Consider the following compounds
1. Phenol
2. o - chlorophenol
3. m - chloropheonl
4. p - chlorophenol
Place these compounds in the decreasing order of acidity

Options

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

Answer

To determine the decreasing order of acidity for the given compounds (Phenol, o-chlorophenol, m-chlorophenol, and p-chlorophenol), we need to analyze the effects of the chlorine substituent on the acidity of phenol. The acidity of these compounds can be assessed based on the stability of their conjugate bases after losing a proton (H⁺).

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Phenol (C₆H₅OH)
- o-Chlorophenol (C₆H₄ClOH)
- m-Chlorophenol (C₆H₄ClOH)
- p-Chlorophenol (C₆H₄ClOH)

2. **Understand Acidity**:
- Acidity is determined by the stability of the conjugate base formed after the compound donates a proton (H⁺).
- The more stable the conjugate base, the stronger the acid.

3. **Analyze the Effects of Chlorine**:
- Chlorine is an electron-withdrawing group (EWG) due to its -I (inductive) effect. This increases acidity by stabilizing the negative charge on the conjugate base.
- Chlorine can also exhibit a +M (mesomeric) effect, which can destabilize the conjugate base if it donates electron density.

4. **Stability of Conjugate Bases**:
- **Phenol**: No substituents, so it has a baseline acidity.
- **o-Chlorophenol**: The chlorine at the ortho position has both -I and +M effects. The +M effect destabilizes the conjugate base, but the -I effect stabilizes it somewhat.
- **m-Chlorophenol**: The chlorine at the meta position only exhibits the -I effect, which stabilizes the conjugate base without any destabilizing +M effect.
- **p-Chlorophenol**: The chlorine at the para position exhibits both -I and +M effects, similar to o-chlorophenol, but the distance reduces the +M effect's destabilizing influence compared to the ortho position.

5. **Order of Acidity**:
- The most acidic compound will be the one with the most stable conjugate base.
- The order of acidity based on the stability of the conjugate bases is:
- **m-Chlorophenol** (most stable conjugate base due to only -I effect)
- **o-Chlorophenol** (stabilized by -I but destabilized by +M)
- **p-Chlorophenol** (stabilized by -I, but +M effect is less pronounced than in o-chlorophenol)
- **Phenol** (least acidic as it has no substituents to stabilize the conjugate base)

### Final Order of Acidity:
The final order of acidity from strongest to weakest is:
**m-Chlorophenol > o-Chlorophenol > p-Chlorophenol > Phenol**