What will happen when `Br_2//C Cl_4` react with (a) cis But-2-ene (b) trans But-2-ene.

Answer

To determine the products formed when `Br2` in `CCl4` reacts with (a) cis-but-2-ene and (b) trans-but-2-ene, we will analyze each reaction step by step.

### (a) Reaction of cis-but-2-ene with Br2/CCl4

1. **Structure of cis-but-2-ene**: The structure of cis-but-2-ene can be represented as:
```
CH3
|
H2C = C
|
CH3
```

2. **Formation of Bromonium Ion**: When cis-but-2-ene reacts with bromine (Br2), the double bond attacks one of the bromine atoms, leading to the formation of a bromonium ion. The other bromine atom leaves as a bromide ion (Br-). The bromonium ion has a three-membered ring structure:
```
CH3
|
H2C - C^+ - Br
|
CH3
```

3. **Nucleophilic Attack**: The bromide ion (Br-) can attack the more substituted carbon atom of the bromonium ion from the opposite side (backside attack) due to steric hindrance. This results in two possible products due to the different attack positions:
- **Product 1**:
```
CH3
|
H2C - C - Br
|
CH3
```
- **Product 2**:
```
CH3
|
H2C - C - Br
|
H
```

4. **Formation of Enantiomers**: The two products formed are enantiomers (mirror images) of each other. Thus, the reaction of cis-but-2-ene with Br2/CCl4 leads to a racemic mixture of 2,3-dibromobutane.

### (b) Reaction of trans-but-2-ene with Br2/CCl4

1. **Structure of trans-but-2-ene**: The structure of trans-but-2-ene can be represented as:
```
CH3
|
H2C = C
|
CH3
```

2. **Formation of Bromonium Ion**: Similar to the cis isomer, when trans-but-2-ene reacts with bromine (Br2), a bromonium ion is formed:
```
CH3
|
H2C - C^+ - Br
|
CH3
```

3. **Nucleophilic Attack**: The bromide ion (Br-) can attack either of the carbon atoms in the bromonium ion. However, due to the symmetry of the trans isomer, both attack positions yield the same product:
- **Product 1**:
```
CH3
|
H2C - C - Br
|
H
```
- **Product 2**:
```
CH3
|
H2C - C - Br
|
H
```

4. **Formation of Mesocompounds**: The two products formed from the reaction of trans-but-2-ene are not enantiomers but rather meso compounds, which have a plane of symmetry. Therefore, the reaction of trans-but-2-ene with Br2/CCl4 leads to a single meso compound, 2,3-dibromobutane.

### Summary of Products
- **cis-but-2-ene + Br2/CCl4**: Forms a racemic mixture of enantiomers (2,3-dibromobutane).
- **trans-but-2-ene + Br2/CCl4**: Forms a single meso compound (2,3-dibromobutane).