Give the decreasing order of hydrate formation of the following.
I. HCHO II. PhCHO
III. Chloral IV. Acetone

Options

A. `(III) gt (I) gt (II) gt (IV)`
B. `(IV) gt (II) gt (I) gt (III)`
C. `(I) gt (III) gt (II) gt (IV)`
D. `(IV) gt (II) gt (III) gt (I)`

Explanation

To determine the decreasing order of hydrate formation for the compounds given (I. HCHO, II. PhCHO, III. Chloral, IV. Acetone), we need to analyze the nucleophilic addition reactions of these carbonyl compounds. The formation of hydrates occurs when a nucleophile attacks the electrophilic carbon atom of the carbonyl group. The extent of this reaction depends on the positive charge on the carbonyl carbon, which is influenced by the substituents attached to it.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- HCHO (Formaldehyde)
- PhCHO (Benzaldehyde)
- Chloral (CCl₃CHO)
- Acetone (CH₃COCH₃)

2. **Understand Nucleophilic Addition**:
- Nucleophilic addition occurs at the carbonyl carbon, which is electrophilic due to the partial positive charge created by the electronegativity of the oxygen atom.

3. **Evaluate Electron-Withdrawing and Electron-Donating Effects**:
- **Formaldehyde (HCHO)**: No significant electron-withdrawing or donating groups. It has a moderate positive charge on the carbonyl carbon.
- **Benzaldehyde (PhCHO)**: The phenyl group (Ph) has a -I (inductive) effect, which withdraws electron density from the carbonyl carbon, increasing its positive charge slightly compared to formaldehyde.
- **Chloral (CCl₃CHO)**: The three chlorine atoms are strong electron-withdrawing groups. They significantly increase the positive charge on the carbonyl carbon, making it very electrophilic and enhancing nucleophilic addition.
- **Acetone (CH₃COCH₃)**: The two methyl groups are electron-donating (due to +I effect), which reduces the positive charge on the carbonyl carbon, making it less electrophilic and less favorable for nucleophilic addition.

4. **Rank the Compounds Based on Electrophilicity**:
- **Chloral**: Highest electrophilicity due to strong electron-withdrawing effect from three chlorine atoms.
- **Formaldehyde**: Moderate electrophilicity; better than benzaldehyde due to less steric hindrance.
- **Benzaldehyde**: Slightly less electrophilic than formaldehyde due to the presence of the phenyl group.
- **Acetone**: Lowest electrophilicity due to electron-donating methyl groups.

5. **Final Order of Hydrate Formation**:
- **Chloral > Formaldehyde > Benzaldehyde > Acetone**

### Conclusion:
The decreasing order of hydrate formation is:
**III. Chloral > I. HCHO (Formaldehyde) > II. PhCHO (Benzaldehyde) > IV. Acetone**