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) `CH_(3)Br` (II) `CH_(3)F` (III) `CH_(3)OH` (IV) `CH_(3)OSO_(2)CF_(3)`
The correct order of decreasing reactivity of the above compounds towards `CH_(3)O^(-)` in an `S_(N)2` reaction is:

Options

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

Answer

To determine the correct order of decreasing reactivity of the given compounds towards `CH3O^(-)` in an `S_N2` reaction, we need to analyze the leaving groups in each compound based on their stability and basicity.

### Step-by-Step Solution:

1. **Identify the Compounds and Their Leaving Groups**:
- (I) `CH3Br` - Leaving group: `Br^(-)`
- (II) `CH3F` - Leaving group: `F^(-)`
- (III) `CH3OH` - Leaving group: `OH^(-)`
- (IV) `CH3OSO2CF3` - Leaving group: `OSO2CF3^(-)`

2. **Evaluate the Stability of Leaving Groups**:
- A good leaving group is one that can stabilize a negative charge effectively after it departs.
- The order of leaving group ability generally follows the trend of acidity: the more stable the leaving group as an ion, the better it is as a leaving group.

3. **Determine the Acidity of the Leaving Groups**:
- `Br^(-)` is a weak base and thus a good leaving group.
- `F^(-)` is a strong base and a poor leaving group.
- `OH^(-)` is also a strong base and a poor leaving group.
- `OSO2CF3^(-)` (the triflate ion) is a very stable leaving group due to resonance stabilization.

4. **Rank the Leaving Groups**:
- Based on the above analysis, we can rank the leaving groups from best to worst:
- `OSO2CF3^(-)` > `Br^(-)` > `OH^(-)` > `F^(-)`

5. **Determine the Reactivity Order in `S_N2` Reactions**:
- Since `S_N2` reactions are directly proportional to the quality of the leaving group, we can now establish the order of reactivity:
- `CH3OSO2CF3` > `CH3Br` > `CH3OH` > `CH3F`

6. **Final Order**:
- Therefore, the correct order of decreasing reactivity of the compounds towards `CH3O^(-)` in an `S_N2` reaction is:
- **(IV) > (I) > (III) > (II)**

### Final Answer:
The correct order of decreasing reactivity is: **`CH3OSO2CF3` > `CH3Br` > `CH3OH` > `CH3F`**.