Q.Draw the structure of H3PO2 and account for its reducing character.
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Oxoacids of Phosphorus – From Intuition to Precision
Think of phosphorus as an atom that loves to bond with oxygen. But unlike carbon or nitrogen, phosphorus has a quirk: it can form P–H bonds that are unusually stable. That single fact is the key to understanding why oxoacids of phosphorus behave so differently from, say, oxoacids of sulphur or chlorine.
When you see an oxoacid of phosphorus, the first question is not "how many hydrogens does it have?" but "how many of those hydrogens are attached to oxygen, and how many are attached directly to phosphorus?" That distinction decides everything — the acid's basicity, its reducing power, and its structure.
The core idea: P–OH vs P–H
In an oxoacid, the acidic hydrogens are always the ones on –OH groups. A hydrogen attached directly to phosphorus (P–H) is not acidic — it does not ionise in water. So the basicity of a phosphorus oxoacid is simply the number of P–OH groups, not the total number of hydrogens.
Take two famous examples:
- H3PO4 (orthophosphoric acid) — all three hydrogens are on oxygen: HO–P(=O)(OH)2. Basicity = 3.
- H3PO3 (phosphorous acid) — only two hydrogens are on oxygen; the third is directly on phosphorus: HP(=O)(OH)2. Basicity = 2.
A common mistake is to assume H3PO3 is tribasic because it has three hydrogens. It is dibasic — the P–H hydrogen never dissociates.
The structural pattern
All oxoacids of phosphorus share a common skeleton: a central phosphorus atom with at least one P=O double bond and one or more P–OH groups. The remaining valency is satisfied by either P–H bonds or additional P–OH groups.
Here is the family in order of increasing oxidation state of phosphorus:
| Acid | Formula | Oxidation state of P | Number of P–OH | Number of P–H | Basicity |
|---|---|---|---|---|---|
| Hypophosphorous | H3PO2 | +1 | 1 | 2 | 1 |
| Phosphorous | H3PO3 | +3 | 2 | 1 | 2 |
| Orthophosphoric | H3PO4 | +5 | 3 | 0 | 3 |
| Pyrophosphoric | H4P2O7 | +5 | 4 | 0 | 4 |
| Hypophosphoric | H4P2O6 | +4 | 4 | 0 | 4 |
As the oxidation state increases, P–H bonds disappear. Only the lower oxoacids (H3PO2 and H3PO3) have P–H bonds. These are also good reducing agents — the P–H bond is easily oxidised.
Why does this happen?
Phosphorus in its +3 or +1 oxidation state is not fully oxidised. It can still be oxidised further, and the P–H bond provides a ready source of electrons. That is why H3PO3 reduces Ag+ to silver metal, while H3PO4 does not.
The structure also explains the dibasic nature of H3PO3 experimentally. Titrate it with NaOH: you get only two equivalence points, not three. The third hydrogen simply refuses to leave.
Basicity of a phosphorus oxoacid = number of P–OH groups.
Never count total hydrogens. Always draw the structure first.
How to draw the structures (the quick way) …
Hypophosphorous acid, H3PO2, has phosphorus bonded to one =O, one -OH group, and two hydrogen atoms attached directly to the phosphorus atom itself.
…
H3PO2 (hypophosphorous acid) has two direct P-H bonds; these are the source of its strong reducing character.
Structure of H3PO2: the central P atom is tetrahedrally surrounded by one =O (double bond), one -OH group, and two -H atoms bonded directly to phosphorus:
O
‖
H — P — OH
|
H
Of the three hydrogen atoms in the molecular formula H3PO2, only ONE (the one on -OH) is acidic/ionisable; the molecule is therefore MONOBASIC, even though its formula suggests three replaceable hydrogens.
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- CBSE 2026Set A1 markMCQQ.The extensive use of P4O10 is done as(a) Reducing agent(b) Preservative(c) Oxidising agent(d) Dehydrating agent
›Reveal solutionSolution
P4O10 is one of the strongest dehydrating agents; it even removes water from many oxoacids.
Phosphorus(V) oxide, P4O10, has an intense affinity for water:
P4O10 + 6 H2O -> 4 H3PO4
Because of this it is used to dry gases and to abstract chemically combined water, e.g. it converts HNO3 to N2O5 and H2SO4 to SO3:
…
- CBSE 2025Set D1 markMCQQ.Which of the following is the structural formula of hypophosphorous acid?(a) HO-P with H (top), =O and H (i.e. P bonded to OH, H, =O, H)(b) HO-P with =O (top), H and OH (i.e. P bonded to OH, =O, H, OH)(c) HO-P with =O (top), OOH and OH (i.e. P bonded to OH, =O, OOH, OH)(d) HO-P with =O (top), OH and OH (i.e. P bonded to OH, =O, OH, OH)
›Reveal solutionSolution
Hypophosphorous acid H3PO2 has P bonded to =O, one -OH and two P-H bonds (option A).
Hypophosphorous acid has the formula H3PO2. Around the central phosphorus atom the bonding is:
- one P=O double bond,
- one P-OH group (the single ionisable/acidic hydrogen),
- two P-H bonds (hydrogen bonded directly to phosphorus). …
- CBSE 2024Set D1 markMCQQ.Which of the following is the molecular formula of Orthophosphoric acid?(a) H3PO3(b) H3PO4(c) HPO3(d) H4P2O7
›Reveal solutionSolution
Orthophosphoric acid is H3PO4 (phosphorus +5, three replaceable OH hydrogens).
Common oxoacids of phosphorus:
- H3PO3 = orthophosphorous acid (P in +3)
- H3PO4 = orthophosphoric acid (P in +5)
- HPO3 = metaphosphoric acid …
- CBSE 2024Set ANNUAL1 markMCQQ.Phosphonic acid (H3PO3) is -(a) monobasic(b) dibasic(c) tribasic(d) tetrabasic
›Reveal solutionSolution
Only the H atoms bonded to O (via O-H bonds) in an oxoacid are ionizable; H atoms bonded directly to P are not, so basicity = number of O-H bonds, not the total H count.
Phosphonic acid (also called phosphorous acid), H3PO3, has the structure:
HP(=O)(OH)2
It has:
- 1 P–H bond (directly bonded hydrogen — not ionizable, since P-H is a non-polar/weakly polar covalent bond that does not release H+ in water)
- 2 P–O–H bonds (these hydrogens ARE ionizable as H+) …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following acids is a tribasic acid?(a) H3PO3(b) H3PO2(c) HPO3(d) H3PO4
›Reveal solutionSolution
Basicity of an oxoacid equals the number of ionizable (-OH bonded) hydrogens, not the total hydrogen atoms in the formula.
- H3PO4 (orthophosphoric acid): P bonded to 3 -OH groups and one =O; all 3 H are ionizable -> tribasic.
- H3PO3 (phosphorous acid): P bonded to 2 -OH, one =O, and one H directly on P (non-ionizable) -> dibasic. …
- CBSE 2022Set ANNUAL1 markQ.How do you account for the reducing behaviour of H3PO2 on the basis of its structure?
›Reveal solutionSolution
Hypophosphorous acid's structure contains two direct P–H bonds in addition to one P–OH and one P=O; because P–H bonds (unlike P–OH) are easily oxidised, this structural feature makes H3PO2 a strong reducing agent.
Structure of H3PO2
Despite its formula suggesting three replaceable/ionisable hydrogens, structure determination shows hypophosphorous acid has the central P atom bonded to: one =O (double bond), one −OH group, and two direct P−H bonds. Only the hydrogen on the −OH group is acidic/ionisable (attached to the highly electronegative oxygen), so H3PO2 is actually monobasic, not tribasic, even though its formula contains 3 hydrogens.
Why this structure causes reducing behaviour
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