The increasing order of the rate of HCN addition to compound A-D is
(A) HCHO
(B) `CH_(3)COOH_(3)`
( C) `PhCOCH_(3)`
(D) `PhCOPh`

Options

A. `A lt B lt C lt D`
B. `D lt B lt C lt A`
C. `D lt C lt B lt A`
D. `C lt D lt B lt A`

Answer

To determine the increasing order of the rate of HCN addition to the given compounds (A-D), we need to analyze the structure of each compound and the effect of substituents on the carbonyl carbon's electrophilicity.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- (A) HCHO (formaldehyde)
- (B) CH₃COOH (acetic acid)
- (C) PhCOCH₃ (acetophenone)
- (D) PhCOPh (benzophenone)

2. **Understand the Mechanism**:
- The reaction involves HCN acting as a nucleophile that attacks the electrophilic carbon of the carbonyl group (C=O).
- The rate of this nucleophilic addition reaction depends on the electrophilicity of the carbonyl carbon, which is influenced by the substituents attached to it.

3. **Analyze the Electrophilicity**:
- **Formaldehyde (A)**: Has no electron-donating groups, making the carbonyl carbon highly electrophilic. Thus, it reacts quickly with HCN.
- **Acetic Acid (B)**: The -COOH group is an electron-withdrawing group, but the -CH₃ group is weakly electron-donating. The overall effect is that the carbonyl carbon is less electrophilic than in formaldehyde but more than in ketones.
- **Acetophenone (C)**: The phenyl group (Ph) is a weak electron-donating group, which decreases the electrophilicity of the carbonyl carbon compared to acetic acid.
- **Benzophenone (D)**: Both phenyl groups are electron-donating, making the carbonyl carbon the least electrophilic among the four compounds. Therefore, it reacts the slowest with HCN.

4. **Order of Reactivity**:
- Based on the analysis, the increasing order of the rate of HCN addition is:
- (D) PhCOPh < (C) PhCOCH₃ < (B) CH₃COOH < (A) HCHO

5. **Final Answer**:
- The increasing order of the rate of HCN addition to compounds A-D is:
- (D) < (C) < (B) < (A)