The correct order of acidic strength of the following compound is :

Question

Accepted Answer

The correct order of acidic strength for the given compounds is provided as Option 3: $CH_3COOH > H_2CO_3 > C_6H_5OH > H_2O > C_2H_2$. Let's delve into the reasoning behind this order and why it is correct, while also explaining why the other options are incorrect.

### Explanation for Option 3:

1. **Acetic Acid ($CH_3COOH$):** Acetic acid is a carboxylic acid, which is known for its relatively high acidic strength due to the resonance stabilization of its conjugate base (acetate ion). The carboxylate ion formed after losing a proton is stabilized by resonance, making acetic acid a strong acid among the given compounds.

2. **Carbonic Acid ($H_2CO_3$):** Carbonic acid is a weak acid, but it is stronger than phenol and water. It can dissociate to form bicarbonate and carbonate ions, which are relatively stable. This makes carbonic acid more acidic than phenol.

3. **Phenol ($C_6H_5OH$):** Phenol is more acidic than water due to the resonance stabilization of the phenoxide ion formed after deprotonation. However, it is less acidic than carbonic acid and acetic acid because the resonance stabilization is not as effective as in the carboxylate ion.

4. **Water ($H_2O$):** Water is a very weak acid. It can donate a proton to form hydroxide ions, but this occurs to a very small extent. It is less acidic than phenol.

5. **Acetylene ($C_2H_2$):** Acetylene is the least acidic among the given compounds. It is a terminal alkyne and can donate a proton to form an acetylide ion, but this is not favored compared to the other compounds listed.

### Why Other Options are Incorrect:

- **Option 1 ($CH_3COOH > C_6H_5OH > H_2CO_3 > H_2O > C_2H_2$):** This option incorrectly places phenol ($C_6H_5OH$) as more acidic than carbonic acid ($H_2CO_3$). Carbonic acid is stronger than phenol due to its ability to form stable bicarbonate and carbonate ions.

- **Option 2 ($CH_3COOH > H_2CO_3 > C_6H_5OH > C_2H_2 > H_2O$):** This option incorrectly places acetylene ($C_2H_2$) as more acidic than water ($H_2O$). Water is more acidic than acetylene because the hydroxide ion is more stable than the acetylide ion.

- **Option 4 ($C_6H_5OH > CH_3COOH > H_2CO_3 > C_2H_2 > H_2O$):** This option incorrectly places phenol ($C_6H_5OH$) as more acidic than acetic acid ($CH_3COOH$). Acetic acid is more acidic due to the resonance stabilization of its conjugate base.

In conclusion, Option 3 correctly orders the compounds based on their acidic strength, taking into account the stability of their conjugate bases and the nature of the acidic hydrogen.