What of the following is expected to be most highly ionised in water ?

Options

A. `CH_2 ClCH_2 CH_2 COOH`
B. `CH_3 CHClCH_2 COOH`
C. `CH_3CH_2 C Cl_2 COOH`
D. `CH_3CH_2 CHClCOOH`

Explanation

To determine which compound is expected to be the most highly ionized in water, we need to analyze the acidic strength of the given compounds based on the presence of electron-withdrawing groups. Here’s a step-by-step solution:

### Step 1: Identify the Compounds
The compounds given in the question are:
1. A: CH2ClCH2C(=O)OH
2. B: CH3CHClC(=O)OH
3. C: CH3CH2CCl2C(=O)OH
4. D: CH3CH2CHClC(=O)OH

### Step 2: Understand Ionization and Acidity
To be highly ionized in water, a compound must be acidic. The acidity of carboxylic acids can be enhanced by the presence of electron-withdrawing groups (EWGs) such as halogens. The more EWGs present, the stronger the acid, and thus the more ionized it will be in solution.

### Step 3: Analyze Each Compound
- **Compound A (CH2ClCH2C(=O)OH)**: Contains one chlorine atom, which has a -I (inductive) effect. However, it is positioned two carbons away from the carboxylic acid group, which reduces its effectiveness in increasing acidity.

- **Compound B (CH3CHClC(=O)OH)**: Contains one chlorine atom, but it is positioned one carbon away from the carboxylic acid group. This proximity increases its -I effect compared to Compound A, making it a stronger acid.

- **Compound C (CH3CH2CCl2C(=O)OH)**: Contains two chlorine atoms, both of which exert a -I effect. Since both chlorines are on the alpha carbon (one carbon away from the carboxylic acid), the combined -I effect is stronger than in A or B, making it a stronger acid.

- **Compound D (CH3CH2CHClC(=O)OH)**: Contains one chlorine atom, but it is positioned two carbons away from the carboxylic acid group, similar to Compound A. Therefore, it has a weaker -I effect.

### Step 4: Compare the Acidity
Based on the analysis:
- Compound A has the weakest acidity due to the far position of Cl.
- Compound B is stronger than A due to the closer position of Cl.
- Compound D is similar to A in terms of acidity.
- Compound C is the strongest due to the presence of two chlorine atoms, both exerting a -I effect from the alpha position.

### Conclusion
The compound expected to be the most highly ionized in water is **Compound C (CH3CH2CCl2C(=O)OH)**, as it has the highest acidity due to the presence of two electron-withdrawing chlorine atoms.

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