Nucleophilic substitution reactions generally expressed as
`Nu^(-) +R-L rarr R-Nu +L^(-)`
Where `Nu^(-) rarr` Nucleophile , `R-L rarr` substrate, `L rarr` leaving group
The best leaving groups are those that become the most stable ions after they depart. since most leaving group leave as a negatibe ion, the best leaving groups are those ions that stabilize a negative charge most effectively. A good leaving group should be
(a) electron-withdrawing to polarize the carbon
(b) stable once it has left (not a strong base)
(c) polaristable to maintain partial bonding with the carbon in the transition state (both `S_(N)1` and `S_(N)2)`. This bonding helps to stabilise the transition state and reduces the activation energy.
(I) `CI^(-)` (II) `CH_(3)O^(-)` (III) `CH_(3)S^(-)` (IV) `I^(-)`
The correct order of increasing leaving group capability of above anions

Options

A. `III lt IV lt II lt I`
B. `II lt III lt I lt IV`
C. `II lt IV lt III lt I`
D. `I lt III lt II lt IV`

Answer

To determine the correct order of increasing leaving group capability of the given anions (I: CI^(-), II: CH₃O^(-), III: CH₃S^(-), IV: I^(-)), we need to analyze their acidity and basicity. The leaving group ability is directly related to the acidity of the corresponding acid; the more stable the leaving group (which corresponds to a more acidic acid), the better it is as a leaving group.

### Step-by-Step Solution:

1. **Understanding Leaving Group Stability**:
- A good leaving group is one that can stabilize the negative charge effectively after it leaves. This is often related to the acidity of the corresponding conjugate acid.

2. **Analyzing the Given Anions**:
- **I: CI^(-)**: This is the conjugate base of hydrochloric acid (HCl). Chlorine is a halogen and has a relatively stable negative charge.
- **II: CH₃O^(-)**: This is the conjugate base of methanol (CH₃OH). Oxygen is less stable with a negative charge compared to halogens.
- **III: CH₃S^(-)**: This is the conjugate base of methanethiol (CH₃SH). Sulfur can stabilize the negative charge better than oxygen but not as well as halogens.
- **IV: I^(-)**: This is the conjugate base of hydroiodic acid (HI). Iodine is larger and can stabilize the negative charge very effectively.

3. **Order of Acidity**:
- The order of acidity for the corresponding acids is: HI > HCl > CH₃SH > CH₃OH.
- This means the order of stability for the leaving groups is: I^(-) > Cl^(-) > CH₃S^(-) > CH₃O^(-).

4. **Order of Leaving Group Capability**:
- Since leaving group capability increases with acidity, the order of increasing leaving group capability is the reverse of the order of acidity:
- CH₃O^(-) < CH₃S^(-) < Cl^(-) < I^(-).

5. **Final Answer**:
- The correct order of increasing leaving group capability is:
- CH₃O^(-) < CH₃S^(-) < Cl^(-) < I^(-).