(i) `F_(3)C-COOH,` (ii) `CH_(3)COOH`, (iii) `C_(6)H_(5)COOH,` (iv) `CH_(3)CH_(2)COOH`
Correct order of `pK_(a)` value is :

Options

A. 1 gt 3 gt 2 gt 4
B. 4 gt 2 gt 3 gt 1
C. 4 gt 3 gt 2 gt 1
D. 1 gt 2 gt 4 gt 3

Answer

To determine the correct order of pKa values for the given compounds, we need to analyze their acidic strengths based on the presence of electron-withdrawing or electron-donating groups. The compounds given are:

1. CF3COOH (Trifluoroacetic acid)
2. CH3COOH (Acetic acid)
3. C6H5COOH (Benzoic acid)
4. CH3CH2COOH (Propanoic acid)

### Step 1: Understand the relationship between pKa and acidity
- The pKa value is inversely proportional to the acidity of a compound. A lower pKa value indicates a stronger acid.

### Step 2: Identify electron-withdrawing and electron-donating effects
- Electron-withdrawing groups (EWGs) increase acidity by stabilizing the negative charge on the conjugate base.
- Electron-donating groups (EDGs) decrease acidity by destabilizing the negative charge on the conjugate base.

### Step 3: Analyze each compound
1. **CF3COOH**: The trifluoromethyl group (CF3) is a strong electron-withdrawing group due to its -I (inductive) effect. This makes CF3COOH very acidic.
2. **CH3COOH**: The methyl group (CH3) is an electron-donating group (+I effect), which makes acetic acid less acidic than CF3COOH.
3. **C6H5COOH**: The phenyl group (C6H5) has a resonance effect that can stabilize the conjugate base, but it is less effective than the -I effect of CF3. Thus, benzoic acid is more acidic than acetic acid but less acidic than trifluoroacetic acid.
4. **CH3CH2COOH**: The ethyl group (CH3CH2) is also an electron-donating group (+I effect), making propanoic acid the least acidic of the four.

### Step 4: Order the compounds based on acidity
- Based on the analysis:
- CF3COOH is the most acidic (lowest pKa).
- C6H5COOH is next (higher pKa than CF3COOH but lower than CH3COOH).
- CH3COOH follows.
- CH3CH2COOH is the least acidic (highest pKa).

### Step 5: Write the order of pKa values
- The correct order of pKa values from lowest to highest is:
1. CF3COOH (lowest pKa)
2. C6H5COOH
3. CH3COOH
4. CH3CH2COOH (highest pKa)

### Final Answer
The correct order of pKa values is:
**CF3COOH < C6H5COOH < CH3COOH < CH3CH2COOH**