Which of the following is correct order of acidity ?

Question

Accepted Answer

Option 4 is the correct order of acidity: $CF_{3}COOH > CNCH_{2}COOH > HCOOH > CH_{3}COOH$. Let's analyze why this order is correct and why the other options are incorrect.

### Explanation for Option 4:

1. **$CF_{3}COOH$ (Trifluoroacetic acid):**
   - Trifluoroacetic acid is the most acidic among the given compounds. The presence of three highly electronegative fluorine atoms in $CF_{3}COOH$ increases the acidity significantly. These fluorine atoms exert a strong electron-withdrawing effect through the inductive effect, stabilizing the negative charge on the carboxylate ion after deprotonation. This stabilization makes it easier for the acid to donate a proton, thus increasing its acidity.

2. **$CNCH_{2}COOH$ (Cyanoacetic acid):**
   - Cyanoacetic acid is the second most acidic. The cyano group ($-CN$) is a strong electron-withdrawing group due to its electronegativity and resonance effects. This group stabilizes the carboxylate ion formed after deprotonation, making the acid more acidic than formic acid ($HCOOH$) and acetic acid ($CH_{3}COOH$).

3. **$HCOOH$ (Formic acid):**
   - Formic acid is more acidic than acetic acid but less acidic than cyanoacetic acid. It lacks additional electron-withdrawing groups compared to the other acids, but it is still more acidic than acetic acid due to the absence of electron-donating alkyl groups.

4. **$CH_{3}COOH$ (Acetic acid):**
   - Acetic acid is the least acidic among the given compounds. The methyl group ($-CH_3$) is an electron-donating group, which slightly destabilizes the carboxylate ion formed after deprotonation. This makes acetic acid less acidic compared to the other acids listed.

### Why Other Options are Incorrect:

1. **Option 1: $CH_3COOH > HCOOH > CNCH_{2}COOH > CF_{3}COOH$**
   - This order is incorrect because it suggests that acetic acid is more acidic than formic acid, cyanoacetic acid, and trifluoroacetic acid, which contradicts the electron-withdrawing effects that increase acidity.

2. **Option 2: $CF_{3}COOH > CNCH_{2}COOH > CH_{3}COOH > HCOOH$**
   - This order incorrectly places acetic acid as more acidic than formic acid. Formic acid is more acidic than acetic acid due to the absence of an electron-donating methyl group.

3. **Option 3: $CNCH_{2}COOH > CH_{3}COOH > CF_{3}COOH > HCOOH$**
   - This order incorrectly places cyanoacetic acid as more acidic than trifluoroacetic acid and suggests that acetic acid is more acidic than trifluoroacetic acid and formic acid, which is not supported by the electron-withdrawing effects.

In conclusion, Option 4 correctly reflects the order of acidity based on the electron-withdrawing effects of substituents on the carboxylic acids, making it the correct answer.