What is the increasing order of reactivity of the following towards HCN addition?

Options

A. `PhCOCH_(3)ltCH_(3)COCH_(3)ltHCHO`
B. `PhCOCH_(3)ltHCHOltCH_(3)COCH_(3)`
C. `HCHOltCH_(3)COCH_(3)ltPhCOCH_(3)`
D. `HCHOltPhCOCH_(3)ltCH_(3)COCH_(3)`

Answer

To determine the increasing order of reactivity of the given compounds towards HCN addition, we need to analyze the electrophilicity of the carbonyl carbon in each compound. The more electrophilic the carbonyl carbon is, the more reactive it will be towards nucleophilic attack by HCN.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's denote the compounds as follows:
- Compound 1: HCHO (formaldehyde)
- Compound 2: CH3CHO (acetaldehyde)
- Compound 3: PhCHO (benzaldehyde)
- Compound 4: CH3COCH3 (acetone)

2. **Understand Electrophilicity**:
The carbonyl carbon (C=O) is electrophilic due to the presence of the electronegative oxygen atom, which pulls electron density away from the carbon, creating a partial positive charge on the carbon. The greater the positive charge on the carbon, the more reactive it will be towards nucleophiles like HCN.

3. **Analyze Each Compound**:
- **HCHO (Formaldehyde)**: This compound has no alkyl groups attached to the carbonyl carbon, making it highly electrophilic.
- **CH3CHO (Acetaldehyde)**: The presence of one methyl group (which has a +I effect) slightly reduces the electrophilicity compared to formaldehyde but still remains quite reactive.
- **PhCHO (Benzaldehyde)**: The phenyl group has a +R effect (resonance) that delocalizes the positive charge, making the carbonyl carbon less electrophilic than in acetaldehyde.
- **CH3COCH3 (Acetone)**: This compound has two methyl groups, which exert a +I effect, significantly decreasing the electrophilicity of the carbonyl carbon.

4. **Order of Electrophilicity**:
Based on the analysis:
- HCHO (most electrophilic)
- CH3CHO
- PhCHO
- CH3COCH3 (least electrophilic)

5. **Increasing Order of Reactivity**:
Therefore, the increasing order of reactivity towards HCN addition is:
\[ \text{CH}_3\text{COCH}_3 < \text{PhCHO} < \text{CH}_3\text{CHO} < \text{HCHO} \]

### Final Answer:
The increasing order of reactivity of the compounds towards HCN addition is:
\[ \text{CH}_3\text{COCH}_3 < \text{PhCHO} < \text{CH}_3\text{CHO} < \text{HCHO} \]