Which Intermediate formed during addition of halogen on alkenes ?

Options

A. Carbocation
B. Carbanion
C. cyclic halonium ion
D. halide ion

Answer

To solve the question regarding the intermediate formed during the addition of halogen to alkenes, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reaction**: The addition of halogens (like bromine, Cl2, etc.) to alkenes involves the reaction of a double bond with halogen molecules.

**Hint**: Remember that alkenes have a double bond which makes them reactive towards electrophiles.

2. **Formation of the Carbocation**: When halogen is added to an alkene, the first step involves the formation of a carbocation intermediate. For example, in the case of ethene (CH2=CH2), the double bond attacks a bromine molecule, leading to the formation of a bromonium ion.

**Hint**: The double bond acts as a nucleophile and attacks the halogen, leading to the formation of a positively charged intermediate.

3. **Formation of the Cyclic Halonium Ion**: Instead of a simple carbocation, the reaction leads to the formation of a cyclic halonium ion. This is a three-membered ring structure that includes the halogen atom and the two carbon atoms from the alkene.

**Hint**: Look for a structure where the halogen is part of a cyclic arrangement with the carbons, indicating a more stable intermediate.

4. **Nucleophilic Attack**: The bromide ion (Br-) then attacks the cyclic halonium ion from the backside due to steric hindrance, leading to the formation of a trans-dihalide product.

**Hint**: The backside attack is crucial for the stereochemistry of the product, resulting in a trans configuration.

5. **Final Product Formation**: The final product of the reaction is a vicinal dibromide (trans-dihalide), where the two bromine atoms are added to adjacent carbon atoms.

**Hint**: Keep in mind that the stereochemistry of the product is influenced by the cyclic nature of the intermediate.

### Conclusion:
The intermediate formed during the addition of halogen to alkenes is the **cyclic halonium ion**. This intermediate is crucial for the stereospecific formation of the final product, which is typically a trans-dihalide.