A carbonyl compound (P) which gives positive iodoform test, undergoes reaction with MeMgBr followed by dehydration to give an olefin (Q). Ozonolysis of (Q) leads to a dicarbonyl compound (R ) which undergors intramolecular aldol reaction to give predominantly (S).
`Poverset(1. MeMgBr)underset{:(2.H^(+)","H_(2)O),(3.H_(2)SO_(4)","Delta):}rarr Q underset(2. Zn"," H_(2)O)overset(1. O_(3))rarr R underset(2. Delta)overset(1. OH^(-))rarr S`
The structure of products (Q) and (R ), respectively, are:

Options

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B.
C.
D.

Answer

To solve the problem step by step, we need to identify the structures of the compounds P, Q, R, and S based on the given reactions and tests.

### Step 1: Identify Compound P
- **Given Information**: Compound P is a carbonyl compound that gives a positive iodoform test.
- **Analysis**: The iodoform test is positive for compounds that have a methyl ketone (a carbonyl group adjacent to a CH3 group) or ethanol. Therefore, P must be a methyl ketone.
- **Structure of P**: The simplest methyl ketone is acetone (CH3COCH3).

### Step 2: Reaction of P with MeMgBr
- **Reaction**: P (acetone) reacts with methyl magnesium bromide (MeMgBr), which is a Grignard reagent.
- **Mechanism**: The Grignard reagent adds to the carbonyl carbon of acetone, forming a tertiary alcohol after hydrolysis.
- **Product Q**: The product after the reaction and subsequent dehydration (removal of water) will be an alkene. In this case, the product will be 2-methylpropene (isobutylene).

### Step 3: Ozonolysis of Q
- **Reaction**: Q (2-methylpropene) undergoes ozonolysis, which cleaves the double bond and forms carbonyl compounds.
- **Product R**: The ozonolysis of 2-methylpropene will yield two carbonyl compounds: acetaldehyde (CH3CHO) and acetone (CH3COCH3). However, since we are looking for a dicarbonyl compound, we can consider the product to be 3-oxobutan-2-one (which has two carbonyl groups).

### Step 4: Intramolecular Aldol Reaction of R
- **Reaction**: The dicarbonyl compound R (3-oxobutan-2-one) undergoes an intramolecular aldol reaction.
- **Mechanism**: In this reaction, one of the carbonyl groups acts as an electrophile, while the adjacent carbon with an alpha hydrogen acts as a nucleophile, leading to the formation of a cyclic compound.
- **Product S**: The predominant product S from this reaction will be 2,5-dimethyl-2,5-dihydro-3-furanone.

### Summary of Products
- **P**: Acetone (CH3COCH3)
- **Q**: 2-methylpropene (isobutylene)
- **R**: 3-oxobutan-2-one (dicarbonyl compound)
- **S**: 2,5-dimethyl-2,5-dihydro-3-furanone

### Final Answer
The structures of products (Q) and (R) are:
- **Q**: 2-methylpropene (isobutylene)
- **R**: 3-oxobutan-2-one