# Why is hypophosphorous acid (H3PO2) a strong reducing agent?

Why is hypophosphorous acid $$(\ce{H3PO2})$$ a strong reducing agent? the oxidation state of phosphorous in $$\ce{H3PO2}$$ is $$+5$$, as hydrogen and oxygen both are more electronegative than phosphorous, and phosphorous can never increase its oxidation state to more than $$+5$$, so how can it be a strong reducing agent?

If you render the oxidation state of hypophosphorous as +5, then the electrons in the phosphorous-hydrogen bonds are attributed to the hydrogen atoms in those bonds. You can do that, but then the hydrogen atoms involved have oxidation state -1 not +1. So the reducing action of hypophosphorous acid is attributed to the negative oxidation state hydrogens.

• so its all because of the negative partial charge on hydrogen that will be responsible for the reducing ability of the acid? – Geet Suri Feb 18 '18 at 21:56
• Hyrogen in a negative oxidation state (-1) is reducing. – Oscar Lanzi Feb 18 '18 at 22:15
• can u plss explain how negative oxidation state of hydrogen is reducing? everytime i encountered with oxidation problems i always used to see the oxidation state of the central atom and then judge the oxidizing and reducing nature of the compound in a reaction.. – Geet Suri Feb 18 '18 at 22:24
• The central atom is often decisive, but not always. You really have to consider the oxidation state of all atoms. – Oscar Lanzi Feb 18 '18 at 22:32

The oxidation state of phosphorus in H3PO4 is indeed +5, but in this case you have only 2 and not 4 oxygens. This means that your P is only +1. The reason it's a reducing is because that P really wants to "become" +5.

Oscar Lanzi in his answer suggests that you can assume the H is H-, but this introduces a complication of this being a hydride, and these are not usually stable in solution. Note that this distinction is artificial. Eventually, this is all covalently bonded and the use oxidation numbers suggests ionic bonding.

• If you assume phosphorous is +5, then hydrogen attached to it is -1. You can assume instead that all the H is +1 but must then accept P being +1. No matter how you slice it, you render something in a state that makes the molecule strongly reducing. – Oscar Lanzi Feb 19 '18 at 13:32
• $\ce{H3PO2}$ not $\ce{H3PO4}$ – A.K. Oct 12 '18 at 3:15
• @A.K. yea I know, that's what I said "only 2 and not 4 oxygens". – Gimelist Oct 12 '18 at 3:30