Assertion: `PCl_(3)` on hydrolysis gives `OH-underset(H)underset(|)overset(O)overset(||)(P)-OH` and not `OH-underset(OH)underset(|)(P)-OH` as major product.
Reason: `H_(3)PO_(3)` exists in two tautomeric forms `:OH-underset(OH)underset(|)(P)-OH hArr HO-underset(OH)underset(|)overset(O)overset(||)(P)-OH`

Options

A. If both (A) and (R) are true and (R) is the correct explanation of (A)
B. if both (A) and (R) are true but (R) is not correct explanation of (A)
C. If (A) is true but (R) is false
D. If (A) is false but (R) is false

Answer

To solve the question, we need to analyze both the assertion and the reason provided:

### Step 1: Understand the Assertion
The assertion states that PCl₃ on hydrolysis gives a specific form of phosphorous acid, denoted as \( \text{OH} \underset{(H)}{\underset{|}{(P)}} \overset{(O)}{\overset{||}{(P)}} - \text{OH} \), and not the other tautomer \( \text{OH} \underset{(OH)}{\underset{|}{(P)}} - \text{OH} \) as the major product.

**Hint:** Identify the structural differences between the two forms of phosphorous acid and understand the hydrolysis process of PCl₃.

### Step 2: Hydrolysis of PCl₃
When PCl₃ reacts with water, it undergoes hydrolysis to form phosphorous acid (H₃PO₃) and hydrochloric acid (HCl). The reaction can be represented as:
\[ \text{PCl}_3 + 3 \text{H}_2\text{O} \rightarrow \text{H}_3\text{PO}_3 + 3 \text{HCl} \]

**Hint:** Recall the role of water in hydrolysis and how it leads to the formation of H₃PO₃.

### Step 3: Identify the Major Product
The major product formed during the hydrolysis is the form of phosphorous acid where phosphorus is in the +5 oxidation state. This is because the oxidation state of phosphorus in the tautomer \( \text{OH} \underset{(H)}{\underset{|}{(P)}} \overset{(O)}{\overset{||}{(P)}} - \text{OH} \) is more stable than in the other tautomer, which has phosphorus in the +3 oxidation state.

**Hint:** Consider the stability of oxidation states and how they influence the predominant form of the product.

### Step 4: Understand the Reason
The reason states that H₃PO₃ exists in two tautomeric forms. The two forms are:
1. \( \text{OH} \underset{(OH)}{\underset{|}{(P)}} - \text{OH} \) (with phosphorus in +3 oxidation state)
2. \( \text{OH} \underset{(H)}{\underset{|}{(P)}} \overset{(O)}{\overset{||}{(P)}} - \text{OH} \) (with phosphorus in +5 oxidation state)

**Hint:** Recognize the concept of tautomerism and how it applies to the structure of phosphorous acid.

### Step 5: Conclusion
Both the assertion and the reason are correct. The assertion correctly identifies the major product of the hydrolysis of PCl₃, and the reason accurately describes the existence of two tautomeric forms of H₃PO₃. Therefore, the reason is a correct explanation of the assertion.

**Final Answer:** Both assertion and reason are correct, and the reason is the correct explanation of the assertion.