pK, value of trifluoroacetic acid, benzoic acid and acetic acid are respectively

Options

A. 4.76, 4:19 and 0:23
B. 4-19, 4-76 and 0-23
C. 0-23, 4-76 and 4-19
D. 0:23, 4-19 and 4.76

Answer

To determine the pK_a values of trifluoroacetic acid, benzoic acid, and acetic acid, we need to understand the factors that affect the acidity of these compounds. The pK_a value is the negative logarithm of the acid dissociation constant (K_a), and a lower pK_a value indicates a stronger acid.

### Step-by-Step Solution:

1. **Identify the Acids**:
- Trifluoroacetic acid (CF₃COOH)
- Benzoic acid (C₆H₅COOH)
- Acetic acid (CH₃COOH)

2. **Understand the Structure**:
- Trifluoroacetic acid has three fluorine atoms attached to the carbon adjacent to the carboxylic group. The electronegative fluorine atoms increase the acidity by stabilizing the conjugate base (CF₃COO⁻).
- Benzoic acid has an aromatic ring, which can stabilize the conjugate base (C₆H₅COO⁻) through resonance.
- Acetic acid has a methyl group, which is less electronegative and does not stabilize the conjugate base as effectively as the other two acids.

3. **Compare Acidity**:
- **Trifluoroacetic Acid**: The presence of three electronegative fluorine atoms significantly increases the acidity, making it the strongest acid among the three. The conjugate base is highly stabilized due to the inductive effect of the fluorine atoms.
- **Benzoic Acid**: The resonance stabilization of the conjugate base makes benzoic acid more acidic than acetic acid, but less acidic than trifluoroacetic acid.
- **Acetic Acid**: It is the weakest acid among the three due to the electron-donating effect of the methyl group, which destabilizes the conjugate base.

4. **Determine the pK_a Values**:
- Trifluoroacetic acid has the lowest pK_a value (strongest acid).
- Benzoic acid has an intermediate pK_a value.
- Acetic acid has the highest pK_a value (weakest acid).

5. **Final Order of pK_a Values**:
- pK_a (Trifluoroacetic acid) < pK_a (Benzoic acid) < pK_a (Acetic acid)

### Summary of pK_a Values:
- Trifluoroacetic acid: **pK_a ≈ 0.5**
- Benzoic acid: **pK_a ≈ 4.2**
- Acetic acid: **pK_a ≈ 4.76**