The product of following reaction is: $CH_{3}CH_{2}CH_{2}COOH\xrightarrow[ (ii)H_{2}O]{(i)Cl_{2}/red P}$

Question

Accepted Answer

The reaction in question involves the conversion of butanoic acid ($CH_3CH_2CH_2COOH$) using chlorine ($Cl_2$) in the presence of red phosphorus, followed by hydrolysis with water ($H_2O$). This is a classic example of the Hell-Volhard-Zelinsky (HVZ) reaction, which is used to halogenate carboxylic acids at the alpha position.

1. **Explanation for Option 1 (2-chlorobutanoic acid):**
   - The HVZ reaction specifically targets the alpha carbon of a carboxylic acid. In butanoic acid, the alpha carbon is the one adjacent to the carboxyl group ($COOH$). 
   - The reaction proceeds with the formation of an acyl chloride intermediate, which is then halogenated at the alpha position. The presence of red phosphorus facilitates the formation of the acyl chloride.
   - After halogenation, hydrolysis with water regenerates the carboxylic acid, now with a chlorine atom at the alpha position.
   - Therefore, the product is 2-chlorobutanoic acid, where the chlorine is attached to the second carbon of the butanoic acid chain.

2. **Explanation for why other options are incorrect:**
   - **Option 2 (3-chlorobutanoic acid):** This would imply that the chlorine is attached to the third carbon in the chain. However, the HVZ reaction specifically targets the alpha carbon, not the beta or gamma carbons. Therefore, 3-chlorobutanoic acid is not the product of this reaction.
   - **Option 3 (4-chlorobutanoic acid):** This suggests that the chlorine is attached to the fourth carbon, which is the terminal carbon in the chain. The HVZ reaction does not affect the terminal carbon; it specifically halogenates the alpha position. Thus, 4-chlorobutanoic acid cannot be the product.
   - **Option 4 (Butanoyl chloride):** This option suggests the formation of an acyl chloride without further reaction. While an acyl chloride intermediate is formed during the HVZ reaction, the final step involves hydrolysis, which converts the acyl chloride back to a carboxylic acid with the halogen at the alpha position. Therefore, butanoyl chloride is not the final product.

In conclusion, the correct product of the given reaction is 2-chlorobutanoic acid, as supported by the mechanism of the HVZ reaction.