Identify the expected major organic product from the reaction shown below. Br2 / H20 ? Br OH Br OH ?? Br OH Br Br III A 1 B 11 III D IV V
The Correct Answer and Explanation is :
The reaction shown involves the addition of bromine (Br₂) and water (H₂O) to an alkene. This is a well-known halohydrin formation reaction, which follows anti-addition and regioselectivity due to the formation of a bromonium ion intermediate.
Step 1: Formation of the Bromonium Ion
- When Br₂ is added to the alkene, a three-membered bromonium ion intermediate forms. This structure prevents free rotation and leads to stereospecific addition.
Step 2: Nucleophilic Attack by Water
- Water (H₂O), acting as a nucleophile, attacks the more substituted carbon of the bromonium ion due to Markovnikov’s rule (since the more substituted carbon can better stabilize the positive charge).
- This attack occurs anti to the bromine due to steric hindrance, resulting in an anti-addition.
Step 3: Formation of the Halohydrin
- After deprotonation, the major product is a halohydrin where a hydroxyl (-OH) group attaches to the more substituted carbon and a bromine (Br) remains on the less substituted carbon.
Product Selection
Given the multiple-choice options in the image:
- The correct product should show Br and OH in anti orientation.
- The hydroxyl (-OH) should be on the more substituted carbon.
- The bromine (Br) should be on the less substituted carbon.
From the given options, Option II (B) appears to be the correct answer since it follows both anti-addition and regioselectivity rules.
Conclusion
The reaction of an alkene with Br₂ and H₂O leads to a halohydrin product with anti-stereochemistry. The bromonium ion intermediate dictates the regioselectivity, leading to the hydroxyl (-OH) attacking the more substituted carbon. Thus, the correct major organic product is II (B).