Which of the following statement are true ?

Options

A. Bridgehead halide are inert for both `S_N1` and `S_N2` reaction.
B. The fist step in both `S_N1` and E1 reaction is same
C. `S_N2` reaction proceed with total retention of configuration .
D. E2 elimination are favoured by weak base

Explanation

To determine which of the statements are true, let's analyze each statement step by step based on the information provided in the video transcript.

### Step-by-Step Solution:

1. **Statement A: Bridgehead halides are inert for SN1 and SN2 reactions.**
- **Analysis:** In a bridgehead position, the formation of a carbocation (which is necessary for SN1) is not stable due to steric hindrance and the nature of the bridgehead structure. The SN2 mechanism also cannot occur effectively because the nucleophile cannot attack from the backside due to steric hindrance. Therefore, this statement is **true**.

2. **Statement B: The first step in both SN1 and elimination reactions are the same.**
- **Analysis:** In both SN1 and elimination reactions, the first step involves the removal of a good leaving group, leading to the formation of a carbocation. This is a common feature in both reaction mechanisms. Therefore, this statement is **true**.

3. **Statement C: SN2 reactions proceed by total retention of configuration.**
- **Analysis:** SN2 reactions are characterized by an inversion of configuration at the carbon center where the nucleophile attacks. This means that the configuration is not retained but inverted. Therefore, this statement is **false**.

4. **Statement D: E2 eliminations are favored by weak bases.**
- **Analysis:** E2 reactions typically require strong bases to abstract a proton and facilitate elimination. Weak bases do not favor E2 eliminations. Therefore, this statement is **false**.

### Conclusion:
The true statements are **A and B**.