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I was practicing a DPP for JEE Mains & Advanced exam. There is a question in organic chemistry, a reaction sequence was given whose answer was oxalate anion. But the next question was what is the degree of unsaturation of the compound formed in the previous question . I simply did it 2 because there are 2 C=O bond but the answer was given 3. Please help me in making out, why it is so.

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    $\begingroup$ Please be more specific how you have calculated the degree of unsaturation. in that way we can see your basis for the problem. $\endgroup$ – Martin - マーチン May 8 '20 at 17:16
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The Degree of Unsaturation can be considered as the number of Pairs of Hydrogen Atoms that could be added to the structure. One can simplify the structure to consist of two carbon-oxygen double bonds and two carbon-oxygen single bonds in one resonance contributor. Note that the single bonded oxygens have a negative charge and thus could accept a hydrogen each. This accounts for 1 degree of unsaturation. There are also two double bonds, and a pair of hydrogens could be added across these also. This gives two degrees of unsaturation. Hence there are 3 degrees of unsaturation in total (as the book says).

In order to calculate the degree of unsaturation for any compound, one can use the formula: ((2* Number of Carbons) + 2 - (Number of Hydrogens and halides) + (number of nitrogens)) / 2. In this case, the formula gives 3, which is the degree of unsaturation.

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  • $\begingroup$ Then you mean any anion would atleast have a degree of unsaturation ? $\endgroup$ – zombie May 8 '20 at 17:42
  • $\begingroup$ In principle, for any negative charge there is half a degree of unsaturation. So a double negative charge is one degree of unsaturation. If it was a single negative charge, that would be half a degree of unsaturation. However I think it is mostly applied to organic compounds. $\endgroup$ – user93414 May 8 '20 at 17:51
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    $\begingroup$ What is a pair of hydrogen atoms in this context? You can't really add three pairs of hydrogen atoms; to get the (theoretical) saturated compound $\ce{C2H6O4}$ two of the hydrogn atoms must be cations. That shouldn't count since those do not bring in electrons. Seems more realistic to require maintaining the charge, so the saturated species would be $\ce{C2H4O4^{2-}}$ and the degree of saturation would be two. $\endgroup$ – Oscar Lanzi May 11 '20 at 13:07

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