Arrange the following in order of their reactivity towards `CH_(3)MgBr`?
(I) `H_(3)C-underset(O)underset(||)(C)-H` (II) `C Cl_(3)-underset(O)underset(||)(C)-H`
(III) `H_(3)C-underset(O)underset(||)(C)-CH_(3)` (IV) `Ph-underset(O)underset(||)(C)-CH_(3)`

Options

A. `IgtIVgtIIIgtII`
B. `IIgtIgtIIIgtIV`
C. `IgtIIgtIIIgtIV`
D. `IgtIIIgtIIgtIV`

Answer

To arrange the given compounds in order of their reactivity towards `CH₃MgBr`, we need to analyze the structure of each compound and the effects of substituents on the electrophilic carbon atom. `CH₃MgBr` acts as a nucleophile due to the presence of the `CH₃⁻` ion.

### Step 1: Identify the compounds
1. **(I)** `H₃C-C(=O)-H` (Acetaldehyde)
2. **(II)** `CCl₃-C(=O)-H` (Trichloroacetaldehyde)
3. **(III)** `H₃C-C(=O)-CH₃` (Acetone)
4. **(IV)** `Ph-C(=O)-CH₃` (Acetophenone)

### Step 2: Analyze the substituents
- **(I)** Acetaldehyde has a hydrogen atom attached to the carbonyl carbon, which does not exert any significant inductive or resonance effects.
- **(II)** Trichloroacetaldehyde has three chlorine atoms attached to the carbon adjacent to the carbonyl group. Chlorine is a strong electron-withdrawing group due to its -I (inductive) effect, which increases the electrophilicity of the carbonyl carbon.
- **(III)** Acetone has two methyl groups attached to the carbonyl carbon. Methyl groups are +I (inductive) groups, which decrease the electrophilicity of the carbonyl carbon.
- **(IV)** Acetophenone has a phenyl group attached to the carbonyl carbon. The phenyl group has a +M (mesomeric) effect, which can stabilize the carbonyl but also decreases its electrophilicity compared to a compound with strong -I groups.

### Step 3: Determine the order of reactivity
- The reactivity of the carbonyl compounds towards `CH₃MgBr` is influenced by the electron-withdrawing or electron-donating effects of the substituents:
- **(II)** Trichloroacetaldehyde will be the most reactive due to the strong -I effect from the three chlorine atoms.
- **(I)** Acetaldehyde will be next as it has no significant electron-donating or withdrawing groups.
- **(IV)** Acetophenone will follow, as the phenyl group has a +M effect, which decreases reactivity.
- **(III)** Acetone will be the least reactive due to the presence of two +I methyl groups.

### Final Order of Reactivity
Thus, the order of reactivity towards `CH₃MgBr` is:
**(II) > (I) > (IV) > (III)**