Help in drawing resonance structures in case of polybasic acids

While drawing resonance structure of dibasic or tribasic acid, e.g. should I take out two $$\ce{H+}$$ (in dibasic) or three $$\ce{H+}$$ (in tribasic) together or one by while drawing resonance structure?

Do dibasic acids give both $$\ce{H+}$$ at once or one by one?

Actually, I was drawing resonance structure of $$\ce{H3PO4},$$ but got confused. Should I remove all three protons at once or only one?

• please forgive me for poor editing & framing of this question.Poor Editing- because i'm new.Poor Framing - because English isn't my native language & i'm a science student not literature. Sorry. – Math Geek Sep 22 '19 at 14:56
• No need to apologize, showing you effort towards improving is what counts. I bet English isn't a native language for about one third of Chemistry.SE users (not for me, either), nevertheless, we do manage to communicate somehow. Please visit this page, this page and this one on how to format your future posts better with MathJax and Markdown. – andselisk Sep 22 '19 at 15:01
• – andselisk Sep 22 '19 at 15:05
• @andselisk my real doubt is in this part." should I take out two H+ (in dibasic) or three H+ (in tribasic) together or one while drawing resonance structure? " – Math Geek Sep 22 '19 at 15:09

It depends on what specifically the question is asking you to do.

In reality, a polyprotic acid such as $$\ce{H3PO4}$$ becomes deprotonated in a sequential way; the first proton is the easiest to remove (most acidic), then the second, then finally the third is hardest to remove (least acidic).

So there are four different species that the question might be asking you about: $$\ce{H3PO4}$$, $$\ce{H2PO4-}$$, $$\ce{HPO4^2-}$$, and $$\ce{PO4^3-}$$

Each of the three anionic species above has multiple resonance structures.

• I almost upvoted this but the last sentence is wrong. Each of those four species has only one significantly contributing resonance structure. Note: proton shuffling (possible in all except phosphate) is not resonance but consitutional isomerism. – Jan Sep 24 '19 at 2:25
• The most commonly taught convention for the structures of these phosphate bases involves a P=O double bond. In that case, my statement was correct and multiple resonance structures exist. If you draw them with ionized P-O single bonds, then you are correct per the structures in your example. While I think this is more realistic (AFAIK computational/spectroscopic data don't support the "expanded octet" for phosphorus) it is less commonly taught at the undergraduate level, and my money is on OP's class using P=O structures and the phosphates as examples of resonance stabilized conjugate bases. – Greg D Sep 24 '19 at 14:14
• Your bet on OP’s level and what OP got taught is probably correct. However, I refuse to use outdated and superceded concepts in my SE answers and I refuse to upvote answers that use them in the hope that they may ultimately finally die out. – Jan Sep 24 '19 at 14:15

Resonance structures can only be drawn for single compounds. Rephrased: if a set of resonance structures are to be valid, then all atoms (nuclei) must be at exactly the same positions.

Thus, when asked to draw resonance structures of e.g. phenol or nitric acid, you cannot deprotonate or transfer protons. All protons must be exactly at the same spot in all structures, only electrons may move—thus, the rightmost structure of nitric acid drawn in scheme 1 is not a resonance structure. Distinct from the examples above, you can draw (a different set of) resonance structures for phenolate or nitrate after removing a single proton (see also scheme 1).

Scheme 1: possible resonance structures of (top to bottom) phenol, nitric acid, phenolate, nitrate. Lesser contributors to phenol and nitric acid in brackets. For nitric acid, an isomer that is not a resonance structure is shown.

Expanding on this, if you happen to have a diprotic or triprotic acid for which you can draw resonance structures such as carbonic acid or salicylic acid, your resonance structures must have the proton connected to the identical oxygen atom for the resonance structures to be correct, compare the examples in scheme 2.

Scheme 2: Resonance structures of carbonic acid and its conjugate bases (top three) and salicylic acid and its conjugate bases (bottom three). Further possible resonance structures of salicyldiate are omitted for brevity.

Finally note that your example is poorly chosen. Phosphoric acid and its conjugate bases only have one significant resonance structure each as depicted in scheme 3.

Scheme 3: Structures of phosphoric acid, dihydrogenphosphate, hydrogenphosphate and phosphate (from left to right). No other significant resonance contributors exist.