Question

Bromine adds to cis- and trans-2-butene to give different diastereomers of 2,3 -dibromobutane. What does this say about the mode of addition of bromine to this alkene? CC=CCBr CC=CCBr We discuss the addition of bromine to alkenes in Chapter \(6 .\)

Short answer

Answer: The addition of bromine to cis- and trans-2-butene results in the formation of different diastereomers of 2,3-dibromobutane. These diastereomers are stereoisomers that are not mirror images of each other. The stereochemistry of the final product is determined by the initial stereochemistry of the alkene. The bromine addition to cis-2-butene results in 2,3-dibromobutane with both bromines on the same side of the molecule, while the addition to trans-2-butene results in 2,3-dibromobutane with bromines on the opposite sides of the molecule.

Step-by-step solution

Identify cis- and trans-2-butene

To begin with, we should sketch the structure of cis- and trans-2-butene. Cis-2-butene has both methyl groups on the same side of the double bond, while trans-2-butene has the methyl groups on opposite sides of the double bond.

Understand the concept of diastereomers

Diastereomers are stereoisomers that are not mirror images of each other. In this case, they are formed by the addition of bromine to cis- and trans-2-butene, resulting in molecules with the same molecular formula but different arrangement of atoms in space.

Show the mechanism of bromine addition

Bromine addition to alkenes is a two-step process. First, the alkene reacts with the bromine molecule (Br2) to form a cyclic bromonium ion intermediate. Next, a second bromine ion (Br-) attacks the more stable carbocation (less hindered) from the opposite side of the bromonium ion, resulting in the formation of 2,3-dibromobutane.

Bromine addition to cis-2-butene

When bromine adds to cis-2-butene, the cyclic bromonium ion intermediate will have the same relative configuration as the starting cis-alkene. As the second bromine ion attacks from the opposite side of the bromonium ion, the resulting 2,3-dibromobutane will have one spatial configuration, with both bromines on the same side of the molecule.

Bromine addition to trans-2-butene

When bromine adds to trans-2-butene, the cyclic bromonium ion intermediate will have the same relative configuration as the starting trans-alkene. As the second bromine ion attacks from the opposite side of the bromonium ion, the resulting 2,3-dibromobutane will have a different spatial configuration, with both bromines on the opposite sides of the molecule.

Conclusion

The mode of addition of bromine to 2-butene (both cis- and trans-) is a two-step process involving the formation of a bromonium ion intermediate and subsequent attack by a bromine ion from the opposite side. The different diastereomers of 2,3-dibromobutane suggest that the addition of bromine to alkenes is a stereospecific reaction, as the initial stereochemistry of the alkene determines the stereochemistry of the final product.