My basic knowledge about nomenclature is: A mineral acid is called hypo when it has one less oxygen than the original one,

  • a pyro acid when 2 molecules of acid add up with release of $\ce{H2O}$
  • a meta acid when one molecule releases $\ce{H2O}$ and
  • a per acid when it has one extra $\ce{O}$ in the structure connected with a peroxide bond.

    Question 1

    I was taught in my class that this structure:

    is correct because when we are drawing structures of a hypo-,per-,meta- or pyro- oxyacid, one must try to maximise the covalency.

    Why is that so?

    Also, usually in pyro- acid we find a $\ce{M-O-M}$ bond where $\ce{M}$ is the central atom(s). Like $\ce{P-O-P}$ in pyrophosphoric acid as show below:

    Then shouldn't the structure of pyrosulfurous acid be like this:

    Why is the structure not similar to pyrophosphoric acid?

    Question 2

    We know that in hypo-acids, there is one less $\ce{O}$ in the empirical formula of the acid i.e. Hypophosphoric acid should have one less $\ce{O}$ than in $\ce{H3PO4}$ i.e. it should be something like $\ce{(H3PO3)_$n$}$. If we put $n=2$ we have $\ce{H6P2O6}$.

    But why is its formula $\ce{H4P2O6}$?

    I might lack some information about it or maybe I've understood the concept wrong. Please help.

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    • $\begingroup$ @Mithoron None of them answers my questions. None, I repeat, None of the questions explain why do I need to maximise the covalency when I'm drawing a structure. And why is the structure of pyrosulphurous acid not the one I think it should be. And the formula for Hypophosphoric acid is different. My questions is not "How to name a mineral acid? " or "Why is meta ,hypo or pyro used? ". I couldn't find anything more than I already know $\endgroup$
      – Quark
      Jan 17, 2016 at 10:54
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      $\begingroup$ Compounds simply are as they are and have to be named somehow. $\endgroup$
      – Mithoron
      Jan 17, 2016 at 15:51
    • $\begingroup$ I know. What would you name the structure that I've made? It has the same formula H2S2O5. Is that structure not possible? If yes, then why? $\endgroup$
      – Quark
      Jan 17, 2016 at 15:59

    3 Answers 3


    The English names are not very systematic, they just give hints. In principle you have to either memorize the name-structure correspondence, or know "all" acids of sulfur, so based on the name you can guess what the structure it corresponds to.

    In addition, the hypo having one oxygen less is just a shortcut. What the names refer to is the oxidation state of central atom from lowest to highest. Broadest range of acids are for chlorine: hypchlorous - chlorous - chloric - perchloric.

    Answer 1: Please refer to the Wikipedia page on sulphur oxo-acids . The $\ce{H2S2O5}$ acid drawn is hypothetical one, but it's fine to have a name for it. Note, that sulfur is in low oxidation state. In the pyrophosphoric acid, the phosphorus is in high oxidation state and has extra oxygen in between them.

    For the summary formula $\ce{H2S2O3}$ you try to mix these properties, putting "extra" oxygen between sulfurs in low oxidation state. This won't work. The acid you should get when you attempt to obtain this one will be the thiosulfuric acid, where one sulfur mimics oxygen as $\ce{S^2-}$.

    Answer 2: Chemistry of phosphorus is distinctly different from sulfur. Phosphoric acid and Phosphorus acid for the acids with one phosphorus atom.
    Your formula $\ce{(H3PO3)_{$n$}}$ with $n=2$ is not really justified, the name corresponds just to the formula on hypophosphoric acid.


    Answer 1 - Covalency is maximized in any molecule formation because bonding electron pair is considered to be in noble state while lone pair always distorts the symmetry or it causes strain in molecular structure.

    Answer 2 - Often we use hypo-, per-, etc. to show various oxidation state of the central atom. Well in your case, in hypo-, one $\ce{-OH}$ is removed.Or you can say that there is one less oxygen than the -ite.

    For clarification in nomenclature you can see the IUPAC 1971 pdf.


    Then shouldn't the structure of Pyrosulfurous acid be something like:

    enter image description here


    See page 472 of Advanced Inorganic Chemistry, Volume 1 by Satya Prakash and see page 514 of Inorganic Chemistry by J. E. House

    Furthermore, in NOMENCLATURE OF INORGANIC CHEMISTRY IUPAC Recommendations 2005 at page 130 it is stated

    $\ce{[(HO)(O)2SS(O)OH]}$ disulfurous acid

    is the acceptable name but adds the footnote:

    This common name presents a problem because the unsymmetrical structure is not the structure which would otherwise be associated with the ‘diacid’ construction (disulfurous acid would systematically be $\ce{[HO(O)SOS(O)OH]}$). The use of an additive name eliminates this potential confusion, but the problem with the use of disulfurous acid as a parent name persists in the naming of organic derivatives.

    and on page 129:

    $\ce{H4P2O6}$ = $\ce{[(HO)2P(O)P(O)(OH)2]}$ hypodiphosphoric acid

    All these names are considered "common names" rather than following any system by the IUPAC recommendations


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