Arrange the following compounds in decreasing order of acidity.
(i) `ClCH_(2)CH_(2)CH_(2)COOH`
(ii) `CH_(3)CHClCH_(2)COOH`
(iii) `CH_(3)CH_(2)CHClCOOH`

Options

A. `I gt II gt III`
B. `III gt II gt I`
C. `I gt III gt II`
D. `III gt I gt II`

Answer

To arrange the given compounds in decreasing order of acidity, we need to analyze the effect of the chlorine (Cl) substituent on the acidity of the carboxylic acids. The presence of electronegative atoms such as chlorine can influence acidity through the inductive effect.

### Step-by-Step Solution:

1. **Identify the Structures**:
- (i) `ClCH₂CH₂CH₂COOH` (Compound A)
- (ii) `CH₃CHClCH₂COOH` (Compound B)
- (iii) `CH₃CH₂CHClCOOH` (Compound C)

2. **Understand the Inductive Effect**:
- The acidity of carboxylic acids is influenced by the stability of the carboxylate ion formed after deprotonation. The more stable the carboxylate ion, the stronger the acid.
- Chlorine is an electronegative atom and exerts a -I (inductive) effect, which pulls electron density away from the carboxyl group, increasing the positive charge density on the hydrogen atom of the carboxylic acid, thus increasing acidity.

3. **Analyze Each Compound**:
- **Compound A (`ClCH₂CH₂CH₂COOH`)**: The Cl is located at the end of the carbon chain, which means it has the least effect on the acidity of the carboxylic acid since it is farthest from the carboxyl group.
- **Compound B (`CH₃CHClCH₂COOH`)**: The Cl is located on the second carbon, which is closer to the carboxyl group compared to Compound A. This means it has a stronger -I effect, increasing the acidity more than Compound A.
- **Compound C (`CH₃CH₂CHClCOOH`)**: The Cl is on the third carbon, which is also relatively close to the carboxyl group, but not as close as in Compound B. Therefore, it will have a moderate -I effect.

4. **Rank the Acidity**:
- Compound B has the strongest acidity due to the Cl being closest to the carboxyl group.
- Compound C is next, as the Cl is still relatively close but not as effective as in Compound B.
- Compound A has the weakest acidity since the Cl is farthest from the carboxyl group.

### Final Order of Acidity:
1. **(ii) `CH₃CHClCH₂COOH` (Compound B)**
2. **(iii) `CH₃CH₂CHClCOOH` (Compound C)**
3. **(i) `ClCH₂CH₂CH₂COOH` (Compound A)**

Thus, the final order of acidity in decreasing order is:
**B > C > A**