The addition of `Br_2` to ( E)-but-2-ene gives

Options

A. (R,R)-2-3-dibromobutane
B. (S,R)-2-3-dibromobutane
C. (R,S)-2-3-dibromobutane
D. A mixture of (R,R) and (S,S) 2,3 dibromobutane

Answer

To solve the question regarding the addition of \( \text{Br}_2 \) to (E)-but-2-ene, we will follow these steps:

### Step-by-Step Solution:

1. **Identify the Structure of (E)-but-2-ene**:
- (E)-but-2-ene has the structure where the two methyl groups (CH₃) are on opposite sides of the double bond.
- The structure can be represented as:
\[ \text{CH}_3 - \text{C} = \text{C} - \text{CH}_3 \]

2. **Mechanism of Addition of \( \text{Br}_2 \)**:
- The addition of \( \text{Br}_2 \) to alkenes occurs via an electrophilic addition mechanism.
- The π electrons from the double bond attack one of the bromine atoms, leading to the formation of a bromonium ion intermediate.

3. **Formation of Bromonium Ion**:
- When \( \text{Br}_2 \) approaches the double bond, one bromine atom forms a bond with one of the carbon atoms, creating a cyclic bromonium ion.
- The structure of the bromonium ion can be depicted as:
\[ \text{Br}^+ - \text{C} - \text{C} - \text{Br}^- \]

4. **Nucleophilic Attack**:
- The bromonium ion is then attacked by a bromide ion (\( \text{Br}^- \)) at the more substituted carbon atom (due to sterics and stability), leading to the formation of a dibromide.
- The attack occurs from the opposite side of the bromonium ion, resulting in anti-addition.

5. **Final Product Structure**:
- The final product of the reaction will be 2,3-dibromobutane. Since the starting material was (E)-but-2-ene, the product will have the bromine atoms added to opposite sides (anti) of the double bond.
- The structure can be represented as:
\[ \text{CH}_3 - \text{C}(\text{Br}) - \text{C}(\text{Br}) - \text{CH}_3 \]

6. **Assigning Configuration**:
- The product will have two stereocenters, and we can assign the configurations (R/S) to each carbon.
- For the first stereocenter, the priority is assigned based on the atomic number of the groups attached. The configuration can be determined to be (R) or (S) based on the Cahn-Ingold-Prelog priority rules.

7. **Final Answer**:
- The final product of the addition of \( \text{Br}_2 \) to (E)-but-2-ene is **(2R,3S)-2,3-dibromobutane**.