Question

Thionyl chloride on reaction with white phosphorus gives a compound of phosphorus 'C' which on hydrolysis gives an oxo acid 'O'. The correct statements about C and O are
I. Shape of 'C' is pyramidal
II. 'O' is a dibasic acid
III. 'O' is a monobasic acid
IV. 'C' on reaction with acetic acid gives 'O'

• A. I \& II only
• B. II \& IV only
• C. I, III \& IV only
• D. I, II \& IV only

Hint:

The basicity of phosphorus oxoacids is a common point of confusion. It's determined by the number of P-OH bonds, not the total number of H atoms. Draw the structures to be sure: - H₃PO₂ (Hypophosphorous): 1 P-OH bond \(\rightarrow\) monobasic. - H₃PO₃ (Phosphorous): 2 P-OH bonds \(\rightarrow\) dibasic. - H₃PO₄ (Phosphoric): 3 P-OH bonds \(\rightarrow\) tribasic.

Answer 1

The Correct Option is D

Step 1: Identifying Compounds C and O:
- Reaction 1: White phosphorus (P₄) reacts with thionyl chloride (SOCl₂) to produce phosphorus trichloride (PCl₃) as the main phosphorus-containing product.
\[ \text{P}_4 + 8\text{SOCl}_2 \rightarrow 4\text{PCl}_3 + 4\text{SO}_2 + 2\text{S}_2\text{Cl}_2 \] So, compound 'C' is PCl₃.
- Reaction 2: Phosphorus trichloride (PCl₃) undergoes hydrolysis with water to form phosphorous acid (or phosphonic acid) and HCl.
\[ \text{PCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{H}_3\text{PO}_3 + 3\text{HCl} \] So, the oxo acid 'O' is H₃PO₃.
Step 2: Evaluating the Statements:
- I. Shape of 'C' is pyramidal: 'C' is PCl₃. The central phosphorus atom has 5 valence electrons. It forms 3 single bonds with chlorine and has one lone pair of electrons remaining. According to VSEPR theory, a molecule with 3 bond pairs and 1 lone pair (AX₃E₁) has a trigonal pyramidal shape. So, statement I is correct.
- II. 'O' is a dibasic acid: 'O' is H₃PO₃ (phosphorous acid). In oxoacids of phosphorus, only the hydrogen atoms attached directly to oxygen atoms are acidic. The structure of H₃PO₃ contains two P-OH bonds and one P-H bond. Since it has two ionizable hydrogen atoms, it is a dibasic acid. So, statement II is correct.
- III. 'O' is a monobasic acid: This is incorrect. As established above, H₃PO₃ is dibasic. (Hypophosphorous acid, H₃PO₂, is monobasic). So, statement III is incorrect.
- IV. 'C' on reaction with acetic acid gives 'O': 'C' (PCl₃) reacts with carboxylic acids like acetic acid (CH₃COOH) to form an acid chloride (acetyl chloride) and phosphorous acid.
\[ 3\text{CH}_3\text{COOH} + \text{PCl}_3 \rightarrow 3\text{CH}_3\text{COCl} + \text{H}_3\text{PO}_3 \] The reaction indeed produces 'O' (H₃PO₃) as a byproduct. So, statement IV is correct.
Step 3: Final Conclusion:
The correct statements are I, II, and IV.
Step 4: Final Answer:
The combination of correct statements is I, II, and IV. This corresponds to option (D).