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 aldehydes and ketones towards HCN addition, we need to consider two main factors: the inductive effect and the steric effect. Here’s a step-by-step solution:

### Step 1: Identify the Compounds
We have three compounds to consider:
1. Formaldehyde (an aldehyde)
2. Acetone (a ketone)
3. Phenyl ketone (an asymmetric ketone)

### Step 2: Understand the Nucleophilic Addition Reaction
In nucleophilic addition reactions, the nucleophile (CN⁻ from HCN) attacks the carbonyl carbon (C=O) of the aldehyde or ketone. The reactivity of these compounds towards HCN depends on:
- The positive character of the carbonyl carbon (inductive effect)
- The steric hindrance around the carbonyl carbon (steric effect)

### Step 3: Analyze the Inductive Effect
- **Formaldehyde** has no alkyl groups attached, making the carbonyl carbon more positive due to the absence of electron-donating groups.
- **Acetone** has two methyl groups, which are weakly electron-donating. This reduces the positive character of the carbonyl carbon compared to formaldehyde.
- **Phenyl ketone** has a phenyl group, which is a strong electron-withdrawing group due to resonance. This increases the positive character of the carbonyl carbon more than in acetone.

### Step 4: Analyze the Steric Effect
- **Formaldehyde** has the least steric hindrance as it has no bulky groups.
- **Acetone** has two methyl groups, which provide some steric hindrance but are smaller than a phenyl group.
- **Phenyl ketone** has a bulky phenyl group, which creates significant steric hindrance, making it less reactive towards nucleophiles.

### Step 5: Combine the Effects
Considering both the inductive and steric effects:
1. **Formaldehyde** is the most reactive due to its high positive character and low steric hindrance.
2. **Acetone** is less reactive than formaldehyde due to the electron-donating methyl groups and moderate steric hindrance.
3. **Phenyl ketone** is the least reactive because of the strong steric hindrance from the phenyl group, despite having a relatively positive carbonyl carbon.

### Conclusion: Increasing Order of Reactivity
Thus, the increasing order of reactivity towards HCN addition is:
**Phenyl ketone < Acetone < Formaldehyde**