Question

Which one of the following has the maximum number of P–OH bonds?

• A. H$_3$PO$_2$
• B. H$_3$PO$_4$
• C. H$_3$PO$_3$
• D. H$_4$P$_2$O$_5$
• E. H$_4$P$_2$O$_6$

Hint:

Always draw or recall the structure of phosphorus oxyacids: Count only OH groups for acidity — ignore P–H bonds completely.

Answer 1

Answer: (E)

Concept: The number of \(\text{P–OH}\) bonds in oxyacids of phosphorus corresponds to the number of ionisable (acidic) hydrogens. Only those hydrogens which are directly bonded to oxygen atoms (\(-OH\) groups) are counted as P–OH bonds. Hydrogens directly bonded to phosphorus (P–H) are non-acidic and are not counted. Very important rule:
• P–OH bonds → acidic hydrogens (count these)
• P–H bonds → non-acidic (do NOT count)

Step 1: Analyse each compound one by one using structure. (A) H$_3$PO$_2$ (Hypophosphorous acid):
Structure: \[ \text{H–P(=O)(OH)H} \] Here:
• One OH group → 1 P–OH bond
• Two P–H bonds (non-acidic) \[ \Rightarrow \text{Number of P–OH bonds} = 1 \] (B) H$_3$PO$_4$ (Orthophosphoric acid):
Structure: \[ \text{P(=O)(OH)}_3 \] Here:
• Three OH groups → 3 P–OH bonds
• No P–H bonds \[ \Rightarrow \text{Number of P–OH bonds} = 3 \] (C) H$_3$PO$_3$ (Phosphorous acid):
Structure: \[ \text{H–P(=O)(OH)}_2 \] Here:
• Two OH groups → 2 P–OH bonds
• One P–H bond \[ \Rightarrow \text{Number of P–OH bonds} = 2 \] (D) H$_4$P$_2$O$_5$ (Pyrophosphorous acid):
Structure contains two phosphorus atoms connected via oxygen. Typical structure gives:
• Total OH groups = 2
• Remaining hydrogens are P–H \[ \Rightarrow \text{Number of P–OH bonds} = 2 \] (E) H$_4$P$_2$O$_6$ (Hypophosphoric acid):
Structure: \[ \text{(HO)$_2$P–P(OH)$_2$} \] Here:
• Each phosphorus has 2 OH groups
• Total OH groups = 4 \[ \Rightarrow \text{Number of P–OH bonds} = 4 \]

Step 2: Compare all values. \[ \begin{aligned} H_3PO_2 &\rightarrow 1 \\ H_3PO_3 &\rightarrow 2 \\ H_3PO_4 &\rightarrow 3 \\ H_4P_2O_5 &\rightarrow 2 \\ H_4P_2O_6 &\rightarrow 4 \end{aligned} \] Clearly, maximum number of P–OH bonds is: \[ 4 \]

Step 3: Final conclusion. The compound having maximum P–OH bonds is: \[ \boxed{H_4P_2O_6} \]