carboxylic acid group undergoes an acid base reaction, giving away the H from its OH, the pKa of the substance is approximately 5. After the reaction, the molecule is even more stabilized by resonance than it was in its original, unreacted state. Beneath the pKa values of the carbonyl-alcohol group and the carboxylic acid group, McBride notes that the pKa of a substance depends on the energy difference between A-H group and A- H+ ions (for both molecules discussed here, O-H compared to O- and H+).
[edit]Slide 13
Slide 13 points out that uric acid is the only acid Scheele discovered that does not have a carboxylic acid group.
[edit]Slide 14
Slide 14 shows a molecule of uric acid before and after it undergoes an acid base reaction. In both cases, the actual resonance structure of the molecule lies somewhere between the three structures drawn on the slide. Resonance between the structures is shown by drawing double-headed arrows between them. Before the reaction, the first drawn structure has no separation of charge and it shows an unshared pair of electrons on the N. In the second, the N shares its pair of electrons with the upper oxygen, creating a positively charged N and a negatively charged O. In the third structure, the N shares its pair of electrons with the lower oxygen, again creating a positively charged N and a negatively charged O. After the reaction, the H attached to the N has been broken away from the molecule. The first structure shows a negatively charged N with two unshared pair of electrons. The second structure shows the N sharing one of its pairs of electrons with the upper O, making a neutrally charged N and a negatively charged O. In the third structure, the N has shared one of its pairs of electrons with the lower O, again making a neutrally charged N and a negatively charged O. In both cases the nitrogen's high HOMO is stabilized by the low LUMOs of the C=O bonds. McBride looked up the pKa of uric acid in two difference reference books. The chemistry book showed a pKa value of 3.89, slightly lower (more acidic) than that of a carboxylic acid group, while the medical reference showed a pKa value of 5.75, slightly higher (less acidic) than that of a carboxylic acid group.
