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

Options

A. `A gt B gt C gt D`
B. `A gt C gt B gt D`
C. `B gt A gt D gt C`
D. `B gt D gt C gt A`

Explanation

To determine the correct order of decreasing acid strength of trichloroacetic acid (A), trifluoroacetic acid (B), acetic acid (C), and formic acid (D), we can analyze the structures and the effects of substituents on their acidity.

### Step-by-Step Solution:

1. **Identify the Structures**:
- **Trichloroacetic Acid (A)**: CCl₃COOH
- **Trifluoroacetic Acid (B)**: CF₃COOH
- **Acetic Acid (C)**: CH₃COOH
- **Formic Acid (D)**: HCOOH

2. **Understand the Inductive Effect**:
- The acidity of a compound is influenced by the stability of its conjugate base. The more stable the conjugate base, the stronger the acid.
- Electron-withdrawing groups (EWGs) increase acidity by stabilizing the negative charge on the conjugate base through the inductive effect.

3. **Analyze the Substituents**:
- **Trichloroacetic Acid (A)** has three chlorine atoms, which are moderate EWGs.
- **Trifluoroacetic Acid (B)** has three fluorine atoms, which are strong EWGs due to their high electronegativity.
- **Acetic Acid (C)** has a methyl group (CH₃), which is an electron-donating group (EDG) and decreases acidity.
- **Formic Acid (D)** has a hydrogen atom as a substituent, which does not donate electrons and is more acidic than acetic acid.

4. **Compare Acidity**:
- **Trifluoroacetic Acid (B)** is the strongest acid due to the strong electron-withdrawing effect of three fluorine atoms.
- **Trichloroacetic Acid (A)** is next, as chlorine is a weaker electron-withdrawing group compared to fluorine.
- **Formic Acid (D)** is stronger than acetic acid (C) because it lacks the electron-donating methyl group and has a hydrogen atom, which does not destabilize the conjugate base.
- **Acetic Acid (C)** is the weakest acid due to the presence of the methyl group, which donates electrons and destabilizes the conjugate base.

5. **Final Order**:
- The order of decreasing acid strength is: **Trifluoroacetic Acid (B) > Trichloroacetic Acid (A) > Formic Acid (D) > Acetic Acid (C)**.

### Conclusion:
The correct order of decreasing acid strength is **B > A > D > C**.