The correct order of reactivity of PhMgBr with
`underset((I))(Ph-underset(C)overset(O)overset(||)(C)-Ph)" " underset((II))(CH_(3)-overset(O)overset(||)(C)-H)" " underset((III))(CH_(3)-overset(O)overset(||)(C)-CH_(3))` is

Options

A. (I) > (II) > (III)
B. (III) > (II) > (I)
C. (II) > (III) > (I)
D. (I) >(III) >(II)

Explanation

To determine the correct order of reactivity of PhMgBr (a Grignard reagent) with the three given compounds, we need to analyze the steric hindrance and the structure of each compound.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Compound I: Ph-C(=O)-Ph (Diphenyl ketone)
- Compound II: CH3-C(=O)-H (Acetaldehyde)
- Compound III: CH3-C(=O)-CH3 (Acetone)

2. **Understand the Role of Grignard Reagent**:
- Grignard reagents (like PhMgBr) are strong nucleophiles. They react with electrophiles, particularly carbonyl groups (C=O), to form alcohols.

3. **Evaluate Steric Hindrance**:
- The reactivity of Grignard reagents decreases with increasing steric hindrance around the carbonyl carbon. More bulky groups around the carbonyl carbon will hinder the approach of the nucleophile.

4. **Analyze Each Compound**:
- **Compound I (Ph-C(=O)-Ph)**: This compound has two bulky phenyl groups attached to the carbonyl carbon, leading to significant steric hindrance. Therefore, it will be the least reactive towards PhMgBr.
- **Compound II (CH3-C(=O)-H)**: This compound has one methyl group and one hydrogen attached to the carbonyl carbon. The steric hindrance is moderate, making it more reactive than Compound I.
- **Compound III (CH3-C(=O)-CH3)**: This compound has two methyl groups attached to the carbonyl carbon. While it has some steric hindrance, it is less than that of Compound I (which has two phenyl groups). Therefore, it will be the most reactive towards PhMgBr.

5. **Determine the Order of Reactivity**:
- Based on the analysis, the order of reactivity from highest to lowest is:
- Compound II (CH3-C(=O)-H) > Compound III (CH3-C(=O)-CH3) > Compound I (Ph-C(=O)-Ph)

### Final Order of Reactivity:
- **Correct Order**: II > III > I