Rank the following compounds in increasing order of their acidity:
1. 3 - fluorobutanoic acid
2. 3 - chlorobutanoic acid
3. 2 - flurobutanoic acid
4. Butanoic acid
5. 4 - chlorobutanoic acid.

Options

A. `3 lt 1 lt 2 lt 4 lt 5`
B. `4 lt 3 lt 1 lt 2 lt 5`
C. `5 lt 2 lt 1 lt 3 lt 4`
D. `4 lt 5 lt 2 lt 1 lt 3`

Answer

To rank the given compounds in increasing order of their acidity, we will analyze the effects of the substituents on the carboxylic acid group. The presence of electron-withdrawing groups (EWGs) increases acidity, while electron-donating groups (EDGs) decrease acidity.

### Step-by-Step Solution:

1. **Identify the Compounds and Their Structures**:
- 1. 3-fluorobutanoic acid
- 2. 3-chlorobutanoic acid
- 3. 2-fluorobutanoic acid
- 4. Butanoic acid
- 5. 4-chlorobutanoic acid

2. **Understand the Influence of Substituents**:
- Fluorine (F) and chlorine (Cl) are both electron-withdrawing groups due to their electronegativity, but fluorine has a stronger -I (inductive) effect compared to chlorine.
- The position of these substituents relative to the carboxylic acid group is crucial. The closer the substituent is to the carboxylic acid group, the stronger the effect on acidity.

3. **Analyze Each Compound**:
- **3-fluorobutanoic acid (1)**: The fluorine is on the third carbon, which is relatively close to the carboxylic group, enhancing acidity.
- **3-chlorobutanoic acid (2)**: The chlorine is also on the third carbon, but it has a weaker -I effect than fluorine.
- **2-fluorobutanoic acid (3)**: The fluorine is on the second carbon, which is even closer to the carboxylic group, making it the most acidic.
- **Butanoic acid (4)**: This compound has no electron-withdrawing groups, making it the least acidic.
- **4-chlorobutanoic acid (5)**: The chlorine is on the fourth carbon, which is the farthest from the carboxylic group, thus having the least effect on acidity.

4. **Rank the Acidity**:
- Based on the above analysis, the order of acidity from highest to lowest is:
- 2-fluorobutanoic acid (3) > 3-fluorobutanoic acid (1) > 3-chlorobutanoic acid (2) > 4-chlorobutanoic acid (5) > Butanoic acid (4).

5. **Final Order**:
- Therefore, the increasing order of acidity is:
- Butanoic acid (4) < 4-chlorobutanoic acid (5) < 3-chlorobutanoic acid (2) < 3-fluorobutanoic acid (1) < 2-fluorobutanoic acid (3).

### Increasing Order of Acidity:
**4 < 5 < 2 < 1 < 3**