In which of the following reactions, rearrangement is possible ?

Options

A. `E1`
B. `SN^1`
C. `SN^(2')`
D. `E1cB`

Explanation

To determine in which of the following reactions rearrangement is possible, we will analyze each reaction type: E1, SN1, SN2, and E1CB.

### Step-by-Step Solution:

1. **Understanding E1 Reactions**:
- E1 stands for unimolecular elimination. This reaction proceeds through a two-step mechanism where a carbocation intermediate is formed after the leaving group departs.
- Since rearrangement can occur at the carbocation stage (to form a more stable carbocation), rearrangement is possible in E1 reactions.

2. **Understanding SN1 Reactions**:
- SN1 stands for unimolecular nucleophilic substitution. Similar to E1, this reaction also involves a two-step mechanism where a carbocation intermediate is formed.
- Rearrangement can occur at the carbocation stage to form a more stable carbocation, making rearrangement possible in SN1 reactions.

3. **Understanding SN2 Reactions**:
- SN2 stands for bimolecular nucleophilic substitution. This reaction occurs in a single step, where the nucleophile attacks the substrate as the leaving group departs.
- Since there is no carbocation intermediate formed in SN2 reactions, rearrangement does not occur in this type of reaction.

4. **Understanding E1CB Reactions**:
- E1CB stands for unimolecular elimination via a conjugate base. This reaction occurs under basic conditions and involves the formation of a carbanion intermediate.
- The carbanion formed is typically stable and does not undergo rearrangement because it is an irreversible step. Thus, rearrangement is not possible in E1CB reactions.

### Conclusion:
From the analysis above, rearrangement is possible in E1 and SN1 reactions due to the presence of carbocation intermediates. Rearrangement is not possible in SN2 and E1CB reactions.

**Correct Options**: E1 and SN1 (Options A and B)