Question

Describe the stereochemistry of the bromohydrin formed in each reaction (each reaction is stereospecific). (a) cis-3-Hexene \(+\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\) (b) trans-3-Hexene \(+\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\)

Short answer

Answer: The stereochemical products formed are (2S,3S)-3-bromo-3-hydroxyhexane for cis-3-Hexene and (2R,3R)-3-bromo-3-hydroxyhexane for trans-3-Hexene.

Step-by-step solution

Identify the reactant configuration

cis-3-Hexene has a cis configuration, meaning that the double bond is between C3 and C4 and both of the alkyl groups are on the same side of the molecule.

Bromohydration Mechanism

The reaction of an alkene with Br₂/H₂O involves a three-step process: 1. Formation of the bromonium ion: Br₂ attacks the double bond, resulting in the formation of a bromonium ion (a cyclic intermediate with a positive charge on the bromine atom). 2. Nucleophilic attack: H₂O acts as a nucleophile to attack the more substituted carbon atom opened the cyclic intermediate. 3. Deprotonation: A base deprotonates the oxygen atom, leading to the formation of a bromohydrin.

Determine the stereochemistry of the product

Since the bromonium ion formation leads to a cyclic transition state, the nucleophilic attack of H₂O occurs anti to the bromine atom. So, the bromohydrin formed from cis-3-Hexene will have anti stereochemistry, with the OH group attached to the more substituted carbon. Thus, the product formed is (2S,3S)-3-bromo-3-hydroxyhexane. (b) trans-3-Hexene + Br₂/H₂O

Identify the reactant configuration

trans-3-Hexene has a trans configuration, meaning that the double bond is between C3 and C4, and the alkyl groups are on the opposite sides of the molecule.

Bromohydration Mechanism

The reaction mechanism is the same as in part (a). We will follow the same steps: bromonium ion formation, nucleophilic attack, and deprotonation.

Determine the stereochemistry of the product

The bromohydrin product will also have anti stereochemistry due to the cyclic bromonium ion. The OH group will be attached to the more substituted carbon. The product formed is (2R,3R)-3-bromo-3-hydroxyhexane.