Question

The structure of the starting compound P used in the reaction given below is: 

 

• A. A
• B. B
• C. C
• D. D

Hint:

Recognizing named reactions is a key skill. When you see reagents like I\(_2\)/NaOH, Br\(_2\)/NaOH, or NaOCl, immediately think of the haloform reaction. Then, identify the product's R-group and attach a `-CO-CH₃` to it to find the reactant.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
We are given the product of a reaction and the reagents used. We need to deduce the structure of the starting material P.
Step 2: Key Formula or Approach:
The reagents are 1. NaOCl (sodium hypochlorite) and 2. H\(_3\)O\(^+\) (acid workup). This set of reagents is used for the haloform reaction. The haloform reaction is a characteristic reaction of methyl ketones (compounds containing the R-C(=O)-CH\(_3\) group) or alcohols that can be oxidized to methyl ketones. The reaction converts a methyl ketone into a carboxylate salt and a haloform (in this case, chloroform, CHCl\(_3\)). The subsequent acid workup protonates the carboxylate to give a carboxylic acid. \[ \text{R-C(=O)CH}_3 \xrightarrow{\text{1. NaOCl}} \text{R-COO}^- + \text{CHCl}_3 \xrightarrow{\text{2. H}_3\text{O}^+} \text{R-COOH} \] Step 3: Detailed Explanation:
We can identify the structure of the starting material P by working backward from the product. The given product is an \(\alpha,\beta\)-unsaturated carboxylic acid. \[ \text{Product} = \text{CH}_3\text{-CH=C(CH}_3)\text{-COOH} \] This product has the structure R-COOH, where the 'R' group is: \[ \text{R} = \text{CH}_3\text{-CH=C(CH}_3)\text{-} \] Since the haloform reaction converts a starting material of the form R-C(=O)CH\(_3\) into R-COOH, the starting material P must have had this 'R' group attached to a methyl ketone group (-C(=O)CH\(_3\)). Let's construct the structure of P: \[ \text{P} = \underbrace{\text{CH}_3\text{-CH=C(CH}_3)\text{-}}_{\text{R group}} \underbrace{\text{C(=O)CH}_3}_{\text{Methyl ketone group}} \] The starting material P is 3,4-dimethylpent-3-en-2-one.
Step 4: Final Answer:
Comparing this deduced structure with the options, it matches the structure in option (A).