The correct order of decreasing acid strength of trichloroacetic acid (A), trifluoroacetic acid
(B) acetic acid
(C) and formic acid
(D) is:

Options

A. `AgtBgtCgtD`
B. `AgtCgtBgtD`
C. `BgtAgtDgtC`
D. `BgtDgtCgtA`

Answer

To determine the correct order of decreasing acid strength among trichloroacetic acid (A), trifluoroacetic acid (B), acetic acid (C), and formic acid (D), we need to analyze the effects of substituents on the acidity of these carboxylic acids.

### Step-by-Step Solution:

1. **Identify the Acids and Their Structures**:
- Trichloroacetic acid (A): Cl₃C-COOH
- Trifluoroacetic acid (B): F₃C-COOH
- Acetic acid (C): CH₃-COOH
- Formic acid (D): HCOOH

2. **Understand Acid Strength**:
The strength of a carboxylic acid is influenced by the stability of its conjugate base (the carboxylate ion formed after deprotonation). The more stable the conjugate base, the stronger the acid.

3. **Consider Electron-Withdrawing Groups (EWGs)**:
- Both trichloroacetic acid and trifluoroacetic acid have strong electron-withdrawing groups (Cl and F, respectively).
- Fluorine is a stronger electron-withdrawing group than chlorine due to its higher electronegativity.

4. **Analyze Each Acid**:
- **Trifluoroacetic acid (B)**: The trifluoro group (F₃) significantly stabilizes the conjugate base due to its strong -I (inductive) effect, making this acid very strong.
- **Trichloroacetic acid (A)**: The trichloro group (Cl₃) also stabilizes the conjugate base, but not as effectively as trifluoroacetic acid. Therefore, it is less acidic than trifluoroacetic acid.
- **Formic acid (D)**: This acid has no strong electron-withdrawing groups, and its conjugate base is less stable compared to A and B.
- **Acetic acid (C)**: The methyl group (CH₃) is an electron-donating group (+I effect), which destabilizes the conjugate base, making acetic acid weaker than both formic acid and the other two acids.

5. **Rank the Acids**:
Based on the analysis:
- Trifluoroacetic acid (B) is the strongest acid.
- Trichloroacetic acid (A) is next.
- Formic acid (D) is stronger than acetic acid (C).

Thus, the order of decreasing acid strength is:
**B > A > D > C**

### Final Answer:
The correct order of decreasing acid strength is:
**Trifluoroacetic acid (B) > Trichloroacetic acid (A) > Formic acid (D) > Acetic acid (C)**.