The leaving group is that functional group which is ejected with `overline e^, s` of the `sigma` -bond in a reaction. Better the leaving group, faster is the reaction. The relative leaving ability of the leaving group `X` in `(R - X)` is increased by :
(i) The polarisability of `(R - X)` bond.
(ii) The stability of `X^(Ө)`.
(iii) The degree of stabilisation through solvation of `X`.
(iv) The strength of `(R - X)` bond.
The leaving group tendency is also called fugacity.
Which of the following undergoes `SN` reaction easily ?

Options

A. Vinyl bromide
B. Bromobenzene
C. p-Nitro bromobenzene
D.

Explanation

To determine which compound undergoes an SN reaction easily, we need to analyze the four options provided in the question. The SN (substitution nucleophilic) reaction can be influenced by the structure of the leaving group and the stability of the resulting carbocation or intermediate.

### Step-by-step Solution:

1. **Identify the Compounds**: The question provides four options:
- (i) Vinyl bromide
- (ii) Bromobenzene
- (iii) Para-nitrobromobenzene
- (iv) Bridgehead halide

2. **Analyze Vinyl Bromide**:
- Vinyl bromide is a vinyl halide, which typically does not undergo SN1 or SN2 reactions due to the stability of the vinyl cation being low. Therefore, this option is not correct.

3. **Analyze Bromobenzene**:
- Bromobenzene is an aryl halide. Aryl halides generally do not undergo SN reactions unless the benzene ring is activated by an electron-withdrawing group (EWG). Since bromobenzene lacks such activation, this option is also not correct.

4. **Analyze Para-nitrobromobenzene**:
- Para-nitrobromobenzene has a nitro group (NO2) that is an electron-withdrawing group. This activation allows the benzene ring to undergo SN reactions. Therefore, this compound can undergo an SN reaction easily, making this option correct.

5. **Analyze Bridgehead Halide**:
- Bridgehead halides are typically inert to both SN1 and SN2 reactions due to steric hindrance and the inability to form stable intermediates. Thus, this option is not correct.

### Conclusion:
The compound that undergoes an SN reaction easily is **para-nitrobromobenzene** (option iii).

### Summary of Steps:
- **Step 1**: Identify the compounds.
- **Step 2**: Analyze vinyl bromide; it does not undergo SN reactions.
- **Step 3**: Analyze bromobenzene; it does not undergo SN reactions without activation.
- **Step 4**: Analyze para-nitrobromobenzene; it can undergo SN reactions due to the nitro group.
- **Step 5**: Analyze bridgehead halide; it does not undergo SN reactions.