Arrange the following in order of their leaving group tendency ?
`underset((I))(Br)^(-)` , `H_(3)C-underset((II))overset(O)overset(||)C-(O)^(-)` , `CH_(3)CH_(2)O^(-)` , `Ph-O^(-)`

Options

A. `IgtIIgtIIIgtIV`
B. `IgtIIIgtIIgtIV`
C. `IgtIIgtIVgtIII`
D. `IgtIVgtIIIgtII`

Answer

To arrange the given leaving groups in order of their leaving group tendency, we need to analyze the stability of the leaving groups when they leave as anions. The more stable the leaving group is as an anion, the better it is as a leaving group.

### Step 1: Identify the leaving groups
The leaving groups we need to compare are:
1. \( Br^- \) (Bromide)
2. \( CH_3C(=O)O^- \) (Acetate ion)
3. \( CH_3CH_2O^- \) (Ethoxide ion)
4. \( PhO^- \) (Phenoxide ion)

### Step 2: Analyze the stability of each leaving group
1. **Bromide ion (\( Br^- \))**:
- Bromide is a halide ion and is relatively stable due to its large size and ability to stabilize the negative charge through dispersion.
- **Stability**: High

2. **Acetate ion (\( CH_3C(=O)O^- \))**:
- The acetate ion can stabilize the negative charge through resonance. The negative charge can be delocalized between the two oxygen atoms.
- **Stability**: Moderate to High

3. **Ethoxide ion (\( CH_3CH_2O^- \))**:
- The ethoxide ion has a negative charge localized on the oxygen atom, and the presence of alkyl groups (ethyl in this case) can cause steric hindrance and destabilization of the negative charge.
- **Stability**: Low

4. **Phenoxide ion (\( PhO^- \))**:
- The phenoxide ion has some resonance stabilization due to the aromatic ring, but the aromatic system is relatively less reactive towards nucleophilic substitution, making it less favorable as a leaving group compared to bromide and acetate.
- **Stability**: Moderate

### Step 3: Rank the leaving groups based on their stability
Based on the analysis:
1. \( Br^- \) (most stable, best leaving group)
2. \( CH_3C(=O)O^- \) (stable due to resonance)
3. \( PhO^- \) (moderate stability due to resonance but less reactive)
4. \( CH_3CH_2O^- \) (least stable, poor leaving group)

### Final Order of Leaving Group Tendency
Thus, the order of leaving group tendency from best to worst is:
\[ Br^- > CH_3C(=O)O^- > PhO^- > CH_3CH_2O^- \]