Select the correct description of the anti addition of bromine to cyclopentene.
a. After one bromine atom adds to a carbon atom, the other bromine atom adds to the number 3 carbon atom.
b. The bromine atoms add to the opposite faces of the double bond.
c. The bromine atoms add to the same face of the double bond.
d. Both bromine atoms add to the same carbon atom.
The Correct Answer and Explanation is :
The correct answer is:
b. The bromine atoms add to the opposite faces of the double bond.
Explanation:
The reaction described is an example of anti addition, which is a type of addition reaction that occurs when two substituents are added to opposite sides (or faces) of a double bond. In the case of cyclopentene and bromine, the addition of bromine occurs through a three-membered bromonium ion intermediate.
Here’s a step-by-step breakdown of the process:
- Bromine molecule approaches the cyclopentene: The electron-rich double bond of cyclopentene attacks the bromine (Br2) molecule, which leads to the formation of a bromonium ion. The double bond breaks, and one of the bromine atoms forms a three-membered ring with the two carbon atoms that were previously involved in the double bond.
- Formation of the bromonium ion: The bromonium ion is positively charged and has a three-membered ring structure with the bromine atom bonded to the two carbon atoms that were part of the original double bond. This intermediate is electrophilic, meaning it is highly reactive and unstable.
- Nucleophilic attack by bromide ion: A bromide ion (Br−) formed during the reaction now attacks one of the carbon atoms in the bromonium ion. Because the carbon atoms are part of a three-membered ring, the attack occurs at the more substituted carbon atom, following Markovnikov’s rule.
- Anti addition: The key feature of this reaction is that the two bromine atoms add to opposite faces (or sides) of the double bond. This means that the bromine atoms end up on opposite sides of the cyclopentene ring.
This anti addition occurs because of the mechanism involving the bromonium ion, which is a three-membered ring intermediate. The addition of the second bromine atom occurs from the opposite side of the bromonium ring to relieve the strain in the intermediate.
I’ll generate an image to visually explain this reaction mechanism.
Here is the diagram illustrating the anti addition of bromine to cyclopentene. It shows the formation of the bromonium ion and how the two bromine atoms add to opposite faces of the double bond.
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