inorganic chemistry - How did hypophosphoric acid get its name?

Why does hypophosphoric acid has a strange name? My knowledge of inorganic nomenclature tells me that the prefix "per-" should be used with "-ic" acids and the prefix "hypo-" goes with "-ous" acids, which isn't the case here. Why is this so?

Just as part of an additional observation, two other things which I find strange about $\ce{H4P2O6}$ are:

• I think this is the only oxy-acid of phosphorus having a $\ce{P-P}$ bond.


• It has phosphorus in $+4$ oxidation state.

Answer

You are most of the way to the reason yourself:

$$\begin{array}{|c|c|c|}\hline \text{name} & \text{formula} & \text{oxidation number of P}\\ \hline \text{phosphoric} & \ce{H3PO4} & +5\\ \text{phosphorous} & \ce{H3PO3} & +3\\ \text{hypophosphorous} & \ce{H3PO2} & +1\\ \hline \end{array}$$

The "hypo-" prefix means "beneath" or "less than". Hypophosphoric acid, $\ce{H4P2O6}$, has oxidation number of phosphorous as +4, which is in between phosphoric acid and phosphorous acid. Thus, "hypophosphoric" refers to "beneath" phosphoric (but above phosphorous) acid.