What is the reason why oxalic acid has its second $\ce{-COOH}$ group flipped?
I'm guessing it's something to do with steric repulsions between the $\ce{-COOH}$ groups but I'm not sure.
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$\begingroup$ Draw the charges, then apply Coulomb law. $\endgroup$– permeakraCommented Jun 25, 2014 at 22:27
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3 Answers
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Like ron already stated, the reason for that arrangement is the possibility to form more intermolecular hydrogen bonds. For oxalic acid there are two modifications which depict this quite clearly.
From charge considerations itself it is not obvious, that the periplanar alignment should be better. However one can see, that there might be a little less repulsion. Given here are natural partial charges from NBO/BP86/cc-pVDZ.
A rotational scan reveals that the rotation barrier is just around $7~\mathrm{kJ/mol}$ and the difference between these are just around $2~\mathrm{kJ/mol}$ (BP86/cc-pVDZ). The dihedral angle of $0^\circ$ refers to the hydroxyl moieties being on the same side.
The barrier results mainly from breaking the partial $\pi$ bond between the two carbon centres.
The $\ce{O...H}$ intramolecular distance is around $230~\mathrm{pm}$ and in gas phase and higher temperatures hydrogen tunneling should be also considered.
In aqueous solution, exchange rates between water and oxalic acid are very fast, so that all oxygen atoms should be equal - there is no difference observable at this time scale. \begin{aligned}\ce{ (COOH)2 + 2H2O &<=> HOOC-COO- + H3+O + H2O\\ &<=> (COO^{-})2 + 2H3+O\\ }\end{aligned}
answered Jun 26, 2014 at 5:38
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More structure stabilizing hydrogen bonds, both intra- and intermolecular, can be formed in the conformation where the carbonyl groups are anti-periplanar.
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The carbonyls' dipole moments minimize energy by pointing in opposite directions.
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$\begingroup$ This is not necessarily a minimization of energy. 1) The OH group also has a significant dipole moment, and 2) in polar solution (like water) you minimize energy by maximizing molecular dipole moment. $\endgroup$ Commented Apr 27, 2014 at 10:53
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$\begingroup$ No argument. Vacuum, solution (in what?) and solid state all have their special - and exploitable - footnotes. $\endgroup$– Uncle AlCommented Apr 28, 2014 at 19:18
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2$\begingroup$ In a polar solvent, oxalic acid is like carbon dioxide; all the C-O bond lengths are the same. $\endgroup$– LDC3Commented May 26, 2014 at 21:21